Novel interaction of the Hsp90 chaperone machine with Ssl2, an essential DNA helicase in Saccharomyces cerevisiae.

Hsp90 is an essential molecular chaperone that is critical for the activity of diverse cellular proteins. Hsp90 functions with a number of co-chaperone proteins, including Sti1/Hop. We conducted a genetic screen in Saccharomyces cerevisiae to isolate mutations that exhibit enhanced growth defects in the absence of STI1. We obtained mutations in genes encoding components of the Hsp90 chaperone machine, HSC82, CPR7 and YDJ1, and two essential genes, SSL2 and UTP21, not previously linked to Hsp90. Ssl2, the yeast homologue of XPB, is an ATP-dependent DNA helicase that is a component of the TFIIH multiprotein complex and has dual functions in transcription and DNA repair. In order to determine whether Ssl2 function is dependent on Hsp90, we further examined the interaction between Ssl2 and Hsp90. Multiple mutant alleles of SSL2 exhibited a pronounced growth defect when co-expressed with a mutant allele of Hsp90. In addition, isolation of Ssl2 protein resulted in the co-purification of Hsp90 and Sti1, suggesting that Ssl2 and Hsp90 are in the same protein complexes in vivo. These results suggest a novel role for Hsp90 in the essential cellular functions of transcription and DNA repair.