A membrane cytoskeleton from Dictyostelium discoideum. II. Integral proteins mediate the binding of plasma membranes to F-actin affinity beads

In novel, low-speed sedimentation assays, highly purified, sonicated Dictyostelium discoideum plasma membrane fragments bind to F-actin beads (fluorescein-labeled F-actin on antifluorescein IgG-Sephacryl S-1000 beads). Binding was found to be (a) specific, since beads containing bound fluorescein-labeled ovalbumin or beads without bound fluorescein-labeled protein do not bind membranes, (b) saturable at ∼0.6 μg of membrane protein per microgram of bead-bound F-actin, (c) rapid with a t1/2 of 4-20 min, and (d) apparently of reasonable affinity since the off rate is too slow to be measured by present techniques. Using low-speed sedimentation assays, we found that sonicated plasma membrane fragments, after extraction with chaotropes, still bind F-actin beads. Heat-denatured membranes, proteolyzed membranes, and D. discoideum lipid vesicles did not bind F-actin beads. These results indicate that integral membrane proteins are responsible for the binding between sonicated membrane fragments and F-actin on beads. This finding agrees with the previous observation that integral proteins mediate interactions between D. discoideum plasma membranes and F-actin in solution (Luna, E. J., V. M. Fowler, J. Swanson, D. Branton, and D. L. Taylor, 1981, J. Cell Biol., 88:396-409). We conclude that low-speed sedimentation assays using F-actin beads are a reliable method for monitoring the associations between F-actin and membranes. Since these assays are relatively quantitative and require only micrograms of membranes and F-actin, they are a significant improvement over other existing techniques for exploring the biochemical details of F-actin-membrane interactions. Using F-actin beads as an affinity column for actin-binding proteins, we show that at least 12 integral polypeptides in D. discoideum plasma membranes bind to F-actin directly or indirectly. At least four of these polypeptides appear to span the membrane and are thus candidates for direct transmembrane links between the cytoskeleton and the cell surface.