Our work underlines the generality of a 'head-first' membrane insertion pathway at the ER, and shows it operates both in vitro and in cultured mammalian cells.
By contrast, the topology of the endosymbiont membrane has been preserved, necessitating the development of complex pathways for membrane insertion and translocation.
On this basis, we speculate that the membrane insertion step of the TRC40-dependent pathway for TA protein biogenesis may be a purely lipid-mediated event that does not require a dedicated integrase.
Their general ease of expression and refolding and straightforward methods to monitor their degree of folding conspire to make OMPs excellent model systems to investigate how the membrane environment and other biological factors modulating the membrane insertion and folding of OMPs influence the folding pathway.
We have addressed the flexibility of cytosolic components participating in these pathways, and explored the thermodynamic constraints of their membrane insertion, by exploiting recombinant forms of Sec61β and Cytb5 (cytochrome b5) bearing covalent modifications within their TA region.
In the last five years, pathways and components have been identified that are involved in this posttranslational membrane insertion in yeast [4,5] and mammals [6 14].
Tail-anchored (TA) proteins, defined by the presence of a single C-terminal transmembrane domain (TMD), play critical roles throughout the secretory pathway and in mitochondria, yet the machinery responsible for their proper membrane insertion remains poorly characterized.
Moreover, palmitate groups are important for determining the interaction of H-Ras with specific membrane microdomains, its membrane insertion and orientation and its presence in internal membranes, which in turn may influence the engagement of various signaling pathways [8], [9].
The 'twin-arginine repeat' or TAT pathway is involved in the secretion of folded proteins and has not been implicated in membrane insertion [18].
Membrane insertion involves the formation of a dodecameric membrane-embedded intermediate.
