Flux at focal adhesions: slippage clutch, mechanical gauge, or signal depot

Abstract

Focal adhesions provide physical linkages between the interior of an adhesive cell and the extracellular matrix (ECM). They may be involved in mediating such functions as cell migration, anchorage, mechanical interactions with the ECM, and the detection of physical cues in the environment. Cell biologists have long struggled to piece together the complex array of components found at focal adhesions, and the equally complex network of interactions among these components, into a "machine" that performs these putative functions. Two recent studies, however, indicate that focal adhesions may be more amorphous and dynamic than previously envisioned. These studies found different degrees of correlated retrograde movement of various focal adhesion proteins with actin filaments, while integrins remained largely stationary. Such differential movements appear inconsistent with a precisely engineered machine and may reflect a slippage clutch for transmitting a variable amount of contractile force to the substrate for migration, a gauge of physical interactions at adhesive sites, or a mechanism for releasing signals from the adhesive sites to the interior of the cell.