The Hsp90/Cdc37p chaperone system is a determinant of molybdate resistance in Saccharomyces cerevisiae

Saccharomyces cerevisiae lacks enzymes that contain the molybdopterin co-factor and therefore any requirement for molybdenum as a trace mineral supplement. Instead, high molybdate levels are inhibitory to its growth. Low cellular levels of heat shock protein 90 (Hsp90), an essential chaperone, were found to enhance this sensitivity to molybdate. Certain Hsp90 point mutations and co-chaperone protein defects that partially compromise the function of the Hsp90/Cdc37p chaperone system also rendered S. cerevisiae hypersensitive to high molybdate levels. Sensitivity was especially apparent with mutations close to the Hsp90 nucleotide binding site, with the loss of the non-essential co-chaperone Sti1p (the equivalent of mammalian Hop), and with the abolition of residue Ser14 phosphorylation on the essential co-chaperone Cdc37p. While it remains to be proved that these effects reflect direct inhibition of the Hsp90 of the cell by the MoO(4) (2+) oxyanion in vivo; this possibility is suggested by molybdate sensitivity arising with a mutation in the Hsp90 nucleotide binding site that does not generate stress sensitivity or an impaired stress response. Molybdate sensitivity may therefore be a useful phenotype to score when studying mutations in this chaperone system.