14-3-3:Shc Scaffolds Integrate Phosphoserine and Phosphotyrosine Signaling to Regulate Phosphatidylinositol 3-Kinase Activation and Cell Survival

Integrated cascades of protein tyrosine and serine/threonine phosphorylation play an essential role in transducing signals in response to growth factors and cytokines. How adaptor or scaffold proteins assemble signaling complexes through both phosphotyrosine and phosphoserine/threonine residues to regulate specific signaling pathways and biological responses is unclear. We show in multiple cell types that endogenous 14-3-3zeta is phosphorylated on Tyr179 in response to granulocyte macrophage colony stimulating factor. Importantly, 14-3-3zeta can function as an intermolecular bridge that couples to phosphoserine residues and also directly binds the SH2 domain of Shc via Tyr179. The assembly of these 14-3-3:Shc scaffolds is specifically required for the recruitment of a PI 3-kinase signaling complex and the regulation of CTL-EN cell survival in response to cytokine. The biological significance of these findings was further demonstrated using primary bone marrow-derived mast cells from 14-3-3zeta -/- mice. We show that cytokine was able to promote Akt phosphorylation and viability of primary mast cells derived from 14-3-3zeta -/- mice when reconstituted with wild type 14-3-3zeta, but the Akt phosphorylation and survival response was reduced in cells reconstituted with the Tyr179Phe mutant. Together, these results show that 14-3-3:Shc scaffolds can act as multivalent signalling nodes for the integration of both phosphoserine/threonine and phosphotyrosine pathways to regulate specific cellular responses.