Fes1p acts as a nucleotide exchange factor for the ribosome-associated molecular chaperone Ssb1p

The HspBP1 homolog Fes1p was recently identified as a nucleotide exchange factor (NEF) of Ssa1p, a canonical Hsp70 molecular chaperone in the cytosol of Saccharomyces cerevisiae. Besides the Ssa-type Hsp70s, the yeast cytosol contains three additional classes of Hsp70, termed Ssb, Sse and Ssz. Here, we show that Fes1p also functions as NEF for the ribosome-bound Ssb Hsp70s. Sequence analysis indicated that residues important for interaction with Fes1p are highly conserved in Ssa1p and Ssb1p, but not in Sse1p and Ssz1p. Indeed, Fes1p interacts with Ssa1p and Ssb1p with similar affinity, but does not form a complex with Sse1p. Functional analysis showed that Fes1p accelerates the release of the nucleotide analog MABA-ADP from Ssb1p by a factor of 35. In contrast to the interaction between mammalian HspBP1 and Hsp70, however, addition of ATP only moderately decreases the affinity of Fes1p for Ssb1p. Point mutations in Fes1p abolishing complex formation with Ssa1p also prevent the interaction with Ssb1p. The ATPase activity of Ssb1p is stimulated by the ribosome-associated complex of Zuotin and Ssz1p (RAC). Interestingly, Fes1p inhibits the stimulation of Ssb1p ATPase by RAC, suggesting a complex regulatory role of Fes1p in modulating the function of Ssb Hsp70s in co-translational protein folding.