text: Earlier we showed altered expression of Hsp40s, nucleotide exchange factors (NEFs) and several TPR-domain proteins affect prion propagation in wild type and Hsp70 mutant cells. We also identified several Hsp40 and TPR protein mutant alleles that impair or enhance prion propagation. Together our data imply that many, if not all the observed effects of co-chaperones on prions are mediated by their regulation of Hsp70 activities. In many instances the same conditions producing a significant effect on one prion have little or no effect on a different prion, pointing to a prion preference or specificity of the Hsp70/co-chaperone pairings. Targeting the two major yeast Hsp40s (Ydj1p and Sis1p) , we have identified mutations that alter prion propagation in both positive and negative ways. Characterization of these mutants is ongoing. We showed that elimination of the URE3 prion by overproduced Ydj1p requires only its ability to regulate Hsp70, which demonstrated that "curing" was indirect and required Hsp40/Hsp70 interaction, but not Hsp40/amyloid interaction. We also showed that while the intact Sis1p is unable to cure cells of URE3, the Hsp70-regulating J-domain of Sis1p could do so when separated from the remainder of the protein, highlighting both specificity of Hsp40 regulation of Hsp70 and functional redundancy of Hsp40 subdomains. We also showed that curing efficiency depended on the particular isoform of Hsp70. More recently we identified certain activities of Sis1p that specifically affect the ability of Hsp70 to interfere with propagation of the PSI+ prion. Similar alrerations of Sis1p have no detectable effect on prions in wild type cells. These findings show the dependency of the mutant Hsp70 on Sis1p in order for it to exert its anti-prion effects, and the lack of a requirement of individual Hsp40 activities of Sis1p for prion propagation. The data also confirm a link between the anti-prion effects of specifically altered Hsp70 function and the alteration of another chaperone Hsp104. This work identifies specific Hsp40/Hsp70 pairings that influence prion propagation in defined ways, and links certain anti-prion effects to the Hsp104 chaperone machinery.