Essential roles of Crk and CrkL in fibroblast structure and motility.

Cytosolic proteins containing SH2 and SH3 domains, such as Crk and Crk-like (CrkL), are broadly expressed adapters that interact with a variety of proteins to fulfill key roles in signal transduction pathways triggered by activation of receptor and non-receptor tyrosine kinases. Crk and CrkL are similar to each other in structure and biochemical function, although they provide both distinct, as well as overlapping, biological roles during development. We developed a systematic approach to investigate Crk family functions at the cellular level by generating a conditional knock-out system for ablation of Crk and CrkL in cultured fibroblasts. The loss of both Crk and CrkL from fibroblasts resulted in reduced cell surface area and adoption of a rounded, refractile cellular phenotype. These morphological alterations were accompanied by a decrease in focal adhesion sites, reduced actin stress fibers and a collapse of microtubule structures. In addition, cells exhibited decreases in spontaneous motility and wound-healing behavior. Reduced p130Cas phosphorylation and actin levels closely followed the loss of Crk and CrkL, and stabilization of polymerized actin by jasplakinolide suppressed the morphological conversion. Ablation of Crk or CrkL alone conferred a much more modest phenotype suggesting that Crk and CrkL have overlapping functions that are critical for maintaining cell structure. The morphological alterations could be partially rescued by reintroduction of CrkII, and, to a lesser extent, CrkL. Taken together, our results suggest that Crk and CrkL have critical roles in cell structure and motility by maintaining cytoskeletal integrity.