Spectrin/actin complex isolated from sheep erythrocytes accelerates actin polymerization by simple nucleation. Evidence for oligomeric actin in the erythrocyte cytoskeleton.

A complex of spectrin and actin, isolated from sheep erythrocyte ghosts, accelerates the polymerization of actin in buffer containing 0.3 m~ MgC12. The rate of actin polymerization is similarly increased by sonicated F-actin nuclei. At steady state, the critical concentration of actin is lower when polymerization occurs in the presence of spectrin/actin complex than in its absence. Polymerization of actin in the presence of spectrin/actin complex is inhibited by substoichiometric concentrations of cytochalasin D, which is thought to block the net polymerizing ends of growing actin filaments (Brenner, S . L., and Korn, E. D. (1979) J. Biol. Chem 254, 9982-9985), in the same way that cytochalasin D inhibits actin polymerization in the absence of complex. However, actin filaments formed in the presence of complex are more stable to cytochalasin D added after polymerization has occurred than are filaments formed in the absence of complex. We conclude from these data that spectrin/actin complex accelerates actin polymerization by simple nucleation and, therefore, that the complex consists of short actin oligomers cross-linked by spectrin tetramers (Brenner, S. L., and Korn, E. D. (1979) J. Biol. Chem 254,8620-8627) which stabilize the net depolymerizing ends of the actin oligomers. The isolated spectrin/actin complex is a fragment of the erythrocyte cytoskeleton. From our proposal for the structure of the complex and estimates of others for the relative amounts of actin, spectrin, and spectrin/actin complex-related high affinity cytochalasin binding sites in human erythrocyte ghosts, we suggest that the cytoskeletal network contains short oligomers of actin consisting on average of about ten subunits each, with one of every two actin subunits cross-linked by a spectrin tetramer to a subunit of another actin oligomer.