Functionally redundant isoforms of a yeast Hsp70 chaperone subfamily have different antiprion effects.
Why eukaryotes encode multiple Hsp70 isoforms is unclear. Saccharomyces cerevisiae Ssa1p and Ssa2p are constitutive 98% identical Hsp70's. Stress-inducible Ssa3p and Ssa4p are 80% identical to Ssa1/2p. We show Ssa1p-4p have distinct functions affecting [PSI(+)] and [URE3] prions. When expressed as the only Ssa, Ssa1p antagonized [URE3] and Ssa2p antagonized [PSI(+)]. Ssa3p and Ssa4p influenced [URE3] and [PSI(+)] somewhat differently but overall their effects paralleled those of Ssa1p and Ssa2p, respectively. Additionally, Ssa3p suppressed a prion-inhibitory effect of elevated temperature. Our previously described Ssa1-21p mutant weakens [PSI(+)] in SSA1-21 SSA2 cells and abolishes it in SSA1-21 ssa2Delta cells. To test if the same mutation affected other prions or altered Ssa2p similarly, we compared effects of a constructed Ssa2-21p mutant and Ssa1-21p on both prions. Surprisingly, [URE3] was unaffected in SSA1-21 SSA2 cells and could propagate in SSA1-21 ssa2Delta cells. Ssa2-21p impaired [URE3] considerably and weakened [PSI(+)] strongly but in a manner distinct from Ssa1-21p, highlighting functional differences between these nearly identical Hsp70's. Our data uncover exquisite functional differences among isoforms of a highly homologous cytosolic Hsp70 subfamily and point to a possibility that variations in Hsp70 function that might improve fitness under optimal conditions are also important during stress.
Sharma
Deepak
D
Laboratory of Biochemistry and Genetics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0851, USA.
Masison
Daniel C
DC
D000483:Alleles; D017353:Gene Deletion; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism*; D010641:Phenotype; D011328:Prions / Q000378:metabolism*; D020033:Protein Isoforms / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D012995:Solubility; D013696:Temperature
D018840:HSP70 Heat-Shock Proteins; D011328:Prions; D020033:Protein Isoforms; D029701:Saccharomyces cerevisiae Proteins