Paxillin is a multi-domain protein that localizes in cultured cells primarily to sites of cell adhesion to the extracellular matrix (ECM) called focal adhesions. Focal adhesions form a structural link between the ECM and the actin cytoskeleton and are also important sites of signal transduction; their components propagate signals arising from the activation of integrins following their engagement with ECM proteins, such as fibronectin, collagen and laminin. Importantly, focal adhesion proteins including paxillin also serve as a point of convergence for signals resulting from stimulation of various classes of growth factor receptor.
Paxillin localizes to focal adhesions through its LIM domains, possibly through a direct association with ß-integrin tails or an intermediate protein ‘X’. In lymphoid cells it can bind directly to ɑ4-integrin (not shown). Its primary function is as a molecular adapter or scaffold protein that provides multiple docking sites at the plasma membrane for an array of signaling and structural proteins. For example, it provides a platform for protein tyrosine kinases such as FAK and SRC, which are activated as a result of adhesion or growth factor stimulation. Phosphorylation of residues in the N-terminus of paxillin by these kinases permits the regulated recruitment of downstream effector molecules such as CRK, which (via association with CAS) is important for transduction of external signals into changes in cell motility and for modulation of gene expression by the various MAP kinase cascades. LIM-domain-associated kinases regulate recruitment of paxillin to focal adhesions. In addition, negative regulators of these pathways, including CSK (an inhibitor of SRC activity) and PTP-PEST (a phosphatase that dephosphorylates p130Cas), bind directly to paxillin, thereby bringing them into close proximity with their targets.
Paxillin binds to many proteins that are involved in effecting changes in the organization of the actin cytoskeleton, which are necessary for cell motility events associated with embryonic development, wound repair and tumor metastasis. These range from structural proteins such as vinculin and actopaxin that bind actin directly to regulators of actin cytoskeletal dynamics such as the ARF GAP, PKL, the exchange factor PIX and the p21-activated kinase, PAK. These proteins serve as modulators/effectors of the ARF and RHO GTPase families.
Several of the paxillin-binding proteins have oncogenic equivalents, such as v-Src, v-Crk and BCR-ABL (not shown). These proteins probably use paxillin both as a substrate and as a docking site to perturb, and even bypass, the normal adhesion and growth factor signaling cascades necessary for controlled cell proliferation. Others, such as the E6 protein from Papillomavirus, facilitate transformation by disrupting the normal links between paxillin and the actin cytoskeleton by displacing paxillin-LD-motif-binding proteins.
Abbreviations: CAS, CRK-associated substrate; CH, calponin-homology domain; CSK, C-terminal SRC kinase; E6, Papillomavirus E6 protein; FAK, focal adhesion kinase; GIT, GRK interacter; GPCR, heterotrimeric-G-protein-coupled receptor; GRK, G-protein-coupled-receptor kinase; MAPK, mitogen-activated protein kinase (ERK, p38, JNK); PAK, p21-activated kinase; PBS, paxillin-binding subdomain; PIX, PAK-interacting exchange factor; PKL, paxillin kinase linker; POR1, partner of Rac; PS, phosphoserine; PT, phosphothreonine; PY, phosphotyrosine; RTK, growth factor receptor tyrosine kinase; SH, SRC-homology domain.
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