Eukaryotic initiation factor eIF2B is a guanine nucleotide exchange protein involved in regulation of translation initiation. Phosphorylation of the ε-subunit is thought to be important in insulin-mediated changes in eIF2B activity. However, elucidation of insulin's action has proven elusive, primarily because eIF2Bε is a substrate in vitro for at least three different protein kinases. In the present study, we observed changes in eIF2Bε kinase activity only in those muscles previously shown to exhibit alterations in protein synthesis in response to insulin. Specifically, eIF2Bε kinase activity was increased in psoas muscle from diabetic rats compared to controls. Treating diabetic rats with insulin rapidly reduced eIF2Bε kinase activity below control values. Changes were not observed in heart. To identify the kinase(s) in psoas responsible for phosphorylating eIF2Bε, the wildtype and two variant forms of the ε-subunit were expressed in and purified from Sf9 insect cells, and were used as substrates in protein kinase assays. The first variant contained a point mutation in the eIF2Bε cDNA that converted the glycogen synthase kinase-3 (GSK-3) phosphorylation site, Ser535, to a nonphosphorylatable Ala residue. In the second variant, the putative GSK-3 'priming' site, Ser539, was converted to Asp. Based on the pattern of phosphorylation of the wildtype and two variant forms of eIF2Bε using casein kinase (CK)-I, CK-II, or GSK-3 as well as that observed with skeletal muscle extracts, we conclude that the predominant eIF2Bε kinase in psoas muscle is GSK-3. Thus, insulin-mediated changes in eIF2B activity are likely to involve GSK-3.
|Original language||English (US)|
|Number of pages||10|
|Journal||International Journal of Biochemistry and Cell Biology|
|State||Published - Jan 1999|
All Science Journal Classification (ASJC) codes
- Cell Biology