Mutation of Tyr138 disrupts the structural coupling between the opposing domains in vertebrate calmodulin

We have used circular dichroism and frequency-domain fluorescence spectroscopy to determine how the site-specific substitution of Tyr(138) With either Phe(138) or Gln(138) affects the structural coupling between the opposing domains of calmodulin (CaM). A double mutant was constructed involving conservative substitution of Tyr(99) --> Trp(99) and Leu(69) --> CyS69 to assess the structural coupling between the opposing domains, as previously described [Sun, H., Yin, D., and Squier, T. C. (1999) Biochemistry 38, 12266-122791. Trp(99) acts as a fluorescence resonance energy transfer (FRET) donor in distance measurements to probe the conformation of the central helix. Cys(69) provides a reactive group for the covalent attachment of 5-((((2-iodoacetyl)amino)ethyl)amino)naphthalene-l-sulfonic acid (IAEDANS), which functions as a FRET acceptor and permits the measurement of the rotational dynamics of the aminoterminal domain. These CaM mutants demonstrate normal calcium-dependent gel-mobility shifts and changes in their near-UV CD spectra, have similar secondary structures to wild-type CaM following calcium activation, and retain the ability to fully activate the plasma membrane Ca-ATPase. The global folds, therefore, of both the carboxyl- and amino-terminal domains in these CaM mutants are similar to that of wild-type CaM. However, in comparison to wild-type CaM, the substitution of Tyr(138) with either Phe(138) or Gln(138) results in (i) alterations in the average spatial separation and increases in the conformational heterogeneity between the opposing globular domains and (ii) the independent rotational dynamics of the amino-terminal domain. These results indicate that alterations in either the hydrogen bond between Tyr(138) and Glu(82) or contact interactions between aromatic amino acid side chains have the potential to initiate the structural collapse of CaM normally associated with target protein binding and activation.