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Volume 271, Number 48, Issue of November 29, 1996 pp. 30336-30339
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Strong Interaction between Caldesmon and Calponin

(Received for publication, June 24, 1996, and in revised form, August 26, 1996)

From the Boston Biomedical Research Institute, Boston, Massachusetts 02114

Caldesmon was labeled at either Cys-153 in the NH₂-terminal domain or Cys-580 in the COOH-terminal domain with a 6-acryloyl-2-dimethylaminonaphthalene (acrylodan) fluorescence probe. The addition of smooth muscle calponin to Cys-580-labeled caldesmon resulted in an 18% drop in fluorescence intensity, which titrated with a stoichiometry of 0.9 and a binding constant of 9.5 × 10⁷ M¹. For Cys-153-labeled caldesmon, there was no change in fluorescence upon adding calponin. These findings indicate strong binding between calponin and the COOH-domain of caldesmon. The association was sensitive to ionic strength, suggesting that ionic interactions between calponin, a basic protein, and caldesmon, an acidic protein, contribute to the stabilization of the protein complex. That non-muscle acidic calponin interacts with caldesmon with a much reduced association constant of 3.5 × 10⁶ M¹ supports such a model. The binding between acidic calponin and caldesmon is strengthened to 1.8 × 10⁷ M¹ in the presence of Ca²⁺, which might bind to acidic residues of the calponin and partially neutralize its negative charge. The strong, specific binding between calponin and caldesmon suggests that this interaction occurs within smooth muscle cells and possibly plays a role in the regulation of contraction.

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