Akt Promotes Cell Survival by Phosphorylating and Inhibiting a Forkhead Transcription Factor

The 14-3-3 family of proteins mediates signal transduction by binding to phosphoserine-containing proteins. Using phosphoserine-oriented peptide libraries to probe all mammalian and yeast 14-3-3s, we identified two different binding motifs, RSXpSXP and RXY/FXpSXP, present in nearly all known 14-3-3 binding proteins. The crystal structure of 14-3-3zeta complexed with the phosphoserine motif in polyoma middle-T was determined to 2.6 A resolution. The bound peptide is in an extended conformation, with a tight turn created by the pS +2 Pro in a cis conformation. Sites of peptide-protein interaction in the complex rationalize the peptide library results. Finally, we show that the 14-3-3 dimer binds tightly to single molecules containing tandem repeats of phosphoserine motifs, implicating bidentate association as a signaling mechanism with molecules such as Raf, BAD, and Cbl.

A signaling pathway was delineated by which insulin-like growth factor 1 (IGF-1) promotes the survival of cerebellar neurons. IGF-1 activation of phosphoinositide 3-kinase (PI3-K) triggered the activation of two protein kinases, the serine-threonine kinase Akt and the p70 ribosomal protein S6 kinase (p70(S6K)). Experiments with pharmacological inhibitors, as well as expression of wild-type and dominant-inhibitory forms of Akt, demonstrated that Akt but not p70(S6K) mediates PI3-K-dependent survival. These findings suggest that in the developing nervous system, Akt is a critical mediator of growth factor-induced neuronal survival.