Earlier we showed that altering the expression of wild type Hsp40s (J-proteins), nucleotide exchange factors (NEFs) and tetratricopeptide repeat (TPR) domain proteins can either enhance or impair propagation of different prions in both wild type and Hsp70 mutant cells. We also identified several mutant alleles of Hsp40s and TPR proteins that impair or enhance propagation of different prions. Our further work pointed to the fact that many, if not all, of the observed effects that these co-chaperones have on prions are mediated by the various ways they can regulate activities of Hsp70. Mammalian genomes encode three major Hsp70 nucleotide exchange factor (NEF) families designated Hsp110, HspBP1 and Bag1. NEFs regulate Hsp70 activity by promoting release of ADP, which facilitates return of Hsp70 to the ATP-bound """"""""open"""""""" state and release of its bound substrate. has Two Saccharomyces cerevisiae NEFs, Sse1/2 and Fes1, which are homologous to the mammalian Hsp110 and HspBP1 families, respectively, have been shown to be required for efficient propagation of prions in yeast. The effects that both have on prions depends on their ability to interact with, and presumably regulate, Hsp70. Snl1 is the only yeast Bag1 family NEF. It is anchored to the membrane of the endoplasmic reticulum and whether it is important for propagation of cytosolic prions is unknown. In work with another group we showed that, unlike SSe1 and Fes1, Snl1 is dispensable for prion propagation. However, elevating expression of a version of Snl1 that contains only the Bag1 homology domain cured cells of URE3 prions. Mutating Snl1 amino acids important for Hsp70 interaction reduced or eliminated the curing, which is consistent with other work showing that effects of Hsp70 co-factors on prions is mediated through interaction with hsp70. A slightly longer version of Snl1 that contains amino acids thought to promote interaction of Snl1 with translating ribosomes failed to cure, but mutating those residues restored curing ability. Thus, it appears that Snl1 must be freely soluble in the cytoplasm for it to cure URE3 and that its effects on prions are independent of any effects it might have on translation. Altering abundance or function of Hsp70 and its co-chaperones reduces pathology in several models of amyloid and other protein folding disorders. Our work provides insight into functions of this chaperone system that can help guide strategies for using chaperones as targets for therapy in such diseases.
