Apocalmodulin finds to the myosin light chain kinase calmodulin target site

The interaction of a 20-residue-long peptide derived from the calmodulin-bi nding domain of the smooth muscle myosin light chain kinase with calcium-fr ee calmodulin (apocalmodulin) was studied using a combination of isothermal titration calorimetry and differential scanning calorimetry. We showed tha t: (i) a significant binding between apocalmodulin and the target peptide ( RS20) exists in the absence of salt (K-a, = 10(6) M-1), (ii) the peptide in teracts with the C-terminal lobe of calmodulin and adopts a partly helical conformation, and (iii) the presence of salt weakens the affinity of the pe ptide for apocalmodulin, emphasizing the importance of electrostatic intera ctions in the complex. Eased on these results and taking into account the w ork of Bayley et al. (Bayley, P. M,, Findlay, W,A,, and Martin, S. R, (1996 ) Protein Sci. 5, 1215-1228), we suggest a physiological role for apocalmod ulin.