The molecular chaperone Ssb from Saccharomyces cerevisiae is a component of the ribosome-nascent chain complex

The 70 kDa heat shock proteins (Hsp70s) are a ubiquitous class of molecular chaperones. The Ssbs of Saccharomyces cerevisiae are an abundant type of Hsp70 found associated with translating ribosomes. To understand better the function of Ssb in association with ribosomes, the Ssb-ribosome interaction was characterized. Incorporation of the aminoacyl-tRNA analog puromycin by translating ribosomes caused the release of Ssb concomitant with the release of nascent chains. In addition, Ssb could be cross-linked to nascent chains containing a modified lysine residue with a photoactivatable cross-linker. Together, these results suggest an interaction of Ssb with the nascent chain. The interaction of Ssb with the ribosome-nascent chain complex was stable, as demonstrated by resistance to treatment with high salt; however, Ssb interaction with the ribosome in the absence of nascent chain was salt sensitive. We propose that Ssb is a core component of the translating ribosome which interacts with both the nascent polypeptide chain and the ribosome. These interactions allow Ssb to function as a chaperone on the ribosome, preventing the misfolding of newly synthesized proteins.