CER1P, A NOVEL HSP70-RELATED PROTEIN REQUIRED FOR POSTTRANSLATIONAL ENDOPLASMIC-RETICULUM TRANSLOCATION IN YEAST

Proteins enter the secretory pathway by translocation across the endop lasmic reticulum (ER) membrane, In Saccharomyces cerevisiae, import of proteins into the ER occurs both cotranslationally and posttranslatio nally, Presumably, the cotranslational targeting to the ER membrane is directed by the signal recognition particle, as demonstrated in other eukaryotic systems, The deletion of a gene, called CER1, inhibits the translocation of proteins that enter the ER posttranslationally, but not those that enter cotranslationally. This translocation defect is m ore pronounced at lower temperatures. A strain possessing a null mutat ion of CER1 in combination with a kar2 temperature-sensitive mutation displays synthetic growth defects, whereas overexpression of the ER Dn aJ homolog Scj1p suppresses the translocation defect in cer1 Delta str ains, CER1 is predicted to encode a 100-kDa polypeptide, residing in t he ER lumen that is related to the hsp70 family of molecular chaperone s.