We characterized the role of protein tyrosine phosphatase (PTP)-␣ in focal adhesion (FA) formation and remodeling using wild-type and PTP␣-deficient (PTP␣ Ϫ/Ϫ ) cells. Compared with wild-type cells, spreading PTP␣ Ϫ/Ϫ fibroblasts displayed fewer leading edges and formed elongated ␣-actinin-enriched FA at the cell periphery. These features suggest the presence of slowly remodeling cell adhesions and were phenocopied in human fibroblasts in which PTP␣ was knocked down using short interfering RNA (siRNA) or in NIH-3T3 fibroblasts expressing catalytically inactive (C433S/C723S) PTP␣. Fluorescence recovery after photobleaching showed slower green fluorescence protein-␣-actinin recovery in the FA of PTP␣ Ϫ/Ϫ than wild-type cells. These alterations correlated with reduced cell spreading, adhesion, and polarization and retarded contraction of extracellular matrices in PTP␣ Ϫ/Ϫ fibroblasts. Activation of Rac1 and its recruitment to FA during spreading were diminished in cells expressing C433S/C723S PTP␣. Rac1Ϫ/Ϫ cells also displayed abnormally elongated and peripherally distributed FA that failed to remodel. Conversely, expression of constitutively active Rac1 restored normal FA remodeling in PTP␣ Ϫ/Ϫ cells. We conclude that PTP␣ is required for remodeling of FA during cell spreading via a pathway involving Rac1. cell spreading; integrins; extracellular matrix; actin cytoskeleton CELL ADHESION AND MOTILITY play pivotal roles in diverse processes, including embryonic development, tissue regeneration, wound healing, and neoplasia (41). As cells explore new regions of the substrate, they interact with extracellular matrix (ECM) molecules, in part through focal adhesions (FA). These integrin-based, multimolecular structures link the ECM to the actin cytoskeleton and comprise Ͼ50 structural and signaling molecules that regulate remodeling of FA, as well as FAdependent signaling (4, 30). In the leading edges (lamellipodia) of motile cells, nascent FA, termed focal complexes, form and are then rapidly remodeled. Focal complexes display a punctuate appearance (32). As the rate of FA assembly and dissipation decreases near the edge of the lamellipodium, highly dynamic focal complexes mature into larger and more stable FA by the recruitment of additional proteins such as ␣-actinin (3,19,22). Mature FA facilitate the transmission of tensile forces to the substrate, and, during locomotion, these structures translocate toward the center of the cell, thus facilitating cell movement relative to the substrate (54). Our understanding of the mechanisms that regulate the assembly and disassembly of FA is incomplete.Members of the Rho family of small GTPases, RhoA, Rac, and Cdc42, are key regulators of adhesion dynamics, in that they couple the formation and breakdown of FA to actin assembly (5). Activation of Cdc42 and Rac1 directs the formation of membrane protrusions, which are associated with increased FA turnover (31, 45). In contrast, active RhoA promotes the formation of actin stress fibers and more stable FA, which enhance cell atta...
