Specific changes to the mechanism of cell locomotion induced by overexpression of beta-actin

Overexpression of beta -actin is known to alter cell morphology, though its effect on cell motility has not been documented previously Here we show; t hat overexpressing p-actin in myoblasts has striking effects on motility, i ncreasing cell speed to almost double that of control cells. This occurs by increasing the areas of protrusion and retraction and is accompanied by ra ised levels of beta -actin in the newly protruded regions. These regions of the cell margin, however, show decreased levels of polymerised actin, indi cating that protrusion can outpace the rate of actin polymerisation in thes e cells, Moreover, the expression of beta*-actin (a G244D mutant, which sho ws defective polymerisation in vitro) is equally effective at increasing sp eed and protrusion. Concomitant changes in actin binding proteins shaw no e vidence of a consistent mechanism for increasing the rate of actin polymeri sation in these actin overexpressing cells. The increase in motility is con fined to poorly spread cells in both cases and the excess motility can be a bolished by blocking myosin function with butanedione monoxime (BDM). Our o bservations on normal myoblasts are consistent with the view that they prot rude by the assembly and cross linking of actin filaments. In contrast, the additional motility shown by cells overexpressing beta -actin appears not to result from an increase in the rate of actin polymerisation but to depen d on myosin function, This suggests that the additional protrusion arises f rom a different mechanism, We discuss the possibility that it is related to retraction-induced protrusion in fibroblasts, In this phenomenon, a wave o f increased protrusion follows a sudden collapse in cell spreading, This vi ew could explain why it is only the additional motility that depends on spr eading, and has implications for understanding the differences in locomotio n that distinguish tissue cells from highly invasive cell types such as leu cocytes and malignant cells.