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Two peripheral proteins of the human eryth- The human erythrocyte membrane provides a model system for investigating protein-membrane associations at the molecular level. Of particular interest is the existence of an extensive cytoskeletal network that may control cell shape and deformability (1-3) and the distribution of intramembrane particles (4) and surface markers (5). Interactions between spectrin, the major cytoskeletal protein, and the cytoplasmic surface of the erythrocyte membrane have been studied in detail and partially characterized (6-10). Bennett and Branton (6) have demonstrated the existence of a class of sites to which spectrin binds with high affinity, and recent evidence (8,11,12) indicates that band 2

Branching filaments with striking perpendicularity form when actin polymerizes in the presence of macrophage actin-binding protein . Actin-binding protein molecules are visible at the branch points . Compared with actin polymerized in the absence of actin-binding proteins, not only do the filaments branch but the average length of the actin filaments decreases from 3 .2 to 0.63 Ittm . Arrowhead complexes formed by addition of heavy meromyosin molecules to the branching actin filaments point toward the branch points . Actin-binding protein also accelerates the onset of actin polymerization . All of these findings show that actin filaments assemble from nucleating sites on actin-binding protein dimers . A branching polymerization of actin filaments from a preexisting lattice of actin filaments joined by actin-binding protein molecules could generate expansion of cortical cytoplasm in amoeboid cells.In 1939, Lewis (16) speculated, on the basis of observations with the light microscope, that transformations between gel and sol states in the peripheral cytoplasm of macrophages direct the movements ofthis cell. Recent studies with cytoplasmic proteins isolated from macrophages and other cells have provided evidence for such transformations and some explanations for this phenomenon at the molecular level. Actin filaments are abundant in the peripheral cytoplasm of macrophages (1, 23), and molecules of actin-binding protein, which cross-link actin filaments into an isotropic gel (4,7,27,28), are concentrated in the macrophage periphery (25). Therefore, cross-linking of actin filaments by actin-binding protein could be responsible for a gel state of macrophage peripheral cytoplasm. Gelsolin, a calcium-binding protein, effectively divides actin filaments in a calcium-dependent manner . This protein, by reversibly severing actin filaments between actin-binding protein cross-links, regulates the consistency of an actin lattice (26,33,34,35).The morphology of gels created by the addition of actinbinding protein to actin filaments when examined with the electron microscope after negative staining is virtually indistinguishable from that of solutions containing only actin filaments, even when actin-binding protein concentrations are 10 times higher than those required for the initial gelation ofactin. High concentrations of actin-binding protein (molar ratio of actin-binding protein to actin of 1 :20) result in the formation ofcross-linked actin filament bundles visible in electron micrographs . Although actin-binding protein molecules can be recognized, cross-linking actin filaments in these bundles formed with high concentrations of actin-binding protein (8, 28), a

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