POSTTRANSLATIONAL PROTEIN-TRANSPORT IN YEAST RECONSTITUTED WITH A PURIFIED COMPLEX OF SEC PROTEINS AND KAR2P

We have reproduced the posttranslational mode of protein translocation across the endoplasmic reticulum membrane with reconstituted proteoli posomes containing a purified complex of seven yeast proteins. This Se c complex includes a heterotrimeric Sec61p complex, homologous to that in mammals, as well as all other membrane proteins found in genetic s creens for translocation components. Efficient posttranslational trans location also requires the addition of lumenal Kar2p (BiP) and ATP. Th e trimeric Sec61p complex also exists as a separate entity that, in co ntrast with the large Sec complex, is associated with membrane-bound r ibosomes. We therefore hypothesize that distinct membrane protein comp lexes function in co- and posttranslational translocation pathways.