We have isolated and purified an activity from amoebae of Physarum polycephalum that reduces the flow birefringence of a solution of F-actin in a Ca2+-dependent manner. The purified activity from 100 g of amoebae consisted of 1 mg of a 40000 mol. wt protein. DNase I-affinity chromatography demonstrated that the protein binds to Physarum actin in a Ca2+-dependent manner, and the binding is not reversed by excess EGTA. Viscometric measurement indicated that the protein (i) accelerates polymerization of G-actin, and (ii) severs F-actin, in a Ca2+-dependent manner. Thus, the protein appeared functionally similar to the fragmin previously isolated from Physarum plasmodia (plasmodial fragmin). However, the two proteins had slightly different mobilities on urea-SDS-PAGE, and antibodies raised against the two proteins scarcely cross-reacted with each other. Hence, we conclude that the two proteins are closely related to but are different from each other, and we have named the novel protein 'myxamoebal fragmin'. Immunoblot analysis indicated that myxamoebal and plasmodial fragmins are specifically present in amoebae and plasmodia, respectively. Results of immunofluorescence staining suggest that the synthesis of plasmodial fragmin is switched on coordinately with the synthesis of the heavy chain of plasmodial myosin and other plasmodium-specific contractile proteins during the apogamic differentiation of amoebae to plasmodia.
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