Chaperone assisted prion replication. Prion polymers (shown as stacks of rectangles) grow by addition of monomers (shown as ovals) to the ends. Monomers are converted into the prion conformation when they join the polymer. Fragmentation of an individual polymer by chaperone action results in formation of two polymers, each of which can grow and continue the cycle. Hsp70/40 probably acts directly on polymers, facilitating Hsp104 interaction. Hsp104 extracts monomers from the polymers by extruding them through its central channel, which results in destabilization and fragmentation of the polymer. Hsp70/40 might also act where peptide exits Hsp104 to help extrude monomers and assist refolding. Many co-chaperones that are known to regulate Hsp70 influence prion propagation, possibly by affecting this reaction. Chaperones might also influence prion propagation by affecting rate of incorporation of monomers into polymers.

Hsp70 reaction cycle and its regulation by co-chaperones. When ATP is bound to the nucleotide-binding domain (light gray) the substrate-binding domain (dark gray) is in an “open” conformation and rapidly exchanges substrate, which is indicated as a short hydrophobic segment of a larger protein. Hydrolysis of ATP to ADP (downward arching arrow) induces a conformational change in the substrate-binding and C-terminal domains that traps substrate. Exchange of nucleotides (upward arrow, ADP release allows rebinding of ATP) restores the open conformation and release of substrate. ATP hydrolysis is rate limiting and is stimulated by Hsp40s, Sti1 and Cns1. ADP release is enhanced by nucleotide exchange factors (NEFs). How Cpr7 influences Hsp70 is unknown but its effects on [PSI⁺], which are additive with those of Sti1, are consistent with Cpr7 stabilizing the ADP-bound state. Altering the cycle to favor the ADP-bound state (e.g., stimulating ATP hydrolysis or inhibiting nucleotide exchange) impairs [PSI⁺].