Dense Precipitate of Brain Tubulin with Skeletal Muscle Myosin1

Abstract

Purified tubulin from porcine brain formed a dense precipitate at 37°C with muscle myosin filaments from rabbit skeletal muscle; this effect was greater than that with partially purified tubulin. ATP or GTP, which prevented the myosin filaments from precipitating, inhibited the formation of the dense precipitate, but did not dissociate the dense precipitate once formed. The dense precipitate was found by thin-section electron microscopy to be composed of side-by-side aggregates of myosin filaments whose projections might be decorated by tubulin. The decoration was also seen by negative-stain electron microscopy. The binding of tubulin to myosin filaments decreased the Mg²⁺- and Ca²⁺-GTPase activity of the myosin by about half, but did not affect either Mg²⁺- or Ca²⁺-ATPase activity. The binding ratio of tubulin to myosin in the presence of 5 mM MgCl², was 2.2 mol/mol using purified tubulin and 1.8 mol/mol using partially purified tubulin. Five mM ATP and GTP in the presence of 5 mM MgCl² decreased the tubulin binding by 1.6–2.0 and 1.1–1.3 mol/mol, respectively, when added before an encounter of tubulin with myosin filaments, but did not cause any decrease when added after such an encounter.