Tryptophan residues in caldesmon are major determinants for calmodulin binding.
Calmodulin has been shown to interact with the COOH-terminal domain of gizzard h-caldesmon at three sites, A (residues 658-666), B (residues 687-695), and B' (residues 717-725), each of which contains a Trp residue [Zhan et al. (1991) J. Biol. Chem. 266, 21810-21814; Marston et al. (1994) J. Biol. Chem. 296, 8134-8139; Mezgueldi et al. (1994) J. Biol. Chem. 269, 12824-12832]. To determine the contribution of each of the three Trp residues in the calmodulin-caldesmon interaction, we have mutated the Trp residues to Ala in the COOH-terminal domain of fibroblast caldesmon (CaD39) and studied the effects on calmodulin binding by fluorescence measurements and using immobilized calmodulin. Wild-type CaD39 binds with a Kd of 0.13 x 10(-6) M and a stoichiometry of 1 mol of calmodulin per mol of caldesmon. Replacing Trp 659 at site A or Trp 692 at site B to Ala reduces binding by 22- and 31-fold (Kd = 2.9 x 10(-6) and 4.0 x 10(-6) M), respectively, and destabilizes the CaD39-calmodulin complex by 1.75 and 1.94 kcal mol-1, respectively. Mutation of both Trp 659 and Trp 692 to Ala further reduces binding with a Kd of 6.1 x 10(-6) M and destabilizes the complex by 2.17 kcal mol-1. On the other hand, mutation of Trp 722 at site B' to Ala causes a much smaller decrease in affinity (Kd = 0.6 x 10(-6) M) and results in a destabilization energy of 0.87 kcal mol-1. To investigate the relative importance of the amino acid residues near each Trp residue in the caldesmon-calmodulin interaction, deletion mutants were constructed lacking site A, site B, and site A + B. Although deletion of site A decreases binding of CaD39 to calmodulin by 13-fold (Kd = 1.7 x 10(-6) M), it results in tighter binding than mutation of Trp 659 to Ala at this site, suggesting that the residues neighboring Trp 659 may contribute negatively to the interaction. Deletion of site B causes a similar reduction in binding (Kd = 4.1 x 10(-6) M) as observed for replacing Trp 692 to Ala at this site, indicating that Trp 692 is the major, if not the only, binding determinant at site B. Deletion of both site A and site B drastically reduces binding by 62-fold. Taken together, these results suggest that Trp 659 and Trp 692 are the major determinants in the caldesmon-calmodulin interaction and that Trp 722 in site B' plays a minor role.
Graether
S P
SP
Department of Biochemistry, Queen's University, Kingston, Ontario, Canada.
Heinonen
T Y
TY
Raharjo
W H
WH
Jin
J P
JP
Mak
A S
AS
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002147:Calmodulin / Q000378:metabolism*; D002148:Calmodulin-Binding Proteins / Q000737:chemistry* / Q000378:metabolism*; D002645:Chickens; D017931:DNA Primers; D005347:Fibroblasts / Q000378:metabolism; D005895:Gizzard, Avian; D006801:Humans; D007700:Kinetics; D009130:Muscle, Smooth / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D017354:Point Mutation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D017384:Sequence Deletion; D013050:Spectrometry, Fluorescence; D014364:Tryptophan
D002147:Calmodulin; D002148:Calmodulin-Binding Proteins; D017931:DNA Primers; D011994:Recombinant Proteins; D014364:Tryptophan