ACTOBINDIN INDUCES THE ACCUMULATION OF ACTIN DIMERS THAT NEITHER NUCLEATE POLYMERIZATION NOR SELF-ASSOCIATE

Actobindin purified from Acanthamoeba castellanii inhibits the nucleat ion, but not the elongation, phase of actin polymerization. Previously , we had speculated that actobindin, which can simultaneously bind two actin monomers (Bubb, M. R., Lewis, M. S., and Kern, E. D. (1991) J. Biol. Chem. 266, 3820-3826), might preferentially interact with small oligomers and inhibit their ability to elongate (Lambooy, P. K., and K ern, E. D. (1988) J. Biol. Chem. 263, 12836-12843). In the accompanyin g paper (Bubb, M. R., Lewis, M. S., and Kern, E. D. (1994) J. Biol. Ch en. 269, 25587-25591), we show that under non-polymerizing conditions, actobindin binds to covalently cross-linked actin dimers with higher affinity than to two actin monomers, The sedimentation velocity and fl uorescence anisotropy experiments described in this paper show that ac tobindin prevents the formation of actin oligomers larger than an acti n dimer under conditions in which, in the absence of actobindin, actin rapidly polymerizes to F-actin with no detectable small oligomers. Mo reover, the molar concentration of actin dimer formed in the presence of actobindin can exceed the total actobindin concentration. These res ults indicate that actobindin does not form a stable complex with nati ve actin dimer but, rather, causes the accumulation of dimers that are unable to nucleate polymerization or self-associate.