New actin-binding proteins from Dictyostelium discoideum

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Abstract
Dictyostelium discoideum contains a soluble actin‐binding protein that caps actin filaments at their fast growing ends. The purified protein consists of two subunits with 34 kd and 32 kd apparent mol. wts. Like similar proteins from Acanthamoeba and bovine brain the capping protein from D. discoideum acts in a Ca2+ ‐independent manner. It lacks severing activity as indicated by its inability to disrupt the stress fibers and the microfilament network in detergent‐extracted cells. Two actin‐binding proteins from a plasma membrane‐enriched fraction were labeled with [125I]actin using a gel overlay technique. One of these proteins, with an apparent mol. wt. of 17 kd in SDS‐polyacrylamide gels, has been purified from high‐salt extracts, the other protein with an apparent mol. wt. of 31 kd has been purified from Triton X‐100 extracted membranes. Monoclonal antibodies were raised against D. discoideum severin, α‐actinin, the larger subunit of the capping protein, and the 17‐kd membrane‐associated protein. Immunoblotting of proteins from whole cell lysates showed that all these actin‐binding proteins were present in both growth phase and aggregation‐competent cells.