Interaction of proteolytic fragments of calmodulin with caldesmon and calponin.

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Interaction of five tryptic fragments of calmodulin with caldesmon and calponin was analysed by native gel electrophoresis. In the presence of Ca2+ intact calmodulin interacts with caldesmon and calponin with apparent Kd values equal to 0.23 and 1.3 microM respectively. The interaction was abolished in the absence of Ca2+. Two large tryptic fragments of calmodulin obtained in the presence of Ca2+ (TR1C, residues 1-77, and TR2C, residues 78-148) interact with caldesmon with apparent Kd values of 11.9 and 4.6 microM. Affinity of TR2C to calponin (Kd 3.8 microM) was comparable with that of native calmodulin and was much higher than the corresponding value for TR1C (Kd 41 microM). The short C-terminal tryptic peptide of calmodulin obtained in the presence of EGTA (TR3E, residues 107-148) interacts with caldesmon and calponin with Kd values of 23.9 and 12.1 microM, whereas the large N-terminal peptide TR1E (residues 1-106) interacts with both caldesmon and calponin with a very low affinity (Kd 60 microM). Thus although both N- and C-terminal domains of calmodulin are involved in the interaction with caldesmon and calponin, the C-terminal part of calmodulin (residues 78-148) is of special importance and has the highest contribution for caldesmon and calponin binding.