About this Research Topic
Bacteria use a broad variety of membrane transport systems to deliver effectors to other cells and to the outer media. At least 9 different secretion pathways have been described so far, numbered from Type I to Type IX secretion systems (T1SS-T9SS). Most of these systems have evolved from or co-evolved with machineries required for other processes such as motility, drug efflux or pili assembly. These multiprotein systems can be categorized in two different groups, depending on the number of steps used to transport the substrate across the bacteria membranes. For example, T1SS, T3SS, T4SS and T6SS can translocate the effectors in a single step, whereas T2SS, T5SS and T9SS effectors use the general Sec or Tat export pathway to cross the inner membrane before the dedicated secretion system to cross the outer membrane starts its action. In addition, these secretions systems can deliver small molecules, peptidoglycan fragments or macromolecules such as proteins and DNA.
These effectors must be specifically recognized, selected and transported by the secretion apparatus, and thus specific signals and cognate receptors have evolved. As a consequence, the components, organization, architecture and mechanism of action of these different secretion systems are highly variable. Finally, some of these secretion systems have also hijacked host factors, such as chaperone, to facilitate their assembly. The biogenesis of these macromolecular machines is sophisticated and, in most cases, not yet understood. However, recent developments in electron microscopy to image large complexes and in fluorescence microscopy to dissect the dynamics of these complexes have provided important insights.
This topic collection welcomes up-to-date descriptions of each transport system and their putative roles in the pathogenicity of common bacteria species.
Keywords: Bacteria, Secretion, Membrane, Transport, Protein Complex, Pathogen
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