FLEXATION OF CALDESMON - EFFECT OF CONFORMATION ON THE PROPERTIES OF CALDESMON

The contribution of the extended and bent forms of caldesmon to its fu nction was investigated by examining chemically modified forms of this protein. The bent 'hairpin' form of caldesmon was enhanced between pH 6.0 and 8.0 and at low ionic strengths, as reported by an increase in excimer fluorescence of pyrene-labelled caldesmon under these conditi ons. The presence of nucleotides also produced significant conformatio nal changes in caldesmon, as detected by fluorescence measurements and protease digestions. Titrations of pyrene caldesmon with actin, heavy meromyosin, and calmodulin resulted in a decrease in excimer fluoresc ence. The function of the bent form of caldesmon was investigated by u sing intramolecular 1-ethyl-3-(3-dimethylamino propyl) carbodiimide-cr osslinked caldesmon. The inhibition of acto-S-1 ATPase activity by cro sslinked caldesmon was less efficient compared with that by pyrene mod ified and control caldesmons. Caldesmon's ability to switch from an ac tivator to an inhibitor of actin-activated ATPase of myosin was also a ffected by the folding. Cosedimentation experiments revealed normal bi nding of crosslinked caldesmon to smooth muscle myosin. These results indicate the importance of caldesmon's transition from extended to fol ded forms and suggest possible functional roles for these different fo rms of caldesmon.