Band 4.9 (a 48,000-mol-wt polypeptide) has been partially purified from human erythrocyte membranes. In solution, band 4.9 polypeptides exist as trimers with an apparent molecular weight of 145,000 and a Stokes radius of 50 A. Electron microscopy shows that the protein is a three-lobed structure with a radius slightly greater than 50 A. When gel-filtered rabbit muscle actin is polymerized in the presence of band 4.9, actin bundles are generated that are similar in appearance to those induced by "vinculin" or fimbrin. The bundles appear brittle and when they are centrifuged small pieces of filaments break off and remain in the supernatant. At low band 4.9 to actin molar ratios (1:30), band 4.9 lowers the apparent steady-state low-shear falling ball viscosity by sequestering filaments into thin bundles; at higher ratios, the bundles become thicker and obstruct the ball's movement leading to an apparent increase in steady-state viscosity. Band 4.9 increases the length of the lag phase and decreases the rate of elongation during actin polymerization as measured by high-shear Ostwald viscometry or by the increase in the fluorescence of pyrene-labeled actin. Band 4.9 does not alter the critical actin monomer concentration. We hypothesize that band 4.9, together with actin, erythrocyte tropomyosin, and spectrin, forms structures in erythroid precursor cells analogous to those formed by fimbrin, actin, tropomyosin, and TW 260/240 in epithelial brush borders. During erythroid development and enucleation, the actin filaments may depolymerize up to the membrane, leaving a membrane skeleton with short stubs of actin bundled by band 4.9 and cross-linked by spectrin.

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