Interaction of chicken gizzard smooth muscle calponin with brain microtubules

Calponin, a major actin-, tropomyosin-, and calmodulin-binding protein in smooth muscle, interacted with tubulin, a main constituent of microtubules, in a concentration-dependent fashion in vitro. The apparent K(d) value of calponin to tubulin was calculated to be 5.2 microM with 2 mol of calponin maximally bound per 1 mol of tubulin. At low ionic strength, tubulin bound to calponin immobilized on Sepharose 4B, and the bound protein was released at about 270 mM NaCl. Chemical cross-linking experiments showed that a 1:1 molar covalent complex of calponin and tubulin was produced. The amount of calponin bound to microtubules decreased with increasing ionic strength or Ca2+ concentration. The addition of calmodulin or S100 to the mixture of calponin and microtubule proteins caused the removal of calponin from microtubules in the presence of Ca2+, but not in the presence of EGTA. Calponin-related proteins including tropomyosin, SM22, and caldesmon had little effect on the calponin binding to microtubules, whereas MAP2 inhibited the binding. Interestingly, there was little, if any, effect of mycalolide B-treated actin on the binding of calponin to microtubules. Furthermore, only about 20% of calponin-F-actin interaction was inhibited in the presence of an excess amount of tubulin (4 mol per mol of calponin), indicating that tubulin binds to calponin at a different site from that of actin. Compared with MAP2, calponin had little effect on microtubule polymerization.