title: Tropomyosin reverses cross-linking of F-actin with microtubule-associated protein-2.
abstract: Brain microtubule-associated protein 2 (MAP2) is known to cross-link muscle F-actin in vitro into a gel or discrete bundles of actin filaments. Previous reports indicate that this cross-linking reverses in the presence of millimolar ATP, while MAP2 molecules remain attached along single filaments of F-actin. Therefore, it is likely that the actin filament has two sets of surface areas with ATP-sensitive and insensitive affinities for MAP2. Using purified preparations of brain MAP2 and skeletal muscle F-actin and tropomyosin, we have studied the effects of tropomyosin on the MAP2-actin interaction by dark-field light microscopy, electron microscopy, sedimentation assay, and low shear viscometry. The results show that cross-linking of F-actin with MAP2 reverses upon addition of a stoichiometric amount of tropomyosin, although MAP2 remains bound to F-actin complex with tropomyosin. The ternary complex does not dissociate noticeably when exposed to a millimolar concentration of ATP. On the basis of these findings, it is concluded that ATP-insensitive MAP2-binding of F-actin is not sterically blocked by tropomyosin, while the ATP-sensitive binding is blocked by it.
mesh_terms: D000199:Actins / Q000032:analysis*; D000255:Adenosine Triphosphate; D000818:Animals; D001923:Brain Chemistry; D004355:Drug Stability; D004591:Electrophoresis, Polyacrylamide Gel; D008854:Microscopy, Electron; D008869:Microtubule-Associated Proteins / Q000032:analysis*; D009132:Muscles / Q000032:analysis; D011817:Rabbits; D014335:Tropomyosin / Q000032:analysis*; D014783:Viscosity
chemical_list: D000199:Actins; D008869:Microtubule-Associated Proteins; D014335:Tropomyosin; D000255:Adenosine Triphosphate