Although Acanthamoeba actobindin binds actin monomers, its inhibition
of actin polymerization differs from that of a simple monomer-sequeste
ring protein in that actobindin inhibits nucleation very much more tha
n elongation [Lambooy, P. K., and Korn, E. D. (1988) J. Biol, Chem. 26
3, 12836-12843] and can induce the accumulation of actin dimers in sto
ichiometric excess of the actobindin concentration [Bubb, M. R., Knuts
on, J. R., Porter, D. M,, and Kern, E. D. (1994) J. Biol. Chem, 269, 2
5592-25597]. We now describe a ''catalytic'' model for the interaction
of actobindin with actin monomer that quantitatively accounts for the
effects of actobindin on the kinetics of actin polymerization de novo
and the elongation of actin filaments. We propose that, in a polymeri
zing buffer, actobindin binds to two actin subunits forming an heterot
rimeric complex that is incompetent for nucleation, self-association,
and elongation. Actobindin can, however, dissociate from this complex,
leaving a novel actin dimer that can participate in elongation but re
mains incompetent for nucleation and self-association. Under appropria
te conditions, the concentration of this novel actin dimer can exceed
the actobindin concentration; thus, the model is catalytic rather than
stoichiometric. The experimentally observed time course of actin poly
merization de novo, the rate of elongation of filaments, and the amoun
t of actin dimer formed as a function of actobindin concentration are
all consistent with the catalytic model and inconsistent with the stoi
chiometric model. The rate of actobindin-induced actin dimer formation
is consistent with the hypothesis that the rate-limiting step is this
pathway is the formation of a precursor heterotrimeric complex.