THE C-TERMINAL HALF-MOLECULAR DOMAIN OF CALMODULIN IS RESPONSIBLE FORHIGH-AFFINITY INTERACTION WITH TARGET ENZYMES

Chimeric proteins from chicken and yeast calmodulin were prepared, and roles of three structural domains, N-domain (1-72), central helix (73 -87) and C-domain (88-148), were evaluated. Mutants with the chicken-t ype C-domain activated the cyclic nucleotide phosphodiesterase with sm all values of K-act (the concentration of calmodulin giving a half-max imal activation), a property of chicken calmodulin. On the other hand mutants with the yeast-type C-domain activated the phosphodiesterase w ith mu M range of K-act, a property of yeast calmodulin. In activation of myosin light chain kinase, introduction of the yeast-type C-domain into chicken calmodulin increased the K-act value by more than 1000-f old with a dramatic decrease in the maximum activity (V-max). On the o ther hand introduction of the chicken-type C-domain led to a profile w ith lower K-act and higher V-max. Minor effects on V-max and K-act wer e observed by substitution of the central helix. Although various smal l contributions of the N-domain were observed, a common role of the ch icken-type C-domain was suggested to catch and maintain the high-affin ity interaction with target enzymes.