ACTIN-BINDING PROTEINS FROM DICTYOSTELIUM DISCOIDEUM
HOCK, RICK S.
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Dictyostelium amoebae exhibit actin-dependent motile behavior such as phagocytosis, capping, pseudopod formation, and chemotaxis. Fractionation of Dictyostelium motile cell extracts has resulted in the identification of a number of actin-binding proteins that may be involved in these processes.;I have isolated two new actin-binding proteins, the 240-kDa protein and the 50-kDa protein, from motile extracts of Dictyostelium amoebae using column chromatographic methods.;The 240-kDa protein increases the low shear viscosity of F-actin at very low molar ratios of dimer to actin (1/500) and cosediments with F-actin. The 240-kDa protein forms networks with F-actin and labels the filaments in these networks. Covalent cross-linking and hydrodynamic studies in high salt demonstrate the presence of an asymmetric dimer (Stokes' radius = 8.6 nm, sedimentation coefficient = 12 S, native molecular weight = 434,000 daltons, and frictional ratio = 1.7). Rotary shadowing electron microscopy demonstrates that the monomer is a flexible rod ca. 70 nm in length that can associate end-to-end to form a dimer ca. 140 nm in length. Dictyostelium 240-kDa protein and chicken gizzard filamin are antigenically related.;The properties of 240-kDa protein suggest it is a member of the filamin class of actin-associated proteins and is involved in actin filament crosslinking.;The 50-kDa protein has a Stokes' radius of 3.1 nm and an apparent molecular weight of 52,000 daltons, indicating that it is monomeric. Copolymerization of actin and fractions containing 50-kDa protein yields actin bundles that can be pelleted at low speed (10,000g-5min). The molar ratio of 50-kDa protein to actin at saturation in the bundles was approximately 1:7. At the electron microscopic level of resolution, the bundles are not cross-striated and the filaments within the bundles are irregularly packed. The interfilament separation is 12.0 (+OR-) 2.7 nm.;The in vitro properties of these proteins indicate that they are partly responsible for the actin crosslinking activity in the motile cell extract. The role of these proteins in actin-dependent motility, their locations in situ, and their association with the cell membrane await further study.