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

Abstract

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.

Firefly luciferase mutants as sensors of proteome stress

“…Grp78 or BiP or Kar2). There are multiple functionally redundant homologs of Hsp70 in the eukaryotic cytosol, such as in S. cerevisiae that contains four non-ribosome associated Hsp70s, Ssa1-4 and three ribosome-associated Hsp70s, called Ssb1, Ssb2 and Ssz1 (Lindquist and Craig, 1988;Nelson et al, 1992;Pfund et al, 1998). The cytosol of higher eukaryotes contains a constitutively expressed Hsp70 homolog called Hsc70 (Heat Shock Cognate 70) and a stress inducible form, Hsp70.…”

Quantitative proteomics and network analysis of SSA1 and SSB1 deletion mutants reveals robustness of chaperone HSP70 network in Saccharomyces cerevisiae

“…Although SSB proteins display over 50% identity with SSA chaperones they are functionally distinct, as demonstrated by a series of experiments 27. Their activity (unlike SSA proteins) is also mainly confined to the ribosome, where they function to facilitate translation and proper protein folding 23, 30 and their transcription is coupled with ribosomal proteins 20. Another notable difference between SSA and SSB families is that SSA1 and SSA2 proteins have been shown to depolymerise clathrin vesicles in vitro, while SSB proteins lack this ability 31.…”

Complex Regulation of the Yeast Heat Shock Transcription Factor

“…The Ssb proteins are regulated quite differently from the Ssa proteins, in that their expression is elevated by growth at low temperature and is repressed during heat shock (Werner-Washburne et al, 1989; Iwahashi et al, 1995;Lopez et al, 1999). The Ssb proteins are ribosome-associated chaperones that associate with nascent peptides and probably participate in early steps in folding (Nelson et al, 1992;Pfund et al, 1998).…”

“…The Ssb proteins are regulated quite differently from the Ssa proteins, in that their expression is elevated by growth at low temperature and is repressed during heat shock (Werner-Washburne et al, 1989; Iwahashi et al, 1995;Lopez et al, 1999). The Ssb proteins are ribosome-associated chaperones that associate with nascent peptides and probably participate in early steps in folding (Nelson et al, 1992;Pfund et al, 1998).HSF (i.e., HSF1) is generally believed to be primarily responsible for modulating the expression of chaperones such as hsp70 during stress and, in yeast, for stress-dependent expression of SSA1 and SSA4 (for review, see Voellmy, 1994). It has been assumed that the main role of HSF is to elevate the expression of chaperones during periods of acute stress, in response to dramatic elevation of the level of partially unfolded proteins.…”

“…Although the idea is not new that HSF activity might be tied to growth rate, the mechanism has been uncertain. The finding that the Ssb members of the hsp70 family can interact with HSF offers a possible mechanism.The Ssb proteins have been shown to function both in binding nascent peptides (Nelson et al, 1992;Pfund et al, 1998) and in the degradation of short-lived proteins (Ohba, 1997). Thus, the availability of Ssb1/2p should be sensitive to changes in the abundance of these substrates.…”