Components of the protein translocation complex in the endoplasmic reticulum (ER) membrane

Protein transport across the ER can occur by two pathways, a co- and post-translational one. In both cases membrane proteins are essential to perform the translocation of the polypeptide across the lipid bilayer. The cotranslational mode is best studied in mammals. For the in vitro reconstitution of this mode only three membrane components of the mammalian ER are needed: 1. the signal recognition particle (SRP) receptor lhat is involved in the SRP-dependent targeting process, 2. the TRAM protein and 3. the Secol-complex that consists of the three subunits Sec61α, Sec61β and Sec61γ. Crosslinking data indicate, that the Secolcomplex is part of the proposed protein-conducting channel. Moreover, it is also a ribosome receptor. Posttranslational transport is more prevalent in yeast. Its reconstitution depends only on one membrane protein complex - the heptameric Sec-complex of S. cerevisiae. Interestingly, the Sec-complex contains a trimeric Secolp-complex that is homolog to the mammalian Secol-complex. As its mammalian counterpart this Secolp-complex exists in yeast also in a ribosome bound form and is therefore likely to be involved in the cotranslational pathway too. The conservation of central components in both transport pathways indicates, that the principle of the translocational mechanism may be similar.