Heat shock proteins and the biogenesis of cellular membranes.

The successful function of cells is importantly contributed by lipid membranes that are more than a simple physical barrier. The major components of cellular membranes are lipids, in particular glycerophospholipids, that have the capacity to assemble spontaneously into vesicles containing a lipid bilayer after exposure to an aqueous milieu due to their amphiphilic characteristics. The lipid capacity to form vesicles and encapsulate substrates has been proposed as a fundamental event during the biogenesis of cells. However, the stability of small vesicles is compromised during their expansion into larger and more complex particles. Recent observations by (Cornell et al. Proc Natl Acad Sci U S A 116:17239-17244, 2019) have shown that the insertion of amino acids into rudimentary vesicles could play a stabilizing role that was critical to the formation of early cells. Fatty acids were likely substituted by glycerophospholipids and amino acids replaced by polypeptides during the evolution of protocells. Thus, archaic peptides displaying lipid-binding and membrane-penetrating capacities could have played a key function in the development of current cells. In this regard, heat shock proteins (HSP), particularly the Hsp70 (HSPA) and small HSP (HSPB) families, could have portrayed that role. Indeed, bacterial DnaK is closest in sequence to the earliest members of the Hsp70 family and inserts into lipid membranes spontaneously. Moreover, extensive studies by the Vigh group have shown that, certainly, Hsp70s stabilize membranes. Thus, the ability of ancestral HSP70s and small HSPs to associate with lipids and stabilize membranes could have been a fundamental event in the genesis of cells.