The Molecular Basis for Membrane – Cytoskeleton Association in Human Erythrocytes

Spectrin, the major cytoskeletal protein in erythrocytes, is localized on the inner membrane surface in association with membrane‐spanning glycoproteins and with intramembrane particles. The presence of a specific, high‐affinity protein binding site for spectrin on the cytoplasmic surface of the membrane has been established by measurement of reassociation of spectrin with spectrin‐depleted inside‐out vesicles. A 72,000 M r proteolytic fragment of this attachment protein has been purified, which bound to spectrin in solution and competed for reassociation of spectrin with vesicles. A 215,000 M r polypeptide has been identified as the precursor of the spectrin‐binding fragment. The membrane attachment protein for spectrin was named ankyrin, and has been purified and characterized. Ankyrin has been demonstrated to be tightly associated in detergent extracts of vesicles with band 3, a major membrane‐spanning polypeptide, and to bind directly to a proteolytic fragment derived from the cytoplasmic domain of band 3. Ankyrin is thus an example of a protein that directly links a cytoplasmic structural protein to an integral membrane protein. The organization of the erythrocyte membrane has implications for more complex cell types since immunoreactive forms of ankyrin distinct from myosin or filamin have been detected by radioimmunoassay in a variety of cells and tissues. Indirect immunofluorescent staining of cultured cells reveals immunoreactive forms of ankyrin in a cytoplasmic meshwork and in a punctate distribution over nuclei. The staining changes dramatically during mitosis, with concentration of stain at the spindle poles in metaphase and intense staining of the cleavage furrow during cytokinesis.