Filamin A, the Arp2/3 complex, and the morphology and function of corticalactin filaments in human melanoma cells

The Arp2/3 complex and filamin A (FLNa) branch actin filaments. To define t he role of these actin-binding proteins in cellular actin architecture, we compared the morphology of Fl-Na-deficient human melanoma (M2) cells and th ree stable derivatives of these cells expressing normal FLNa concentrations . All the cell lines contain similar amounts of the Arp2/3 complex. Serum a ddition causes serum-starved M2 cells to extend flat protrusions transientl y; thereafter, the protrusions turn into spherical blebs and the cells do n ot crawl. The short-lived lamellae of M2 cells contain a dense mat of long actin filaments in contrast to a more three-dimensional orthogonal network of shorter actin filaments in lamellae of identically treated FLNa-expressi ng cells capable of translational locomotion. FLNa-specific antibodies loca lize throughout the leading lamellae of these cells at junctions between or thogonally intersecting actin filaments. Arp2/3 complex-specific antibodies stain diffusely and label a few, although not the same, actin filament ove rlap sites as FLNa antibody. We conclude that FLNa is essential in cells th at express it for stabilizing orthogonal actin networks suitable for locomo tion. Contrary to some proposals, Arp2/3 complex-mediated branching of acti n alone is insufficient for establishing an orthogonal actin organization o r maintaining mechanical stability at the leading edge.