Mammalian Sec61 is associated with Sec62 and Sec63

In yeast, efficient protein transport across the endoplasmic reticulum (ER) membrane may occur co-translationally or post-translationally. The latter process is mediated by a membrane protein complex that consists of the Sec6 1p complex and the Sec62p-Sec63p subcomplex. In contrast, in mammalian cell s protein translocation is almost exclusively co-translational. This transp ort depends on the Sec61 complex, which is homologous to the yeast Sec61p c omplex and has been identified in mammals as a ribosome-bound pore-forming membrane protein complex. We report here the existence of ribosome-free mam malian Sec61 complexes that associate with two ubiquitous proteins of the E R membrane. According to primary sequence analysis both proteins display ho mology to the yeast proteins Sec62p and Sec63p and are therefore named Sec6 2 and Sec63, respectively. The probable function of the mammalian Sec61-Sec 62-Sec63 complex is discussed with respect to its abundance in ER membranes , which, in contrast to yeast ER membranes, apparently lack efficient post- translational translocation activity.