Maintenance of protein homeostasis in eukaryotes during normal growth and stress conditions requires the functions of Hsp70 chaperones and associated co-chaperones. Here we investigate an evolutionarily-conserved serine phosphorylation that occurs at the site of communication between the nucleotide-binding and substrate-binding domains of Hsp70. Ser151 phosphorylation in yeast Hsp70 (Ssa1) is promoted by cyclin-dependent kinase (Cdk1) during normal growth and dramatically affects heat shock

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... onses, a function conserved with HSC70 S153 phosphorylation in human cells. Phospho-mimic forms of Ssa1 (S151D) also fail to relocalize in response to starvation conditions, do not associate in vivo with Hsp40 co-chaperones, Ydj1 and Sis1, and do not catalyze refolding of denatured proteins in vitro in cooperation with Ydj1 and Hsp104. S151 phosphorylation strongly promotes survival of heavy metal exposure and reduces Sup35-dependent [PSI⁺] prion activity, however, consistent with proposed roles for Ssa1 and Hsp104 in generating self-nucleating seeds of misfolded proteins. Taken together, these results suggest that Cdk1 downregulates Hsp70 function during periods of active growth, reducing propagation of aggregated proteins despite potential costs to overall chaperone efficiency.