Inter-Domain Interaction and the Structural Flexibility of Calmodulin in the Connecting Region of the Terminal Two Domains

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Abstract
The calcium-dependent difference absorption spectrum of scallop calmodulin was measured in the presence ofmastoparan. The difference spectrum at 286 run (ΔA286) showed biphasic response to Ca2+concentration. The first change represents the conformational change around Tyr-138 and the second change may respond to an interaction between N-and C-domain of calmodulin which became apparent in the associated state with mastopar-an. Calmodulin-mastoparan complex was eluted from a gel filtration column after free calmodulin in the presence of Ca2+, which indicates a more compact structure of calmodulin-mastoparan complex than of free calmodulin. The biphasic response of ΔA286 was also observed with free calmodulin when the ionic strength was as low as 0.02 M NaCl. In the absence of NaCl, the Ca2+dependence of ΔA288 was monophasic, assuming identical affinity of Ca2+to both domains. Increase in the sensitivity of calmodulin to trypsin was observed with decrease in ionic strength. These results suggest an ionic-strength-dependent decrease in ordered structure of the connecting region. Calmodulin may change shape depending upon the ionic strength by bending at the connecting region. We assumed from the observations that calmodulin in solution may fluctuate between the two extreme shapes of the bent and the dumbbell structure. Target proteins may select and fix the specific bent structure for their activation.