Heme-regulated eukaryotic initiation factor 2α kinase (HRI) functions under conditions of heme shortage caused by blood diseases such as erythropoietic protoporphyria and β-thalassemia, and retains the heme:globin ratio at 1:1 by sensing the heme concentration in reticulocytes. This HRI function is regulated by various factors including autophosphorylation and protein-protein interactions. A heat-shock protein controls HRI function, however, the molecular mechanism of catalytic regulation of HRI by the heat-shock protein is unclear. In the present study, we examined the interactions of HRI with a heat-shock protein, Hsp90, under various conditions, using a pull-down assay and measuring catalytic activity. It was found that  an interaction between Hsp90 and phosphorylated HRI was evident, whereas no interaction was observed between Hsp90 and HRI dephosphorylated by treatment with δ protein phosphatase;  Hsp90 enhanced the kinase activity of phosphorylated HRI but not dephosphorylated HRI, but this enhancement was not observed in the presence of heme; and,  autophosphorylation of HRI was not influenced by Hsp90. Therefore, we propose that autophosphorylation of HRI is critical for catalytic regulation by Hsp90 under heme-shortage conditions.
Keywords: Heme, eIF2α kinase, phosphorylation, heat shock protein 90, HRI, thalassemia, eIF2a, GCN2, PERK, PKR, RNA, ATP, Coomassie Brilliant Blue, Hsp90, SDS-PAGEHeme, eIF2α kinase, phosphorylation, heat shock protein 90, HRI, thalassemia, eIF2a, GCN2, PERK, PKR, RNA, ATP, Coomassie Brilliant Blue, Hsp90, SDS-PAGE
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