A 27,000-D core of the Dictyostelium 34,000-D protein retains Ca(2+)-regulated actin cross-linking but lacks bundling activity.
Actin cross-linking proteins are important for formation of isotropic F-actin networks and anisotropic bundles of filaments in the cytoplasm of eucaryotic cells. A 34,000-D protein from the cellular slime mold Dictyostelium discoideum mediates formation of actin bundles in vitro, and is specifically incorporated into filopodia. The actin cross-linking activity of this protein is inhibited by the presence of micromolar calcium. A 27,000-D fragment obtained by digestion with alpha-chymotrypsin lacks the amino-terminal six amino acids and the carboxyl-terminal 7,000 D of the intact polypeptide. The 27,000-D fragment retains F-actin binding activity assessed by cosedimentation assays and by 125I-[F-actin] blot overlay technique, F-actin cross-linking activity as assessed by viscometry, and calcium binding activity. Ultrastructural analyses indicate that the 27,000-D fragment is deficient in the bundling activity characteristic of the intact 34,000-D protein. Actin filaments are aggregated into microdomains but not bundle in the presence of the 27,000-D fragment. A polarized light scattering assay was used to demonstrate that the 34,000-D protein increases the orientational correlation among F-actin filaments. The 27,000-D fragment does not increase the orientation of the actin filaments as assessed by this technique. A terminal segment(s) of the 34,000-D protein, lacking in the 27,000-D fragment, contributes significantly to the ability to cross-link actin filaments into bundles.
Fechheimer
M
M
Department of Zoology, University of Georgia, Athens 30602.
Furukawa
R
R
D008841:Actin Cytoskeleton / Q000648:ultrastructure; D000199:Actins / Q000737:chemistry*; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002413:Cations, Divalent; D004023:Dictyostelium / Q000737:chemistry*; D005656:Fungal Proteins / Q000737:chemistry / Q000378:metabolism*; D005782:Gels; D008027:Light; D046911:Macromolecular Substances; D008840:Microfilament Proteins / Q000737:chemistry* / Q000378:metabolism; D008970:Molecular Weight; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D012542:Scattering, Radiation; D013329:Structure-Activity Relationship; D014783:Viscosity
D000199:Actins; D002413:Cations, Divalent; D005656:Fungal Proteins; D005782:Gels; D046911:Macromolecular Substances; D008840:Microfilament Proteins; D010446:Peptide Fragments; D002118:Calcium