Severin is a protein from Dictyostelium that severs actin filaments in a Ca2+-dependent manner and remains bound to the filament fragments (Brown, S. S., K. Yamamoto, and J. A. Spudich , 1982, J. Cell Biol., 93:205-210; Yamamoto, K., J. D. Pardee , J. Reidler , L. Stryer , and J. A. Spudich , 1982, J. Cell Biol. 95:711-719). Further characterization of the interaction of severin with actin suggests that it remains bound to the preferred assembly end of the fragmented actin filaments. Addition of severin in molar excess to actin causes total disassembly of the filaments and the formation of a high-affinity complex containing one severin and one actin. This severin -actin complex does not sever actin filaments. The binding of severin to actin, measured directly by fluorescence energy transfer, requires micromolar Ca2+, as does the severing and depolymerizing activity reported previously. Once bound to actin in the presence of greater than 1 microM Ca2+, severin is not released from the actin when the Ca2+ is lowered to less than 0.1 microM by addition of EGTA. Tropomyosin, DNase I, phalloidin, and cytochalasin B have no effect on the ability of severin to bind to or sever actin filaments. Subfragment 1 of myosin, however, significantly inhibits severin activity. Severin binds not only to actin filaments, but also directly to G-actin, as well as to other conformational species of actin.

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