Functional characterization of human erythrocyte spectrin alpha and beta chains: association with actin and erythrocyte protein 4.1.

Human erythrocyte spectrin alpha and beta chains were purified by preparative sodium dodecyl sulfate gel electrophoresis and also by DEAE-cellulose chromatography in the presence of urea. The purified chains behaved as individual monomers on sucrose gradients and did not form homodimers. Recombination of the chains led to the formation of alpha-beta heterodimers with sedimentation characteristics identical with native alpha-beta dimers. The binding of 125I-labeled band 4.1 to alpha and beta chains was measured by sucrose gradient rate zonal sedimentation and by quantitative immunoassay. It was found that both alpha and beta chains associated with 125I-labeled band 4.1 in a nearly identical manner over the range of band 4.1 concentration studied. The association was abolished by heat denaturation of the spectrin chains or by denaturation of band 4.1 with a 40-fold molar excess of N-ethylmaleimide. As expected, purified beta chains but not alpha chains bound to 125I-labeled ankyrin as measured by a quantitative radioimmunoassay. The binding of purified alpha chains, beta chains, and recombinant alpha-beta heterodimers to F-actin was measured in the presence of band 4.1. We found that alpha or beta chains separately exhibited no band 4.1 dependent association with F-actin but that alpha-beta heterodimers formed by recombination of the chains did. We conclude that spectrin binding to F-actin in the presence of band 4.1 requires the participation of both of spectrin's polypeptide chains.