Phosphorylation of elongation factor-2 kinase on serine 499 by cAMP-dependent protein kinase induces Ca2+/calmodulin-independent activity.
Elongation factor-2 kinase (eEF-2K) negatively regulates mRNA translation via the phosphorylation and inactivation of elongation factor-2 (eEF-2). We have shown previously that purified eEF-2K can be phosphorylated in vitro by cAMP-dependent protein kinase (PKA) and that this induces significant Ca(2+)/calmodulin (CaM)-independent eEF-2K activity [Redpath and Proud (1993) Biochem. J. 293, 31-34]. Furthermore, elevation of cAMP levels in adipocytes also increases the level of Ca(2+)/CaM-independent eEF-2K activity to a similar extent, providing a mechanistic link between elevated cAMP and the inhibition of protein synthesis [Diggle, Redpath, Heesom and Denton (1998) Biochem. J. 336, 525-529]. Here we describe the expression of glutathione S-transferase (GST)-eEF-2K fusion protein and the identification of two serine residues that are phosphorylated by PKA in vitro. Endoproteinase Arg-C digestion of GST-eEF-2K produced two phosphopeptides that were separated by HPLC and sequenced. (32)P Radioactivity release from these peptides indicated that the sites of phosphorylation were Ser-365 and Ser-499, both of which lie C-terminal to the catalytic domain. Mutation of these sites to non-phosphorylatable residues indicated that both sites need to be phosphorylated to induce Ca(2+)/CaM-independent eEF-2K activity in vitro. However, expression of Myc-tagged eEF-2K in HEK 293 cells, followed by treatment with chlorophenylthio-cAMP (CPT-cAMP), showed that Ser-499 phosphorylation alone induced Ca(2+)/CaM-independent eEF-2K activity in cells. Co-expression of wild-type eEF-2K with luciferase resulted in a 2-3-fold reduction in luciferase expression. Expression of eEF-2K S499D resulted in a 10-fold reduction in luciferase expression despite the fact that this mutant was expressed at very low levels. This indicates that eEF-2K S499D is constitutively active when expressed in cells, thus leading to the suppression of its own expression. Our data demonstrate an important role for the phosphorylation of Ser-499 in the activation of eEF-2K by PKA and the inhibition of protein synthesis.
Diggle
T A
TA
Department of Biochemistry, University of Leicester, University Road, Leicester LE1 7RH, UK.
Subkhankulova
T
T
Lilley
K S
KS
Shikotra
N
N
Willis
A E
AE
Redpath
N T
NT
D000818:Animals; D001483:Base Sequence; D002118:Calcium / Q000378:metabolism*; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000737:chemistry / Q000378:metabolism*; D002147:Calmodulin / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D017931:DNA Primers; D004591:Electrophoresis, Polyacrylamide Gel / Q000379:methods; D054736:Elongation Factor 2 Kinase; D010449:Peptide Mapping; D010766:Phosphorylation; D011817:Rabbits; D012694:Serine / Q000378:metabolism*
D002147:Calmodulin; D017931:DNA Primers; D012694:Serine; D017868:Cyclic AMP-Dependent Protein Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D054736:Elongation Factor 2 Kinase; D002118:Calcium