Hsp90 regulates p50cdc27 function during the biogenesis of the active conformation of the heme-regulated eIF2α kinase

Recent studies indicate that p50(cdc37) facilitates Hsp90-mediated biogenesis of certain protein kinases. In this report, we examined whether p50(cdc37) is required for the biogenesis of the heme-regulated eIF2 alpha kinase (HRI) in reticulocyte lysate. p50cdc37 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 heme deficiency, but did not activate HRI per se. p50cdc37 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 HspSO. The Hsp90-specific inhibitor geldanamycin disrupted the ability of both HspSO 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 HspSO 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 HspSO.