Calmodulin Fragments Can Not Activate Target Enzymes

Abstract

Under conditions where nM level of calmodulin was able to show full activation of myosin light chain kinase and cyclic-nucleotide phosphodiesterase, the fragments of calmodulin at concentrations as high as 20 μM failed to activate these enzymes in the presence of Ca²⁺. The fragments tested were Ala1-Lys75 (F 12), Ala1-Arg74 (F 12'), Lys 75-Lys148 (F 34''), Met76-Lys148 (F 34'), Asp78-Lys148 (F 34), Alal-Arg106 (F 123), and His107-Lys148 (F 4). Purification of the proteolytic fragments through HPLC was necessary to remove contaminant calmodulin. Among the fragments, that corresponding to the C-terminal half domain inhibited myosin light chain kinase activity with the inhibition constant of 13 μM. The integrated structure of calmodulin consisting of N-terminal half domain, C-terminal half domain, and the linker peptide was indispensable for the enzyme activation. We discuss the functions of the two structural domains (N-domain and C-domain) in the activation of various enzymes.