Flexation of caldesmon: effect of conformation on the properties of caldesmon.

The contribution of the extended and bent forms of caldesmon to its function 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 conditions. The presence of nucleotides also produced significant conformational 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 fluorescence. The function of the bent form of caldesmon was investigated by using intramolecular 1-ethyl-3-(3-dimethylamino propyl) carbodiimide-crosslinked caldesmon. The inhibition of acto-S-1 ATPase activity by crosslinked caldesmon was less efficient compared with that by pyrene modified and control caldesmons. Caldesmon's ability to switch from an activator to an inhibitor of actin-activated ATPase of myosin was also affected by the folding. Cosedimentation experiments revealed normal binding of crosslinked caldesmon to smooth muscle myosin. These results indicate the importance of caldesmon's transition from extended to folded forms and suggest possible functional roles for these different forms of caldesmon.