Phosphorylation of a heme-regulated eukaryotic initiation factor 2α kinase enhances the interaction with heat-shock protein 90 and substantially upregulates kinase activity

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 [1] an interaction between hsp90 and phosphorylated hri was evident,whereas no interaction was observed between hsp90 and hri dephosphorylated by treatment with λ protein phosphatase; [2] hsp90 enhanced the kinase activity of phosphorylated hri but not dephosphorylated hri,but this enhancement was not observed in the presence of heme; and,[3] 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. © 2011 bentham science publishers.