A distinctive role for focal adhesion proteins in three-dimensional cell motility

Focal adhesions are large multi-protein assemblies that form at the basal surface of cells on planar dishes, which mediate cell signaling, force transduction, and adhesion to the substratum. While much is known about focal adhesion components in 2-D systems, their role in migrating cells within a more physiological three-dimensional (3-D) matrix is largely unknown. Live-cell microscopy shows that for cells fully embedded in a 3-D matrix, focal adhesion proteins, including vinculin, paxillin, talin, α-actinin, zyxin, VASP, FAK, and p130Cas, do not form aggregates but are diffusively distributed throughout the cytoplasm. Despite the absence of detectable focal adhesions, focal adhesion proteins still modulate cell motility but in a manner distinct from cells on planar substrates. Rather, focal adhesion proteins in matrix-embedded cells regulate cell speed and persistence by affecting protrusion activity and matrix deformation, two processes that play no direct role in controlling 2-D cell speed. This study shows that membrane protrusions constitute a critical motility/matrix-traction module that drives cell motility in a 3-D matrix.