The stress-activated kinase p38 mediates non-canonical activation of Src and tyrosine phosphorylation of the adapter protein TAB1.

Src is a non-receptor tyrosine kinase that is overexpressed and highly activated in many cancers and is one of the key factors contributing to malignant transformation. According to current concepts, Src activity relies on tyrosine phosphorylation, and phospho(p) Y419 in the activation loop is often regarded as a marker of its activation. However, recent studies have shown that pY419 may contribute to substrate selection. Therefore, the mechanisms underlying Src activation other than classical tyrosine phosphorylation warrant further study. We herein demonstrated that Src phosphorylates a novel substrate, TAB1, directly at Y481 in the TAK1-binding domain, changing the TAK1-TAB1 interaction. p38 enhances Src-mediated TAB1 phosphorylation through the direct phosphorylation of Src at N-terminal S75. Moreover, the mode of substrate recognition by the SH2 domain of Src is different in TAB1 than in FAK, a known SH2-dependent substrate. The present results identify a novel non-canonical Src activation mechanism based on serine phosphorylation and suggest pY481-TAB1 and pS75-Src as improved markers of Src activation, thereby offering alternative modes for assessing the Src activation status of Src-dependent cancers before and during targeted therapy.