MULTIMERIZATION OF HSP42P, A NOVEL HEAT-SHOCK PROTEIN OF SACCHAROMYCES-CEREVISIAE, IS DEPENDENT ON A CONSERVED CARBOXYL-TERMINAL SEQUENCE

Rap1p is a transcriptional regulator of Saccharomyces cerevisiae, whic h plays roles in both transcriptional activation and silencing, To ide ntify proteins involved in Rap1p-dependent regulation of transcription , we used the two-hybrid system to screen for Rap1p-interacting protei ns. Two of the clones isolated from this screen encode a truncated pro tein with homology to small heat shock proteins (HSPs). Here we presen t an analysis of this novel S. cereuisiae HSP, which we name Hsp42p, E xpression of HSP42 is regulated by a range of stress conditions simila r to S. cerevisiae HSP26, with which Hsp42p shares most homology. Howe ver, HSP42 expression is more sensitive to increased salt concentratio n and to starvation and, in contrast to HSP26 is expressed in unstress ed cells. Hsp42p interacts with itself in the two-hybrid assay. This i nteraction is dependent on a hydrophobic region which is conserved amo ng small HSPs, Using bacterially expressed Hsp42p fusion proteins, we demonstrate that this is a direct interaction. Fractionation of yeast protein extracts by size demonstrates that all of the Hsp42p in these extracts is present in complexes with a molecular mass of greater than 200 kDa, suggesting that Hsp42p exists in high molecular mass complex es.