Structural unit of the erythrocyte cytoskeleton. Isolation and electron microscopic examination.

We isolated a protein complex containing major cytoskeletal components from the Triton shell of bovine erythrocytes. This protein complex, which we called the 26-S complex, consisted of three major components, spectrin, band-4.1 protein and actin, and one minor component, band-4.9 protein. The molar ratio of spectrin heterodimer:band 4.1:actin was determined by sodium dodecyl sulfate (SDS) gel electrophoresis to be about 1:2:2, approximately the same as that for the Triton shell. By electron microscopic examinations of rotary-shadowed specimens, it was revealed that the 26-S complex had a "spider-like" morphology with a central core and several spectrin heterodimers radiating from it. The number of spectrin arms in the complex was not constant but was in the range between 3 and 6. The complexes with five spectrin heterodimers were the most numerous. The results showed that the 26-S complex contained on the average five spectrin heterodimers, ten band-4.1 polypeptides and ten actin monomers. As judged from the formation of oligomeric 26-S complexes through spectrin arms, the central core of the complex presumably contains band 4.1 and actin. Supporting this conclusion, the central core acted as a nucleus for actin polymerization when the 26-S complex was mixed with G-actin under an actin-polymerizing condition. The 26-S complex could form large aggregates under a certain condition that spectrin was promoted to associate from dimer to tetramer. We conclude that the 26-S complex is the structural unit of the erythrocyte cytoskeleton.