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 mole cular chaperones. The Ssbs of Saccharomyces cerevisiae are an abundant type of Hsp70 found associated with translating ribosomes, To underst and 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 c ould be cross-linked to nascent chains containing a modified lysine re sidue with a photoactivatable cross-linker. Together, these results su ggest an interaction of Ssb with the nascent chain. The interaction of Ssb with the ribosome-nascent chain complex was stable, as demonstrat ed 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 w hich interacts with both the nascent polypeptide chain and the ribosom e, These interactions allow Ssb to function as a chaperone on the ribo some, preventing the misfolding of newly synthesized proteins.