Minimal features of paxillin that are required for the tyrosine phosphorylation of focal adhesion kinase

Tyrosine phosphorylation of FAK (focal adhesion kinase) regulates signalling that results from the interaction of integrins with extracellular matrix and growth factor receptors. A critical step in this process is the phosphorylation of Tyr397 of FAK, which creates a binding site for Src family kinases, PI3K (phosphoinositide 3-kinase) and Shc (Src homology and collagen homology). An intact Tyr397 site is required for FAK-mediated regulation of cell migration, survival signals and full responsiveness to soluble growth factors. We showed previously that the adaptor protein paxillin is required for the overall tyrosine phosphorylation of FAK in embryonic stem cells [Wade, Bohl and Vande Pol (2002) Oncogene 21, 96-107]. In the present paper, we identify the minimal structural features of paxillin that are required to support overall FAK tyrosine phosphorylation and Tyr397 phosphorylation. Paxillin contains N-terminal leucine-rich LD motifs that bind directly to FAK and four LIM (Lin-11, Isl-1 and Mec-3) domains in the C-terminus. We show that paxillin LIM domains 1, 2 and 3 are each required for FAK tyrosine phosphorylation, while LIM4 is dispensable. In addition to paxillin LIM domains 1, 2 and 3, a single LD motif on paxillin is required to support FAK tyrosine phosphorylation in embryonic stem cells. Both sequence and spatial requirements exist for LD motifs to support FAK tyrosine phosphorylation. Interestingly, synthetic LD motifs that fail to bind FAK in vitro are able to fully support FAK tyrosine phosphorylation, indicating that minimal interactions of LD motifs with FAK suffice. Our results demonstrate at least four distinct structural domains of paxillin support at least three distinct functions that are each required for FAK tyrosine phosphorylation.