Organisms encode multiple homologous heat shock protein (Hsp)-70s, which are essential protein chaperones that play the major role in cellular protein "quality control." Although Hsp70s are functionally redundant and highly homologous, many possess distinct functions. A regulatory motif underlying such distinctions, however, is unknown. The 98% identical cytoplasmic Hsp70s Ssa1p and Ssa2p function differently with regard to propagation of yeast [URE3] prions and in the vacuolar-mediated degradation of gluconeogenesis enzymes, such as FBPase. Here, we show that the Hsp70 nucleotide binding domain (NBD) regulates these functional specificities. We find little difference in ATPase, protein refolding, and amyloid inhibiting activities of purified Ssa1p and Ssa2p, but show that interchanging NBD residue alanine 83 (Ssa1p) and glycine 83 (Ssa2p) switched functions of Ssa1p and Ssa2p in [URE3] propagation and FBPase degradation. Disrupting the degradation pathway did not affect prion propagation, however, indicating these are two distinct processes where Ssa1/2p chaperones function differently. Our results suggest that the primary evolutionary pressure for Hsp70 functional distinctions is not to specify interactions of Hsp70 with substrate, but to specify the regulation of this activity. Our data suggest a rationale for maintaining multiple Hsp70s and suggest that subtle differences among Hsp70s evolved to provide functional specificity without affecting overall enzymatic activity.
