Tubulin-myosin interaction. Some properties of binding between tubulin and myosin

This report presents evidence suggesting the direct binding between tubulin and myosin: (1) coprecipitation of tubulin with myosin occurred at a low ionic strength at which no precipitation of tubulin by itself occurred; (2) the amount of tubulin coprecipitated was unchanged when the coprecipitate was washed thoroughly; (3) about 2 mol of tubulin dimer could bind per mol of myosin at the maximum under our experimental conditions. The binding of about 1 mol of tubulin dimer was influenced by the presence of F-actin, but that of the other 1 mol of tubulin dimer was uninfluenced. In the former binding, tubulin or actin which bound first to myosin was suggested to have a priority. With regard to the priority of the binding, a similar result was obtained from the experiments of tubulin interference in actin activation of myosin Mg2+-ATPase. The tubulin-myosin binding occurred moderately even at 0 degrees C and was not affected by Ca2+ (2 mM), colchicine (200 microM), or Mg-ATP (4 mM), reflecting that the ability of tubulin to bind to myosin was different from the ability of tubulin to form microtubules and that the nature of tubulin-myosin binding was different from that of F-actin-myosin binding. Besides tubulin-myosin interaction, a possible interaction between microtubule-associated proteins (MAPs) and actomyosin was suggested from the data that MAPs activated actomyosin MG2+-ATPase activity while purified tubulin inhibited the activity.