ZUOTIN, A RIBOSOME-ASSOCIATED DNAJ MOLECULAR CHAPERONE

Correct folding of newly synthesized polypeptides is thought to be fac ilitated by Hsp70 molecular chaperones in conjunction with DnaJ cohort proteins. In Saccharomyces cerevisiae, SSB proteins are ribosome-asso ciated Hsp70s which interact with the newly synthesized nascent polype ptide chain. Here we report that the phenotypes of an S.cerevisiae str ain lacking the DnaJ-related protein Zuotin (Zuo1) are very similar to those of a strain lacking Ssb, including sensitivities to low tempera tures, certain protein synthesis inhibitors and high osmolarity. Zuo1, which has been shown previously to be a nucleic acid-binding protein, is also a ribosome-associated protein localized predominantly in the cytosol. Analysis of zuo1 deletion and truncation mutants revealed a p ositive correlation between the ribosome association of Zuo1 and its a bility to bind RNA. We propose that Zuo1 binds to ribosomes, in part, by interaction with ribosomal RNA and that Zuo1 functions with Ssb as a chaperone on the ribosome.