Interaction of protein-bound polysaccharide (PS-K) with microtubule proteins. V. Influence on tubulin-dependent adenosine triphosphatase (ATPase) activity.

Abstract

In the presence of 5 mM Ca2+ or 2mM Mg2+, a protein-bound polysaccharide (PS-K) isolated from Basidiomycetes inhibited the tubulin-dependent adenosine triphosphatase (ATPase) activity of microtubule-associated proteins (MAPs) in a concentration-dependent manner, though it had little effect on the activity in the absence of tubulin. The extent of inhibition decreased at lower concentrations of Ca2+ or Mg2+ and an excess amount of tubulin largely restored the inhibitory effect. When Mn2+ was added to the reaction mixture instead of these divalent cations, the presence of PS-K conversely stimulated the tubulin-dependent ATPase activity of MAPs at lower Mn2+ concentrations (<1-2 mM) under the conditions tested, though PS-K inhibited the ATPase activity at high Mn2+ concentrations. Maximal stimulation and inhibition were about 160% and 70% of the original level (i.e., in the absence of PS-K), respectively. PS-K could also enhance MAPs ATPase activity under conditions where it enhanced the activity in the presence of tubulin, indicating that PS-K itself interacts with MAPs ATPase as tubulin does. The addition of a large amount of ATP under experimental conditions where PS-K inhibited tubulin-dependent Mn2+-ATPase activity of MAPs, on the other hand, led to stimulation by PS-K. Thus, the influence of PS-K on the ATPase activity was dependent on the molar ratio of metal ions to adenosine triphosphate (ATP).