Fragmin: a calcium ion sensitive regulatory factor on the formation of actin filaments.

Physarum actinin previously isolated [Hatano, S., & Owaribe, K. (1976) in Cell Motility (Goldman, R., Pollard, T., & Rosenbaum, J., Eds.) Vol. 3, Book B, p 499, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY] was found to be a 1:1 complex of actin and fragmin which is a regulatory factor in the formation of actin filaments. Since fragmin did not contain a cysteine residue, it was purified from the complex by the selective cleavage of actin with 2-nitro-5-thiocyanobenzoic acid, followed by column chromatography. Fragmin had nearly the same molecular weight as actin, but had a quite different amino acid composition. When added to G-actin before polymerization, fragmin accelerated the initial viscosity increase of actin solutions induced by salts, but kept the final viscosity much lower than normal F-actin. When added to F-actin after polymerization, fragmin drastically reduced the viscosity of actin solutions. In both cases, the final products of reaction of fragmin with actin were short F-actin filaments. The number average length of the filaments decreased with the increasing molar ratio of fragmin to actin, and the length distribution was always exponential. Fragmin required for its activity a concentration of free Ca2+ higher than 10(-6) M. When the concentration of free Ca2+ was lower than 10(-7) M, fragmin affected neither actin polymerization nor F-actin. The regulation by Ca2+ was reversible.