Tropomodulin: a cytoskeletal protein that binds to the end of erythrocyte tropomyosin and inhibits tropomyosin binding to actin.
Human erythrocytes contain a Mr 43,000 tropomyosin-binding protein that is unrelated to actin and that has been proposed to play a role in modulating the association of tropomyosin with spectrin-actin complexes based on its stoichiometry in the membrane skeleton of one Mr 43,000 monomer per short actin filament (Fowler, V. M. 1987. J. Biol. Chem. 262:12792-12800). Here, we describe an improved procedure to purify milligram quantities to 98% homogeneity and we show that this protein inhibits tropomyosin binding to actin by a novel mechanism. We have named this protein tropomodulin. Unlike other proteins that inhibit tropomyosin-actin interactions, tropomodulin itself does not bind to F-actin. EM of rotary-shadowed tropomodulin-tropomyosin complexes reveal that tropomodulin (14.5 +/- 2.4 nm [SD] in diameter) binds to one of the ends of the rod-like tropomyosin molecules (33 nm long). In agreement with this observation, Dixon plots of inhibition curves demonstrate that tropomodulin is a non-competitive inhibitor of tropomyosin binding to F-actin (Ki = 0.7 microM). Hill plots of the binding of the tropomodulin-tropomyosin complex to actin indicate that binding does not exhibit any positive cooperativity (n = 0.9), in contrast to tropomyosin (n = 1.9), and that the apparent affinity of the complex for actin is reduced 20-fold with respect to that of tropomyosin. These results suggest that binding of tropomodulin to tropomyosin may block the ability of tropomyosin to self-associate in a head-to-tail fashion along the actin filament, thereby weakening its binding to actin. Antibodies to tropomodulin cross-react strongly with striated muscle troponin I (but not with troponin T) as well as with a nontroponin Mr 43,000 polypeptide in muscle and in other nonerythroid cells and tissues, including brain, lens, neutrophils, and endothelial cells. Thus, erythrocyte tropomodulin may be one member of a family of tropomyosin-binding proteins that function to regulate tropomyosin-actin interactions in non-muscle cells and tissues.
Fowler
V M
VM
Department of Molecular Biology, Research Institute of Scripps Clinic, La Jolla, California 92041.
D000199:Actins / Q000302:isolation & purification / Q000378:metabolism*; D000596:Amino Acids / Q000032:analysis; D000818:Animals; D003598:Cytoskeletal Proteins / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D004910:Erythrocyte Membrane / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008565:Membrane Proteins / Q000378:metabolism*; D008854:Microscopy, Electron; D009132:Muscles / Q000378:metabolism; D011485:Protein Binding; D011817:Rabbits; D014335:Tropomyosin / Q000302:isolation & purification / Q000378:metabolism* / Q000648:ultrastructure
D000199:Actins; D000596:Amino Acids; D003598:Cytoskeletal Proteins; D008565:Membrane Proteins; D014335:Tropomyosin