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J. Biol. Chem., Vol. 266, Issue 27, 17872-17879, 09, 1991

Myosin subfragment-1 interacts with two G-actin molecules in the absence of ATP

C Valentin-Ranc, C Combeau, MF Carlier and D Pantaloni
Laboratoire d'Enzymologie, Centre National de la Recherche Scientifique, Gif-sur-Yvette, France.

The interaction between G-actin and myosin subfragment-1 (S1) has been monitored by pyrenyl-actin fluorescence and light scattering. In low ionic strength buffer and in the absence of ATP the polymerization of G- actin induced by myosin subfragment-1 is preceded by the formation of binary GS and ternary G2S complexes in which S1 interacts tightly in rapid equilibrium (K greater than 10(7) M-1) with one and two G-actin molecules, respectively. Pyrenyl fluorescence of G-actin is enhanced 4- fold in GS and 3-fold in G2S. At concentrations of G-actin and S1 in the micromolar range and above, G2S is the predominant species at G- actin/S1 ratios equal to or greater than 1. The isomer of myosin subfragment-1 carrying the A1 light chain, S1(A1), forms a tighter ternary complex than the isomer S1(A2). Actin-bound ATP is not hydrolyzed upon formation of GS and G2S. In the presence of one molar equivalent or more of myosin subfragment-1/mol of G-actin, in low ionic strength buffer containing no nucleotides, G-actin polymerizes faster in the presence of S1(A1) than in the presence of S1(A2). The interaction of S1 with G-actin is inhibited by the binding of ATP or ADP to S1, ATP having a higher affinity for S1 than ADP. The possible structural similarity of the G2S complex to the F-acto-S1 complex in the rigor state and the potential significance of a ternary (actin)2- myosin interaction for actomyosin-based motility are discussed.
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