Abstract
Bridge-like lipid transporters (BLTPs) have recently been revealed as key regulators of intraorganellar lipid trafficking, with their loss being associated with defective synaptic signalling and congenital neurological diseases. This group consists of five protein subfamilies [BLTP1-3, autophagy-related 2 (ATG2), and vacuolar protein sorting 13 (VPS13)], which mediate minimally selective lipid transfer between cellular membranes. Deceptively simple in both structure and presumed function, this review addresses open questions as to how bridge-like transporters work, the functional consequences of bulk lipid transfer on cellular signalling, and summarises some recent studies that have shed light on the surprising level of regulation and specificity found in this family of transporters.
Introduction
For several decades, subcellular organelles were considered distinct entities, whose membrane lipid content could only be changed by the action of organelle-targeted enzymes, by subcellular sorting performed by adaptor proteins bending and sorting lipids into specific geometries to be scissioned and trafficked away from its parent organelle, or by admixture of organelle membranes via mechanisms such as SNARE-mediated fusion. Excitingly, recent decades have revealed the existence of non-vesicular lipid transfer by specialised transporters at proteinaceous organelle-to-organelle junctions (membrane contacts) whereby selected membrane lipids are exchanged across a narrow organelle-to-organelle gap.
In the last few years, genetic studies combined with structural insight from protein modelling (; Neuman et al., 2022a) revealed a significant group of structurally similar proteins, where the lipid transporter, rather than fostering an exchange mechanism, forms a physical bridge that spans the gap between a donor and a receiver organelle. These bridge-like transfer proteins (BLTPs) are formed by multiple repeats of one motif: a repeating beta groove (RBG) (Neuman et al., 2022a), which forms an extremely long semi-open tube with a hydrophobic interior (). These repeats and the tube they form are half-open to solvent, whereby lipids are conducted from one end to the other of the organelle gap by a single protein. This gap can span as much as ~30 nm between organelles and is known to have critical roles in establishing membrane function. Our current understanding of the function of BLTPs is relatively crude: they are thought to be comparatively unselective in the lipids they transport and to act essentially as a ‘firehose’ delivering phospholipids to their target membranes. However, as our understanding of these mechanisms develops, researchers have found a surprising amount of subtlety as to how each of the five members of the BLTP family functions, and how lipid transfer via each of these transporters leads to signalling and trafficking deficits in human patients and model organisms.
Understanding the properties, regulation, and function of this class of proteins will provide significant insight into how organelles maintain their identity, along with transmembrane cargo and adaptors, while allowing response to physiological changes. This review discusses some of the key open questions concerning the nature and function of these proteins and presents current evidence as to how this family of proteins works to drive organelle function.
The structure of bridge-like transporters
The group of bridge-like transporters (BLTPs) form a small, but well-conserved (Table 1) group of five protein groups(; Neuman et al., 2022a) BLTP1-3, autophagy-related 2 (ATG2) and vacuolar protein sorting 13 (VPS13), and their paralogues. The group shares some common features: an N-terminal Chorein domain [a scoop-shaped domain with a hydrophobic interior, whose evolutionary origins may extend as far back as a common ancestry with bacteria (Neuman et al., 2022a; )], which directly funnels into the interior of anywhere from 6 to 17 repeats of the Repeating Beta Groove (RBG) motif (Neuman et al., 2022a; ). The RBG motif is a five antiparallel beta-stranded repeat that curves into a U or ‘taco’ shape and terminates in an unstructured loop, which when repeated, spans the distance between donor and acceptor membrane. BLTPs show extensive and conserved cytosolic loops and patches that decorate the cytosolic length of the tube formed by the RBG repeats (), but few specific interactors of these patches are known. At both the N- and C-termini of these proteins, which form the interfaces with donor and acceptor membranes, the proteins become more specialised among individual family members. Nearly every member of this family has been associated with human disease (Ugur et al., 2020; see Table 2), highlighting the important physiological role that this form of lipid transport plays.
Table 1
| H. sapiens | D. melanogaster | C. elegans | S. cerevisiae | D. discoideum | A. thaliana |
|---|---|---|---|---|---|
| BLTP1 (KIAA1109) | Tweek | lpd-3 | CSF1 | DDB_G0289829 | |
| BLTP2 (KIAA0100) | Hobbit | bltp-2 | Fmp27 (HOB1), HOB2 | SABRE, KINKY POLLEN | |
| BLTP3A (UHRF1BP1) BLTP3B (SHIP164/UHRF1BP1L) | CG34126 | C44H4.4 | DDB_G0279089 | AT3G20720 (Q84R14) | |
| ATG2A, ATG2B | Atg2 | atg-2 | ATG2 | Atg2 (DDB_G0277419) DDB_G0282057 | ATG2 (Wang et al., 2011) |
| VPS13A (ChAc) VPS13B, VPS13C VPS13D | Vps13, Vps13B, Vps13D | VPS-13A (T08g11) VPS-13D (C25H3.11) () | VPS13 | VPS13A VPS13B VPS13C (TipC), VPS13D VPS13E VPS13F () | AtVPS13S (Shrubby) AtVPS13M1 AtVPS13M2(Velayos-Baeza et al., 2004) AtVPS13X () |
Known orthologues of BLTP proteins (alternative names for the same gene in brackets).
Table 2
| Protein | Disease | N-terminal membrane (except chorein domain) | C-terminal membrane |
|---|---|---|---|
| BLTP1 (KIAA1109) | Alkuraya-Kucinskas Syndrome (; ) (MIM 617822) | Transmembrane helix () spigot (C1orf43) via chorein and N terminal RBGs () ER | Polybasic patch (expected to bind Phosphoinositide lipids) (Wang et al., 2022) Plasma membrane |
| BLTP2 (KIAA0100) | transmembrane helix (Neuman et al., 2022b) ER | Phosphoinositide lipids (PI, PIP, PIP2, PIP3) (Neuman et al., 2022b) PI4P () FAM102A/B via helical binding domain () Amphiphysin2 via RXP SH3 binding motif () Plasma membrane | |
| BLTP3A (UHRF1BP1) | Systemic lupus erythematosus (Wen et al., 2020) | rab7via RBG2 (Hanna et al., 2025) late endosome/lysosome | Rab45 () Centrosome ? via RBG6 (Hanna et al., 2025) VAMP7-positive vesicles LC3, GABARAP-via LIR motif (Hanna et al., 2025) Stressed/damaged lysosome |
| BLTP3B (SHIP164/UHRF1BP1L) | Parkinson’s disease () | Rab5 () Early endosomes RhoBTB3 (Wang et al., 2024) Golgi | syntaxin 6 (Otto et al., 2010) via LxxYY motif () Early/recycling endosomes DyneinLL1/2 () via C-terminal peptide Rab45 () Centrosome VPS26B (Wang et al., 2024) M6PR-positive endosome (not retromer) |
| ATG2A(BLTP4A) ATG2B (BLTP4B) | ATG2B: Familial susceptibility to myeloproliferative neoplasms (MIM: 616604) (Saliba et al., 2015) | ATG9 (Wang et al., 2024) TMEM41B VMP1 () ER | Liprin-like domain (; Van Vliet et al., 2022) GABARAP/GABARAPL1/LC3A via LIR motif () WIPI4 (; ) via YFS motif (Zheng et al., 2017) ATG9 (Wang et al., 2024; Van Vliet et al., 2022) Phosphoinositide lipids (PI3P) () Phagophore TOM40 via MAM localisation domain (MLD) (Tang et al., 2019) Mitochondria associated ER membrane/phagophore |
| VPS13A (BLTP5A) | Chorea-acanthocytosis (MIM: 200150) (Rampoldi et al., 2001) | VAPA/B via FFAT-motif (Yeshaw et al., 2019) ER | XK () via PH domain () Plasma membrane ? via PH domain () mitochondria SNX5 via VAB (Tornero-Ecija et al., 2023) rab7 (Munoz-Braceras et al., 2019) late endosome/lysosome Lipid membranes via liprin-like domain/amphipathic helices () |
| VPS13B (BLTP5B) | Cohen syndrome (MIM: 216550) () | FAM177A1 (Ugur et al., 2024) via? Golgi | Sec23IP via VAB () ER-exit site (ERES) Phosphoinositide lipids (PI4P) () via PH (PI3P) () via? Rab14 (Pietra et al., 2013) endosomes Syntaxin 6 and syntaxin 13 together () Transferrin receptor-positive early/recycling endosomes rab6A/B/C (Seifert et al., 2015) Golgi |
| VPS13C (BLTP5C) | Autosomal recessive early onset Parkinson’s Disease 23 (; ) (MIM: 616840) Dementia with Lewy bodies (Smolders et al., 2021) (MIM:127750) | VAPA/B via FFAT-motif ER | pT73-rab10 (Schrӧder et al., 2024) lysosomes rab7 via VAB () late endosomes/lysosomes liprin-like domain/amphipathic helices () phosphoinositide lipids |
| VPS13D (BLTP5D) | Autosomal recessive Spinocerebellar Ataxia 4 (Seong et al., 2018; ) (MIM: 607317) | ? via N terminal region (Wang et al., 2021) mitochondria VAPB via pFFAT-motif () ER | Mitofusin 2 (Shen et al., 2021) Miro1/2 via VAB () Mitochondria/peroxisome liprin-like domain (Wang et al., 2021) TSG101 via VAB domain (Wang et al., 2021) lipid droplets K63-linked ubiquitin via UBA domain () p97/VCP via UBA and VAB domains () |
| yeast VPS13 | LIR motif ER | MCP1 to VAB domain via PXXP motif (; ; ) mitochondria spo71 to VAB domain via PXXP motif (; Park et al., 2013) prospore membrane ypt35 to VAB domain via PXXP motif () endosome/vacuole Arf1 via PH domain () Golgi phosphoinositide lipids (PI3P) (Rzepnikowska et al., 2017; ) [PI(4,5)P2] () |
Diseases associated with BLTPs, recruitment factors for N terminal and C terminal domains of BLTPs with specific subdomains if known.
Bold underline, proteins with scramblase activity. Bold italics, small GTPases or dynamin-like GTPases. Interactors that have not been definitively associated with a direct physical interaction with the N or C terminus of BLTPs are omitted or indicated with a question mark. Red, organelle recruitment defined by the specified interactor(s). Please note that other organelle junctions sporting BLTPs have been found where the interaction that defines the recruitment is not known. LIR, LC3 interacting region.
Very loosely, the BLTP family of proteins can be split into two groups: BLTP1-3, which do not have extensive C-terminal interfaces with adaptor proteins, and ATG2/VPS13 families (designated BLTP4/BLTP5 respectively), which have extensive C-terminal specialisations that form a platform to recruit multiple adaptor factors on their target membrane. For a cartoon of the domain structures of BLTP family proteins, see Figure 1. A model of their association with membranes is presented in Figure 2.
Figure 1
Figure 2
BLTP1 and BLTP2 have a N-terminal transmembrane helix that anchors the bridge (
In BLTP1-2, C-terminal specialisations are relatively simple: The C terminus is denoted by a single-helical segment, which has been demonstrated to interact with membrane phosphoinositide lipids (Wang et al., 2022; Neuman et al., 2022b), while peptide motifs in BLTP2 contact membrane adaptor proteins (
In VPS13/ATG2 family proteins, this extra series of C-terminal domains appears to specify an interacting surface for proteins on the adaptor end of the bridge to recruit BLTPs to their target membrane, allowing dynamic refinement of the recruitment of bridge-like transporters to their target membranes. Ultimately, however, the C-terminal specialisations of these proteins are not completely necessary for function, as loss of ATG2 in mammalian cells can be rescued by strong overexpression of the N-terminal portion of ATG2 alone (Valverde et al., 2019). This suggests that the minimum requirement is the ability to retrieve lipids from the donor membrane and some kind of channel to carry the solubilised lipid, which can, if necessary, randomly incorporate into its acceptor membrane.
A brief overview of BLTP functions
Functionally, BLTPs are known to play a crucial role in autophagy, particularly in facilitating the rapid expansion of the phagophore membrane through proteins such as the well-studied ATG2 proteins (recently reviewed here; Vargas Duarte and Reggiori, 2023;
Beyond autophagy, members of this family are implicated in numerous essential cellular processes, such as mitochondrial homeostasis (
Frequently, BLPTs have been associated with inherited disorders (see Table 2), typically linked to recessively inherited disorders of the nervous system, such as the severe neurodevelopmental disorder, Alkuraya-Kučinskas syndrome (BLTP1), a commonly perinatal lethal disorder affecting multiple systems, particularly brain development, where surviving patients suffer seizures, cardiac and renal symptoms, and some degree of intellectual disability (
While this is a rapidly developing field, several recent reviews offer insight into the functional role of this protein family and their effects on membrane traffic and signalling properties in model systems (
Open questions
Is lipid transport through RBG proteins directional?
While it is debated whether all members of the BLTP family mediate lipid transfer in one direction only, most of the bridge-like proteins themselves appear to span membrane contact sites in consistent directions, co-ordinated by interactions with adaptors that are specific to the N and C termini of these proteins. This is most clearly borne out in the interactions of the single VPS13 protein in yeast, where N-terminal ER-to-C-terminal organelle contacts are formed by competitive recruitment of the C-terminus to organelle-specific adaptors (
While the example of BLTP3A is the most striking to date, it is apparent that the selection of membranes that the BLTPs transfer to and from is largely determined by the N and C terminal interacting proteins of these family, as the BLTPs do not display strong intrinsic preferences for target junctions in the absence of other factors recruiting them to membranes. As described in yeast, overexpression of proteins that can recruit VPS13 family proteins [the adaptor proteins mcp1 (
Other proteins of the family appear to be less specialised, and it is unclear whether the distinction between the donor and acceptor ends of the protein (and therefore, the direction of lipid transport over these bridges), or the conditions for RBG proteins to be recruited to organelle junctions, is as highly regulated as it appears to be for the ATG2/VPS13 grouping when expressed at endogenous levels.
In some of these proteins, the selection of a ‘donor’ end of the protein may be fixed: BLTP1 has recently been identified with its N-terminal transmembrane helix in complex with two proteins (spigot/C1orf43, an ER-resident protein that cups the N-terminal chorein-like domain, and intake, a nematode-specific helical transmembrane protein which contacts both BLTP1(C. elegans LPD-3) and spigot) (
Nevertheless, it would appear that at least some members of the BLTP family have specialisations that may favour unidirectional transport, particularly the VPS13/ATG2 grouping. In the case of VPS13 family proteins, donor/acceptor membranes are also largely determined by protein–protein interactions at the C-terminus of the protein (see Table 2). Interestingly, in at least some subcellular organelles, ATG2 and VPS13 proteins are partially redundant (
How selective are RBG transporters for specific cargo lipids?
Generally, evidence shows that the chorein domains of BLTPs can bind and solubilise the major membrane phospholipids [phosphatidylethanolamine (PE), phosphatidylcholine (PC), phosphatidic acid (PA), phosphatidylserine (PS), phosphatidylinositol (PI), and phosphatidylglycerol (PG)] and are a poor selector for cholesterol (reviewed here;
As the research currently stands, selectivity for lipids is more likely to be driven by the recruitment of the complex to the appropriate junction, and regulation by the properties of the two membranes driving a transport gradient. Assays in reconstituted systems show that BLTP3B rapidly retrieves a packet of lipid from the donor liposomes and then remains stable if there is no acceptor liposome to receive transferred lipid, suggesting that donor lipid is moving down an internal gradient within the RBG tube (
X-ray crystallography resolved the S. pombe ATG2 chorein domain in complex with PE (Osawa et al., 2019), where the majority of the interactions were with the acyl chains of the lipid, with what appeared to be a relatively loose selectivity for acyl chain length and little interaction with the lipid headgroup except a weak interaction with the PE phosphoryl group stabilised by an arginine residue in the chorein domain (see Figure 3A). It is also possible that some lipids are incorporated into the flow of lipids into chorein domains as ‘passengers’ and are not necessarily selected for. Molecular dynamics (Wang Y. et al., 2024) models favour an entry mechanism of spontaneous absorption of PC by the hydrophobic chorein domain interior, when the acyl chains of the absorbed lipid have been exposed to the cytosol by membrane bending (e.g., a highly curved membrane modelled in this case by a 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) micelle). This process appears to involve a number of conformational steps in the chorein domain: First, one of the two DOPC acyl chains spontaneously inserts into the hydrophobic chorein domain cavity while the charged headgroup interacts with a charge at the ‘mouth’ of the chorein domain scoop; second, the charged headgroup interacts with other slightly deeper positively charged residues to flip the polar head of PC into the cavity; finally, the second acyl chain trails behind and the lipid is transferred onwards, dragging a new acyl chain from the next lipid to enter behind it, potentially generating a continuous stream of oriented lipid. It is speculated that the energy required to drive these conformational changes in the chorein domain, particularly that needed to untangle one acyl chain from its hydrophobic association with other lipids in the micelle, may be supplied by interacting proteins that lower this energy barrier.
Figure 3

Lipid interaction with BLTPs. (A) X-ray crystallography of a single PE molecule with S. pombe ATG2 chorein domain (PDB:6A9J) showing acyl chain docking to the hydrophobic interior of the chorein domain, and phosphoryl group of PE close to a positive charge at the chorein domain mouth. (B) Computational model of lipid docking to the open ends of RBG repeats shows that there are discontinuous ‘bottlenecks’ where lipid is not docked in the interior of some RBGs. Image reproduced from Srinivasan et al. (2024), under the terms of the Creative Commons Attribution–NonCommercial–ShareAlike 4.0 International license. (C) Model of the C-terminal liprin-like domain of VPS13B interacting with model membranes [Modelarchive ma-xnr8z; (
Are lipids driven through BLTPs by gradients between donor and acceptor?
Current thinking suggests that BLTPs exploit membrane gradients to transfer bulk membrane phospholipids. Unless a gradient is built by the expenditure of energy (e.g., enzymatic activity), BLTPs would exploit passive gradients between membranes, as well as the properties of the transported lipids themselves. Some gradients are easily generated by local phenomena (e.g., placing the BLTP very close to a lipid synthase on the donor membrane (
The disturbance of acyl chains on the donor membrane is likely to facilitate lipid transfer. In models of the interaction of the liprin-like C terminal domain of VPS13 family proteins interacting with membranes, the liprin-like domain disturbs the cytosolic leaflet of the acceptor membrane bilayer, exposing the hydrophobic lipid chains to the ‘incoming’ lipid being donated by the BLTP, thus likely increasing the efficiency of their incorporation in the target membrane. Interestingly, several missense mutations of this liprin-like domain cause disease in human VPS13s, suggesting their relevance to the lipid transfer process (
Consistent with this mechanism being exploited by BLTPs in vivo, examination of subcellular organisation places lipid transfer via ATG2 to the very edge of the growing omegasome (
Other properties, such as lipid packing, can also be sensed and/or rectified via BLTP-mediated transfer, suggesting that gradients of lipid composition or fluidity also drive BLTP function. This is illustrated in the case of the role of C. elegans lpd-3/BLTP1 countering the effect of cold stress on membrane fluidity (Wang et al., 2022), which is discussed below.
An important consideration is that the action of BLTPs is to drive bulk lipids to the cytosolic leaflet of a membrane bilayer, which then may or may not be redistributed by a scramblase. Eukaryotic membranes favour a less fluid, more tightly packed outer membrane leaflet, while the cytosolic face of the membrane bilayer (the target of BLTP-mediated transfer) is more loosely packed, thus allowing spontaneous membrane deformation (
How the nature of the donor membrane composition may impact proteins such as BLTP1, which appears to transfer lipids via its N-terminal helix and partner transmembrane protein C1orf43 (
Can lipid transport through the RBG bridge be regulated?
Several models have been made of lipid interactions with members of the BLTP family, which all favour a continuous stream or bolus of lipid passing through the RBG portion of BLTPs (
As lipids traverse the RBG repeats, modelling and cryo-electron microscopy find that BLTPs are suited to the co-ordinated deposition of multiple lipids at a time, which allows the lipids to orient and group themselves, thus minimising the energy needed to transfer lipids from one membrane to another. Recent cryo-electron microscopy studies (
Recent and elegant molecular dynamic (MD) simulations of lipid docking on open membrane-less models of the RBG portion of BLTPs (Srinivasan et al., 2024), shown in Figure 3B, show no intrinsic preference in terms of direction of entry for the open RBG repeats of ATG2 in MD simulations of free lipids in solution with BLTPs. Simulating sequential docking of multiple free lipids in yeast Atg2, Vps13, and Csf1/BLTP1, the authors show that these proteins can carry in the RBG repeat ‘tube’ at least 15, 49, and 53 phospholipids, respectively, where the charged lipid headgroup faces the solvent. Interestingly, this modelling also showed that the docked lipids were not equally distributed down the length of the RBG tube, instead finding bottlenecks in the hydrophobic track of Atg2 and Csf1/BLTP1 where lipids did not preferentially reside, which may provide an opportunity for transport down the length of the RBG tube to be regulated. These models, where only the open RBG tube is considered, would favour the bi-directional transport of lipids via BLTPs as the free lipids entered from both N and C terminal directions of the modelled open tubes. These bottlenecks in RBG repeats were also found in cryo-EM structures of ATG2A (Wang Y. et al., 2024), suggesting that the passage of lipids through BLTPs is not uninterrupted, and may require changes in conformation to allow passage of lipids. Several possible methods of regulation may exist that would modulate the diameter of the RBG tube during lipid transport, such as locally twisting or compressing RBG repeats, either by the many conserved flexible repeats and motifs that are scattered along the length of RBG proteins (
Other models of BLTP family interaction with lipid in RBG bridges agree with a model of interrupted, and therefore regulatable, lipid transfer. A recent preprint (
Unfortunately, it was not possible to solve the structure of the entire BLTP1 complex ex vivo at the resolution needed to identify docked lipids over the entire length of the membrane bridge, so the nature of the interaction of BLTP1 with the plasma membrane was not observed in this system. Computational models of BLTP1 structure favour a widening of RBG repeats from the N- to C-terminus, until reaching a narrow neck at the C-terminal region of the protein (Wang et al., 2022). However, BLTP1 (Toulmay et al., 2022;
There are arguments, however, that the more specialised members of the BLTP family (ATG2, VPS13) for which the evidence for unidirectional lipid transport is stronger, may have adopted features that make lipid transport favour the N-terminal to C-terminal direction. Our own study modelling interactions of the specialised N- and C-terminal domains of VPS13A-D with donor and acceptor membranes (
Based on our modelling on VPS13s, along with work from others since, we propose two models to regulate the passage of lipids through RBG bridges: (1) a spring-like action of compressing and expanding RBG repeats (which potentially may be regulated by interaction with the many conserved cytosolic loops and motifs down the length of the BLTP proteins), and (2) a transient interaction model, where VPS13 RBGs are rigid, and lipids fall down the gradient between donor and acceptor membranes. In this model, the N-terminal chorein domain is weakly associated with donor membranes via long, flexible tethers to proteins such as VAPA/B, allowing transient ‘gulps’ of donor lipids. This could well be similar in other BLTPs, as current evidence suggests that BLTPs are strongly associated with either their donor (BLTP1-2) or their acceptor (VPS13) membranes, but not both—allowing transient interactions with one side of the intraorganelle bridge to drive pulses of lipid down the RBG tube.
How fast are BLTPs?
The presence of BLTPs, despite their apparent lack of selectivity, also provides some advantages when dynamically manipulating membrane properties, as bulk transfer allows movement of lipids between organelle membranes on the scale of minutes (Wong et al., 2019; Reinisch and Prinz, 2021) thus allowing fast and presumably local response to changes in membranes: This is particularly seen by the action of VPS13C (Schrӧder et al., 2024; Wang X. et al., 2024) BLTP3A(Hanna et al., 2025) and ATG2 (Tan and Finkel, 2022) being recruited to damaged lysosomes for the process of lysosomal repair—a process that is accomplished on the scale of minutes and heavily involves a variety of other LTPs outside of the BLTP family.
In vitro studies using Förster resonance energy transfer (FRET)-based assays in liposomes have measured the transfer of a 2% mix of NBD-PE from donor liposomes at a surprisingly low rate of ~0.017 s-1 per molecule of the BLTP (
In vivo measurements, however, put the lipid transfer rate much higher: ~200 lipids per second per molecule of Atg2 in yeast (
What effect does BLTP-mediated lipid transport have on target membrane geometry?
Insights into the consequences of lipid transfer mediated by BLTPs have been gained through structure–function analyses and knockout studies across various organisms and cell types. Collectively, these findings suggest that the effects of BLTP-mediated lipid transfer depend on both the specific lipid being deposited and the manner in which the donated lipid is incorporated with the acceptor membrane. This process influences the geometry and biophysical properties of the target organelle, ultimately modulating cell signalling pathways and membrane sorting.
A key factor in determining how acceptor membranes are altered lies in the presence of accessory complex components associated with BLTPs. These proteins often function as part of larger macromolecular assemblies, interacting with accessory proteins such as Rabs and adaptor proteins or associating with membrane scramblases, which modulate the impact of the transferred lipid on the target membrane. The specific targeting and interaction partners at each BLTP-mediated intraorganellar contact appear to dictate the functional outcome of BLTP activity at a given membrane junction.
BLTPs are thought to deposit bulk membrane lipids onto their target membrane, but the context and method of deposition have different effects on the membrane properties of the target organelle. Experiments show BLTPs can drive membrane expansion in the case of VPS13/ATG2 proteins (
One key distinction between the actions of BLTPs in their different cellular contexts is the presence or otherwise of associated scramblases as this imposes a geometry as to how the transported lipid will be distributed on the acceptor membrane. These transmembrane proteins facilitate donated lipids accessing the lumenal leaflet of the membrane bilayer, which has the effect of equilibrating the donated lipid across both bilayers of the membrane, which allows expansion of acceptor membranes without much bending. This is best understood for the VPS13/ATG2 grouping of BLTPs. It was noted that VPS13A associates with the scramblase XK (Ryoden et al., 2022) and shares a common disease phenotype (Park and Neiman, 2020) where PS exposure and PC internalisation of the outer face of the plasma membrane are compromised. In yeast, VPS13 forms a complex with the yeast-specific MCP1 protein, which, despite no sequence conservation with XK, also has scramblase activity (
In these cases, the equilibration of the inner and outer leaflets of the target organelle will also drive transfer gradients by spreading the donated lipid over both the inner and outer leaflets of the membrane bilayer, by helping to maintain a concentration gradient between the donor membrane and the acceptor membrane cytosolic leaflet. This method of lipid deposition is supported by the established role of scramblase-associated ATG2 in depositing lipids for phagophore formation (
In proteins such as BLTP3B/SHIP164, which are not known thus far to form complexes with a scramblase, overexpression of the BLTP and a targeting protein (in this case its interactor, syntaxin 6) leads to excessive tubulation of endosomes, possibly driven by the asymmetric delivery of lipids to the outer leaflet of the acceptor membrane, which, if not equilibrated, would need to deform and bend to accommodate the excess lipid (Otto et al., 2010). This change in membrane geometry leads to changes in the trafficking of endosomal cargoes such as cation-independent mannose 6 phosphate receptor and transferrin receptor. This also appears to be the case for the VPS13 family of proteins when these proteins are not interacting with scramblase, where VPS13-mediated transport then favours membrane distortion: Yeast VPS13 directs lipid to lysosomes to accomplish ESCRT-mediated inward budding and membrane sorting to form ILVs (Suzuki et al., 2024) or VPS13B allows the formation of extended tubular ERGIC to accommodate the trafficking and secretion of long extracellular proteins such as procollagen (
Do BLTPs change the signalling properties of membranes?
Although BLTPs transport lipids that are abundant in most membranes, the donor and acceptor membranes connected by BLTP bridges exhibit distinct characteristics, such as the different phosphatidylinositol phosphates (PIPs) that define organelles, and varying levels of cholesterol, which does not appear to be transported as cargo. They also differ in lipid composition (e.g., PC/PE/PS ratio), lipid saturation levels, and the level of asymmetry between the cytosolic and lumenal/extracellular leaflets of the bilayer [reviewed here (
The rapid admixture of lipids from other organellar membranes, such as the relatively unsaturated lipids found in fluid ER, the site of synthesis for most of the lipids known to be transported via BLTPs, is likely to change the properties of both donor and acceptor membranes. These changes may be to the geometry (e.g., cone-shaped lipids such as PE favour membrane bending, and unsaturated lipids favour membrane flexibility), or the capacity of the membrane to form ordered domains for signalling [as, e.g., is found at ER-membrane contacts (
Nevertheless, there is a degree of subtlety as to how each BLTP is able to change membrane properties. In systems where some level of functional redundancy might be anticipated between members of the BLTP family such as BLTP2 and BLTP1—which both mediate transfer between ER and the plasma membrane—certain functional roles are specialised. These roles may be influenced by specific protein interactions with these BLTPs, as well as by the physical context of their membrane junctions and the local curvature induced by lipid transfer. Both BLTP1 and BLTP2 have important roles in homeoviscous adaptation (HVA), that is, maintaining membrane sorting and signalling properties under cold stress (
BLTP2 regulates membrane fluidity by increasing PE levels at PM, facilitating cancer growth (
PE is also a precursor of GPI anchors, and while mutation of yeast Fmp27/BLTP2 does not appreciably affect the synthesis of GPI anchors (
However, the downstream consequences of altering membrane properties, even by manipulation of the highly abundant phospholipids that BLTPs are known to transfer, are considerable. Changing the balance of PE to PI, where PE is a curvature promotor and PI, because of hydrogen bonding between headgroups, favours uncurved membrane (
It is unclear whether these effects on PIP lipid signalling are due to a direct defect in PIP lipid transfer via the RBG bridge, or the deficit in PIP lipids stems from downstream consequences of changes to membrane fluidity and therefore recruitment of specific PIP kinases and phosphatases, which preferentially segregate to lipid microdomains defined by acyl chain composition (Wang and Richards, 2012; Wenk et al., 2003).
Conclusion
Recent studies have identified several common characteristics of BLTPs: (1) a loose association with either donor or acceptor membranes; (2) the presence of internal gradients that promote lipid accumulation in regions with wider RBG repeats; (3) lipid transfer “bottlenecks,” which may serve as regulatory points for lipid transfer; and (4) effects on PIP lipid signalling, likely secondary to the transfer of major lipid constituents such as phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Whether bi-directional or unidirectional, the effects of lipid transfer appear to be driven by both the gradient of phospholipids between donor and acceptor membranes and association with other membrane proteins which drive lipids to equilibrate or not over associated membrane bilayers. The presence of these accessory complex members imposes specific membrane geometries and lipid sorting patterns, with significant implications for protein distribution and membrane signalling. Overall, the studies conducted so far suggest that from the simplest of principles (a scoop that can load lipids and a tube that can conduct it in co-ordinated patches), highly specific, context-dependent changes can be made to membrane function at all levels of membrane sorting and signalling, underpinning neuronal and organismal function. Our growing understanding of such an ancient, seemingly simple, group of lipid transfer proteins suggests that BLTP-mediated regulation of phospholipid gradients can orchestrate extensive and surprisingly sophisticated changes in cellular signalling and interactions with the extracellular environment.
Statements
Author contributions
LS: Writing – original draft, Writing – review & editing.
Funding
The author(s) declare that financial support was received for the research and/or publication of this article. Laura Swan acknowledges funding from the Wellcome Trust (ISSF 204822/Z/16/Z) and AFM Telethon, grant/award no. 22429.
Conflict of interest
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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The author declares that no Gen AI was used in the creation of this manuscript.
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- ATG2
Autophagy-related 2
- BLTP
Bridge-like lipid transporter
- CASM
Conjugation of Atg8 to single membranes
- ChAc
Chorea-acanthocytosis
- ER
Endoplasmic reticulum
- ERGIC
ER-Golgi intermediate compartment
- ESCRT
Endosomal sorting complex required for transport
- FFAT
Two phenylalanines in an acidic tract
- GTPase
Guanosine triphosphatase
- ILV
Intraluminal vesicle
- LC3
Microtubule-associated proteins 1A/1B light chain 3
- LIR
LC3-interacting region
- MLD
Mitochondrial-associated membrane localisation domain
- mTOR
Mechanistic target of rapamycin
- NBD
Nitrobenzoxadiazole (fluorescent lipid label)
- PA
Phosphatidic acid
- PC
Phosphatidylcholine
- PE
Phosphatidylethanolamine
- PG
Phosphatidylglycerol
- PH
Pleckstrin homology
- PI
Phosphatidylinositol
- PM
Plasma membrane
- PS
Phosphatidylserine
- RBG
Repeating beta groove
- RNA
Ribonucleic acid
- RXP
Arginine-X-Proline motif
- SH3
Src homology 3
- SNARE
Soluble NSF attachment protein receptor
- TSG101
Tumour susceptibility gene 101
- UHRF1BP1
Ubiquitin-like with PHD and RING finger domains 1 binding protein 1
- UHRF1BP1L
UHRF1BP1-like protein
- VAB
VPS13 adaptor-binding domain
- VAMP7
Vesicle-associated membrane protein 7
- VAPA/B
Vesicle-associated membrane protein-associated protein A/B
- VMP1
Vacuole membrane protein 1
- VPS13
Vacuolar protein sorting 13
- WD40
Tryptophan (W) aspartate (D) motif of 40 amino acids
- WIPI
WD repeat domain phosphoinositide-interacting protein
- WWE
Trp Trp Glu motif domain
- XK
Kell blood group precursor
‘Glossary
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Summary
Keywords
BLTP, VPS13, ATG2, scramblase activity, lipid transfer activity, membrane contacts
Citation
Swan LE (2025) VPS13 and bridge-like lipid transporters, mechanisms, and mysteries. Front. Neurosci. 19:1534061. doi: 10.3389/fnins.2025.1534061
Received
25 November 2024
Accepted
14 March 2025
Published
28 April 2025
Volume
19 - 2025
Edited by
Andreas Hermann, University Hospital Rostock, Germany
Reviewed by
Jay Penney, University of Prince Edward Island, Canada
Sean Munro, University of Cambridge, United Kingdom
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© 2025 Swan.
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*Correspondence: Laura Elizabeth Swan, laura.swan@liverpool.ac.uk
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