DIFFUSION HINDRANCE AND GEOMETRY OF FILAMENT CROSSINGS ACCOUNT FOR THE COMPLEX INTERACTIONS OF F-ACTIN WITH ALPHA-ACTININ FROM CHICKEN GIZZARD

The interaction of alpha-actinin from chicken gizzard with F-actin is quite complex. The apparent dissociation constant, C, increases with t he increase of actin concentration according to the following expressi on: C = K(o) + a[actin] - c[actin]5/2 . At pH 7.5 and 37-degrees-C, in the presence of 0.1 M KCl and 2 mM MgCl2, the dissociation constant a t infinite actin dilution, K(o), is 2.17 muM. The binding of alpha-act inin to actin is related by the term a[actin] to the diffusion of acti n filaments and by the term c[actin]5/2 to the crossing number concent ration of the F-actin network. Especially at low actin concentration, the binding of alpha-actinin to actin is increased by gelsolin, which fragments actin filaments and increases their diffusion. The different binding isotherms of alpha-actinin to actin filaments and to actin bu ndles are discussed.