Stress-induced regulation of eukaryotic elongation factor 2 kinase by SB 203580-sensitive and -insensitive pathways.

Eukaryotic elongation factor 2 (eEF2) kinase, the enzyme that inactivates eEF2, is controlled by phosphorylation. Previous work showed that stress-activated protein kinase 4 (SAPK4, also called p38delta) inhibits eEF2 kinase in vitro by phosphorylating Ser-359, while ribosomal protein S6 kinases inhibit eEF2 kinase by phosphorylating Ser-366 [Knebel, Morrice and Cohen (2001) EMBO J. 20, 4360-4369; Wang, Li, Williams, Terada, Alessi and Proud (2001) EMBO J. 20, 4370-4379]. In the present study we have examined the effects of the protein synthesis inhibitor anisomycin and tumour necrosis factor-alpha (TNF-alpha) on the phosphorylation of eEF2 kinase. We demonstrate that Ser-359, Ser-366 and two novel sites (Ser-377 and Ser-396) are all phosphorylated in human epithelial KB cells, but only the phosphorylation of Ser-359 and Ser-377 increases in response to these agonists and correlates with the dephosphorylation (activation) of eEF2. Ser-377 is probably a substrate of MAPKAP-K2/K3 (mitogen-activated protein kinase-activated protein kinase 2/kinase 3) in cells, because eEF2 kinase is phosphorylated efficiently by these protein kinases in vitro and phosphorylation of this site, induced by TNF-alpha and low (but not high) concentrations of anisomycin, is prevented by SB 203580, which inhibits SAPK2a/p38, their "upstream" activator. The phosphorylation of Ser-359 induced by high concentrations of anisomycin is probably catalysed by SAPK4/p38delta in cells, because no other stress-activated, proline-directed protein kinase tested phosphorylates this site in vitro and phosphorylation is insensitive to SB 203580. Interestingly, the phosphorylation of Ser-359 induced by TNF-alpha or low concentrations of anisomycin is suppressed by SB 203580, indicating that phosphorylation is also mediated by a novel pathway. Since the phosphorylation of Ser-377 does not inhibit eEF2 kinase in vitro, our results suggest that anisomycin or TNF-alpha inhibit eEF2 kinase via the phosphorylation of Ser-359.