Hsp90 regulates p50^cdc37 function during the biogensis of the active conformation of the heme-regulated eIF2α kinase

Recent studies indicate that p50^cdc37 facilitates Hsp9O-mediated biogenesis of certain protein kinases. In this report, we examined whether p50^cdc37 is required for the biogenesis of the heine-regulated eIF2α kinase (HRI) in reticulocyte lysate. p50^cdc37 interacted with nascent HRI co-translationally and this interaction persisted during the maturation and activation of HRI. p50^cdc37 stimulated HRI's activation in response to heine deficiency, but did not activate HRI per se. p50^cdc37 function was specific to immature and inactive forms of the kinase. Analysis of mutant Cdc37 gene products indicated that the N-terminal portion of p50^cdc37 interacted with immature HRI, but not with Hsp90, while the C-terminal portion of p50^cdc37 interacted with Hsp90. The Hsp90-specific inhibitor geldanamycin disrupted the ability of both Hsp9O and p50^cdc37 to bind HRI and promote its activation, but did not disrupt the native association of p50^cdc37 with Hsp90. A C-terminal truncated mutant of p50^cdc37 inhibited HRI's activation, prevented the interaction of Hsp90 with HRI, and bound to HRI irrespective of geldanamycin treatment. Additionally, native complexes of HRI with p50^cdc37 were detected in cultured K562 erythroleukemia cells. These results suggest that p50^cdc37 provides an activity essential to HRI biogenesis via a process regulated by nucleotide-mediated conformational switching of its partner Hsp90.