-Actinin
But Binds Actin Like Tropomyosin. -Actinin
But Binds Actin Like Tropomyosin. J.M. Ervasti, I.N. Rybakova and K.J. Amann
Dept. of Physiology, Univ. of Wisconsin, Madison, WI 53706.
Based solely on its sequence homology with spectrin and -actinin,
dystrophin was believed to exist as an antiparallel dimer that formed a
hexagonal lattice by crosslinking different actin filaments through oppositely
oriented amino-terminal actin binding domains. To test this hypothesis
experimentally, we quantitatively characterized the actin binding properties
of purified dystrophin-glycoprotein complex. Dystrophin-glycoprotein complex
bound to F-actin with a stoichiometry of 1 dystrophin per 24 actin monomers
and an average apparent Kd for dystrophin of 0.5 mM. However, in contrast
with the other members of the F-actin crosslinking superfamily of proteins,
dystrophin in the glycoprotein complex did not crosslink F-actin. We obtained
evidence for a novel actin binding site located near the middle of the
dystrophin rod domain and observed that dystrophin-glycoprotein complex
significantly protected F-actin from depolymerization. Data from gel permeation
chromatography indicated that the native molecular weight of dystrophin-glycoprotein
complex was only sufficient to contain one copy of each protein in the
complex, including dystrophin. Taken together, these results suggest that
monomeric dystrophin in the dystrophin-glycoprotein complex binds along
side an actin filament with 0.5 mM affinity via two or more lower affinity
binding sites distributed throughout its amino-terminal and rod domains.
Supported by NIH grants AR42423 and AR01985 and the Muscular Dystrophy
Association.