A Novel mTOR-Regulated Phosphorylation Site in Elongation Factor 2 Kinase Modulates the Activity of the Kinase and Its Binding to Calmodulin

Abstract

Eukaryotic elongation factor 2 (eEF2) kinase is an unusual calcium- and calmodulin-dependent protein kinase that is regulated by insulin through the rapamycin-sensitive mTOR pathway. Here we show that insulin decreases the ability of eEF2 kinase to bind calmodulin in a rapamycin-sensitive manner. We identify a novel phosphorylation site in eEF2 kinase (Ser78) that is located immediately next to its calmodulin-binding motif. Phosphorylation of this site is increased by insulin in a rapamycin-sensitive fashion. Regulation of the phosphorylation of Ser78 also requires amino acids and the protein kinase phosphoinositide-dependent kinase 1. Mutation of this site to alanine strongly attenuates the effects of insulin and rapamycin both on the binding of calmodulin to eEF2 kinase and on eEF2 kinase activity. Phosphorylation of Ser78 is thus likely to link insulin and mTOR signaling to the control of eEF2 phosphorylation and chain elongation. This site is not a target for known kinases in the mTOR pathway, e.g., the S6 kinases, implying that it is phosphorylated by a novel mTOR-linked protein kinase that serves to couple hormones and amino acids to the control of translation elongation. eEF2 kinase is thus a target for mTOR signaling independently of previously known downstream components of the pathway.

This work was supported by a project grant from the BBSRC.

We are grateful to Nick Morrice and David Campbell (University of Dundee) for valuable assistance with protein chemistry and mass spectrometry, and to Nick Redpath (formerly of Leicester University), Richard Lamb (Institute of Cancer Research, London, United Kingdom), Grahame Hardie, Dario Alessi, and Axel Knebel (all from the University of Dundee) for generous gifts of antisera and other reagents.