GERp95 belongs to a family of signal-transducing proteins and requires Hsp90 activity for stability and Golgi localization

GERp95 (Golgi-endoplasmic reticulum protein 95 kDa) is part of a large fami ly of highly conserved proteins found in all metazoans and the fission yeas t Schizosaccharomyces pombe. Genetic studies suggest that homologs of GERp9 5 are components of signaling pathways that regulate cellular differentiati on, development, and RNA interference. However, the precise molecular funct ions of these proteins remain unknown. Genetic analysis of GERp95 homologs has been complicated by the presence of multiple genes with overlapping fun ctions in most organisms. Binding partners for members of this protein fami ly have not been identified. The purpose of this study was to identify prot eins that associate with GERp95. Glutathione S-transferase-GERp95 fusions w ere expressed in transfected cells, and proteins that bound to GERp95 were co-purified using glutathione-agarose beads. The amino-terminal region of G ERp95 was found to interact with the specialized chaperone Hsp90 and a numb er of its cognate binding proteins. Inhibition of Hsp90 activity with gelda namycin or radicicol resulted in rapid degradation of newly synthesized GER p95. The membrane-associated pool of GERp95 was not bound to Hsp90, althoug h activity of this chaperone was required for stable association of GERp95 with the Golgi in normal rat kidney cells. These results indicate that GERp 95 engages an Hsp90 chaperone complex prior to association with intracellul ar membranes.