The self-association of human spectrin at high concentration

The self-association of purified human spectrin has been studied at sedimentation equilibrium over a wide range of concentration (0–20 g/L) at 30°C and pH 7.5. Coincidence of apparent weight average molecular weight and Ω( r) plots as a function of total spectrin concentration indicated that equilibrium was attained and that no significant concentration of solute was incapable of participating in the self-association reaction. Under these conditions, no significant dissociation of the heterodimer to component polypeptide chains could be detected. The behavior of spectrin between 0 and 20 g/L can be described reasonably well by a cooperative isodesmic model, in which the protomer for association is the αβ heterodimer. With this model, the equilibrium constant for the heterodimer-tetramer step, K 24, is 2 × 10 6 M −1, and K iso, the equilibrium constant describing all other steps, is approximately 0.2 × 10 6 M −1. The returned value of the second virial coefficient for this model, 1.0 × 10 −7 L mol g −2, is consistent with the lower limit of values calculated for the heterodimer from the charge and Stokes radius of spectrin. On the other hand, the attenuated indefinite association model fails to describe the self-association of spectrin adequately over the range 0–20 g/L. Systematic decreases in the estimates of the second virial coefficient and the equilibrium constants for association beyond the tetramer suggest that the assumption of a single value of the second virial coefficient may not be appropriate for spectrin, and that non-ideality would best be taken into account by consideration of the detailed solution composition.