title: Phosphorylation of a heme-regulated eukaryotic initiation factor 2α kinase enhances the interaction with heat-shock protein 90 and substantially upregulates kinase activity.
abstract: 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.
authors.affiliation: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan.
authors.forename: Kentaro
authors.initials: K
authors.lastname: Mukai
authors.forename: Toru
authors.initials: T
authors.lastname: Shimizu
authors.forename: Jotaro
authors.initials: J
authors.lastname: Igarashi
mesh_terms: D000818:Animals; D018841:HSP90 Heat-Shock Proteins / Q000378:metabolism*; D051379:Mice; D010766:Phosphorylation; D011485:Protein Binding; D019892:eIF-2 Kinase / Q000378:metabolism*
chemical_list: D018841:HSP90 Heat-Shock Proteins; D019892:eIF-2 Kinase