Interactions of gelsolin and gelsolin-actin complexes with actin. Effects of calcium on actin nucleation, filament severing, and end blocking

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Abstract
Gelsolin is a Ca binding protein that shortens actin filaments. This effect occurs in the presence but not in the absence of micromolar Ca2+ concentrations and is partially reversed following removal of Ca2+. Once 2 actin molecules have bound to gelsolin in solutions containing Ca2+ one of the actins remains bound following chelation of Ca so that the reversal of gelsolin''s effect cannot be accounted for simply by its dissociation from the ends of the shortened filaments to allow for elongation. The interactions with actin of the ethylene glycol bis(.beta.-aminoethyl ether)-N,N,-N'',N''-tetraacetic acid (EGTA) stable 1:1 gelsolin-actin complexes are compared with those of free gelsolin. The abilities of free or complexed gelsolin to sever actin filaments, nucleate filament assembly, bind to the fast growing (+) filament ends, and lower the filament size distribution in the presence of either Ca2+ or EGTA were examined. Both free gelsolin-actin complexes apparently are highly dependent on Ca2+ concentration when present in a molar ratio to actin < 1:50. The gelsolin-actin complexes, however, differ from free gelsolin in that they have a higher affinity for (+) filament ends in EGTA and they cannot sever filaments in Ca. The limited reversal of actin-gelsolin binding following removal of Ca and the Ca sensitivity of nucleation by complexes suggest an alternative reannealing of shortened filaments that involves redistribution of actin monomers and may account for the Ca-sensitive functional reversibility of the solation of actin by gelsolin.