Phospholipid binding to the FAK catalytic domain impacts function

Focal adhesion kinase is an essential nonreceptor tyrosine kinase that plays an important role in development, in homeostasis and in the progression of human disease. Multiple stimuli activate FAK, which requires a change in structure from an autoinhibited to activated conformation. In the autoinhibited conformation the FERM domain associates with the catalytic domain of FAK and PI(4,5) P-2 binding to the FERM domain plays a role in the release of autoinhibition, activating the enzyme. An in silico model of FAK/PI(4,5) P2 interaction suggests that residues on the catalytic domain interact with PI(4,5) P2, in addition to the known FERM domain PI(4,5) P2 binding site. This study was undertaken to test the significance of this in silico observation. Mutations designed to disrupt the putative PI(4,5) P2 binding site were engineered into FAK. These mutants exhibited defects in phosphorylation and failed to completely rescue the phenotype associated with fak (-/-) phenotype fibroblasts demonstrating the importance of these residues in FAK function. The catalytic domain of FAK exhibited PI(4,5) P2 binding in vitro and binding activity was lost upon mutation of putative PI(4,5) P2 binding site basic residues. However, binding was not selective for PI(4,5) P2, and the catalytic domain bound to several phosphatidylinositol phosphorylation variants. The mutant exhibiting the most severe biological defect was defective for phosphatidylinositol phosphate binding, supporting the model that catalytic domain phospholipid binding is important for biochemical and biological function.