Paxillin comes of age

Nicholas O. Deakin 0 Christopher E. Turner ) 0 0 Department of Cell and Developmental Biology, SUNY Upstate Medical University , 750 East Adams Street, Syracuse, NY 13210 , USA - Summary Paxillin is a multi-domain scaffold protein that localizes to the intracellular surface of sites of cell adhesion to the extracellular matrix. Through the interactions of its multiple protein-binding modules, many of which are regulated by phosphorylation, paxillin serves as a platform for the recruitment of numerous regulatory and structural proteins that together control the dynamic changes in cell adhesion, cytoskeletal reorganization and gene expression that are necessary for cell migration and survival. In particular, paxillin plays a central role in coordinating the spatial and temporal action of the Rho family of small GTPases, which regulate the actin cytoskeleton, by ce Introduction en The interaction of a cell with its microenvironment provides ic important chemical and spatial cues that contribute to the regulation lS of processes such as embryonic development, wound healing, le immune surveillance and tissue homeostasis through the modulation C of a diverse array of cellular functions, such as migration, fo differentiation and proliferation. The principal cell-surface proteins la that are responsible for regulating the binding of a cell to rn components of the external environment are the integrins. Integrins u are transmembrane proteins that comprise an and a subunit, Jo which together produce 24 distinct heterodimers (Hynes, 2002) that serve as bridges between the extracellular matrix (ECM) and the intracellular signaling machinery and actin cytoskeleton. Upon their interaction with the ECM, integrins cluster and recruit a wide variety of intracellular proteins (Fig. 1). These macromolecular foci were originally observed in cultured fibroblasts as electron-dense regions of the plasma membrane (Abercrombie et al., 1971) that were termed attachment plaques (Heggeness et al., 1978; Hynes and Destree, 1978) and have subsequently been defined as focal complexes, focal adhesions and fibrillar adhesions (Geiger et al., 2001), depending on their size, cellular localization or dependency on different members of the Rho family of GTPases (Chrzanowska-Wodnicka and Burridge, 1992; Nobes and Hall, 1995; Small et al., 1999; Zamir et al., 2000). Integrin cytoplasmic domains have no intrinsic enzymatic activity and so must recruit a variety of proteins to adhesion plaques or contacts to enable these foci to serve as conduits for the transmission of force that is necessary for cell migration and for bidirectional signaling between the cell interior and its local microenvironment. Originally, the list of focal-adhesion-localized proteins was limited to structural proteins, such as talin and vinculin, that were believed to mediate the anchorage of the integrin cytoplasmic domain to the actin cytoskeleton (Burridge et al., 1988). Now, approximately 37 years after the visualization of the first cell-adhesion contact, the apparent molecular complexity of focal adhesions continues to increase. Focal adhesions are now reported to comprise upwards of 125 individual proteins, many of which recruiting an array of GTPase activator, suppressor and effector proteins to cell adhesions. When paxillin was first described 18 years ago, the amazing complexity of cell-adhesion organization, dynamics and signaling was yet to be realized. Herein we highlight our current understanding of how the multiple protein interactions of paxillin contribute to the coordination of celladhesion function. exhibit multiple protein-protein interactions (Turner, 2000; ZaidelBar et al., 2007a). Thus, understanding the interactions that govern focal-adhesion function, the regulation of these multiple interactions and their role in coordinating bidirectional signaling remains a considerable challenge. In 1990, paxillin joined the integrins (Hynes, 1992; Zaidel-Bar et al., 2007a), talin (Burridge and Connell, 1983) and vinculin (Geiger et al., 1980) as one of the earliest known members of the focal adhesion proteome (Turner et al., 1990). Having first been identified as a 68 kDa protein that exhibited increased tyrosine phosphorylation following the transformation of chick embryonic fibroblasts by the Src-expressing Rous sarcoma virus (Glenney and Zokas, 1989), paxillin was subsequently purified from smooth muscle tissue and characterized as a direct binding partner for the vinculin tail domain (Turner et al., 1990). In keeping with the prevailing dogma of the time, in which focal adhesions were believed to function solely as passive, structural links between the ECM and the actin cytoskeleton, the name paxillin was coined: it derives from the Latin term paxillus (a peg or stake) and suggests a function that is somewhat analogous to a tent peg in tethering actin stress-fiber cables to the adhesion site (Brown and Turner, 2004; Turner et al., 1990). The discovery that paxillin, along with the recently described focal adhesion kinase (FAK) (Parsons, 2003; Schaller et al., 1992), was also tyrosine phosphorylated in nontransformed cells upon their adhesion to the ECM (Burridge et al., 1992) and in the developing embryo (Turner, 1991; Turner et al., 1993) provided the first indications that focal-adhesion proteins were actively signaling to the cell interior. The early embryonic lethality of mice deficient in paxillin, FAK and fibronectin further reinforced the essential role that focal adhesion proteins play in vivo (Hagel et al., 2002; Ilic et al., 1995). The subsequent analysis of fibroblasts derived from paxillin-deficient embryos has indicated that defects in cell migration might be the primary cause of the severe developmental phenotype. In this Commentary, we first detail the basic molecular architecture of paxillin and explain its classification as a molecular scaffold. In addition, we discuss how paxillin, through tightly IK L CrkII ERK Paxillin FAK Fig. 1. Focal adhesions provide both a structural and a signaling link between the ECM and the actin cytoskeleton. The adhesion of a cell to the ECM, via transmembrane -integrin heterodimers, leads to integrin activation and the recruitment of numerous intracellular proteins to the plasma membrane. Focal adhesions are now known to comprise over 125 protein species (only selected examples are depicted), which include both structural proteins (which mediate a physical link to the actin cytoskeleton) and regulatory proteins (which have a major role in the modulation of actin dynamics for productive cell migration). Proteins such as paxillin serve as scaffold proteins to facilitate the functional integration of these different categories of focal-adhesion proteins. Paxillin structure and the paxillin interactome The molecular cloning of paxillin and subsequent peptide-sequence analysis revealed that it comprises numerous discrete structural domains (Brown et al., 1996; Tu (...truncated)