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Type I Chaperonin: Mechanism and Beyond

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Chaperone proteins control almost all aspects of proteostasis, such as protein synthesis, translocation, folding, and degradation. Essentially, chaperones accompany almost every protein from its birth until its death. Chaperonins are one subgroup of molecular chaperones that assist in the folding of ...

Chaperone proteins control almost all aspects of proteostasis, such as protein synthesis, translocation, folding, and degradation. Essentially, chaperones accompany almost every protein from its birth until its death. Chaperonins are one subgroup of molecular chaperones that assist in the folding of polypeptide chains to an active conformation upon synthesis, unfolding or following translocation. They can be divided into two subtypes, Type I and Type II chaperonins. The function of the chaperonins is executed by the Hsp60 chaperonin, which serves as a folding chamber for denatured protein, assisted by its 10 kDa co-chaperonin, Hsp10. For Type I chaperonins, the Hsp60 and Hsp10 functions are carried out by two individual proteins, while for Type II chaperonins, the Hsp60 and Hsp10 functions are fused into a single subunit. Several important milestones are worth mentioning that led to our current understanding of the molecular of function of Type I chaperonins. The latter were discovered in the 1970s as bacterial host proteins that are essential for the assembly of phage particles. During the same period, they were found to be overexpressed under heat shock , conditions known to compromise protein folding. Additional in vivo studies showed that chaperonins are key players in the assembly process of RubisCO in plants and that they are important for the folding of newly translocated proteins into the mitochondrial matrix as well.
These discoveries led to general recognition of Type I chaperonins as important protein Nano machines that play a key role in protein folding or assembly, in bacteria mitochondria, and chloroplasts. In vitro reconstitution of their protein folding activity using denatured dimeric RubisCO as model system opened the door to a new field of research, which focused on mechanistic aspects of chaperonin function. The friendly nature of the Escherichia coli chaperonins, in particular the profound stability of the protein oligomers, enabled their extensive investigation, which established them as the prototype chaperonin model system. In the ensuing years, further investigation of chaperonins from chloroplasts, mitochondria and numerous additional bacterial strains, revealed a wide range of divergence from the E. coli paradigm. In the case of chloroplast chaperonins, the most striking observation was that these chaperonins assemble into hetero-oligomeric tetradecamers that are composed of several homologous subunits, in contrast to the homooligomeric nature of bacterial chaperonins. One of them, Cpn20, is unique in that it is composed of two Hsp10 (Cpn10) like domains fused together and was implicated in numerous additional roles in the chloroplast such as activation of FeSOD or abscisic acid signaling. With regard to mitochondrial chaperonins, these were also found to exhibit unique structural properties and retain unexpected extra-organellar moonlighting functions. As such, they were found to function in a variety of processes, including signal transduction events that may regulate immunity and inflammation. The aforementioned research indicates that variations among Type one chaperonins may reflect adaptation to unique cellular environment.

This Research Topic aims at highlighting various central aspects of Type I chaperonins, including mechanistic aspects of its folding cycle, structural and functional divergences from the classic GroEL model and how they relate to moonlighting functions.


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