Actin structural proteins in cell motility.

The machinery for cell locomotion is based in a network of polymerized actin filaments supporting the peripheral cytoplasm. This network or 'gel' consists of actin filaments in a variety of configurations, including cables, loose bundles, and branching arrays; all formed by the interaction of actin-associated proteins with actin filaments. For cell locomotion to occur, this network must be reversibly disassembled or 'solated' to allow protrusion, then re-assembled to stabilize the resulting extension. Thus, proteins to promote both 'solation' and 'gelation' of actin are important for efficient cell locomotion. Because of their distribution, control, and in vitro effects on actin filaments, two such proteins, gelsolin and actin-binding protein (ABP) should play especially important roles in cell motility. Support for this premise is found in in vivo studies of mouse kidney fibroblasts which demonstrated increased translocational locomotion after cytoplasmic gelsolin expression was increased genetically and in melanoma cells missing actin-binding protein which behave as expected for a cell unable to achieve efficient actin gelation. Since malignant transformation is known to affect the expression and distribution of several of these actin structural proteins, including gelsolin, further investigations of the role these proteins play in cell motility will be important to the determination of tumor cell motility and hence metastatic propensity.