Mapping of contact sites in the caldesmon-calmodulin complex

The interaction of intact calmodulin and its four tryptic peptides with deletion mutants of caldesmon was analysed by native gel electrophoresis, fluorescence spectroscopy and zero-length cross-linking. Deletion mutants H2 (containing calmodulin-binding sites A and B) and H9 (containing sites B and B') interacted with intact calmodulin to form complexes whose stoichiometries varied from 2:1 to 1:1. The N-terminal peptides of calmodulin (TR1C, residues 1-77, and TR2E, residues 1-90) bound H2 with higher affinity than H9. At the same time H2 was less effective than H9 in binding to the C-terminal peptides of calmodulin TR2C (residues 78-148) and TR3E (residues 107-148). The N-terminal peptides of calmodulin (TR1C and TR2E) could be cross-linked to intact caldesmon and its deletion mutants H2 and H9. The similarity in the primary structures of sites A and B' of caldesmon and our measurements of the affinities of H2 and H9 to calmodulin and its peptides strongly indicate an orientation of the protein complex where sites A and B' interact with the N-terminal domain of calmodulin, whereas site B interacts with the C-terminal domain of calmodulin. The spatial organization of contact sites in the caldesmon-calmodulin complex agrees with the earlier proposed two-dimensional model of interaction of the two proteins [Huber, El-Mezgueldi, Grabarek, Slatter, Levine and Marston (1996) Biochem. J. 316, 413-420].