Cytoplasmic gels from macrophages. Evidence for the involvement of four proteins in the calcium-dependent solation of actin networks.

A method has been devised to study the influence of Ca2+ on the in vitro formation of actin gel networks. Under appropriate conditions low-Ca2+ cytosolic extracts (less than 1 nM) from macrophages rapidly formed a macromolecular complex composed of actin, filamin, alpha-actinin and two new proteins of 70 kDa and 55 kDa. [Pacaud, M. (1986) Eur. J. Biochem. 156, 521-530]. Increasing concentrations of free Ca2+ to 1-2 microM resulted in complete inhibition of the association of 70-kDa protein, a protein which associates actin filaments into parallel arrays. Concentrations of Ca2+ greater than 3 microM caused incorporation of two additional proteins, gelsolin and a 18-kDa polypeptide, with no change in either the actin or alpha-actinin content of the cytoskeletal structures. Use of a polyacrylamide gel overlay technique with 125I-calmodulin revealed that a high-Mr calmodulin-binding protein analogous to spectrin was also associated with these structures when micromolar Ca2+ was present. Similar assays with 45CaCl2 indicated that the 70-kDa protein binds Ca2+ with high affinity. It is thus suggested that Ca2+ might regulate the dynamic assembly of microfilaments through several target proteins, gelsolin, the 70-kDa protein and calmodulin.