FIG. 7.

Regulation of Hsp70 (Ssa1p) reaction cycle by cochaperones. Hsp70 function is finely tuned by ATP hydrolysis and nucleotide exchange, which regulate substrate binding. It is known that Hsp40 (Ydj1p, Sis1p) and Sti1p stimulate ATP hydrolysis, promoting substrate binding, and that Fes1p accelerates ADP release and nucleotide exchange, promoting substrate release. Our data suggest that enhancing conversion of Hsp70 to the ADP-bound state or stabilizing this state impairs [PSI⁺] propagation. Our data also suggest that Cpr7p and Cns1p might promote or stabilize this step in the cycle.