INTERACTION OF PROTEOLYTIC FRAGMENTS OF CALMODULIN WITH CALDESMON ANDCALPONIN

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 a pparent K-d values equal to 0.23 and 1.3 mu M respectively. The intera ction was abolished in the absence of Ca2+. Two large tryptic fragment s of calmodulin obtained in the presence of Ca2+ (TR1C, residues 1-77, and TR2C, residues 78-148) interact with caldesmon with apparent K-d values of 11.9 and 4.6 mu M. Affinity of TR2C to calponin (K-d 3.8 mu M) comparable with that of native calmodulin and was much higher than the corresponding value for TR1C (K-d 41 mu M). The short C-terminal t ryptic peptide of calmodulin obtained in the presence of EGTA (TR3E, r esidues 107-148) interacts with caldesmon and calponin with K-d values of 23.9 and 12.1 mu M, whereas the large N-terminal peptide TR1E (res idues 1-106) interacts with both caldesmon and calponin with a very lo w affinity (K-d 60 mu M). Thus although both N- and C-terminal domains of calmodulin are involved in the interaction with caldesmon and calp onin, the C-terminal part of calmodulin (residues 78-148) is of specia l importance and has the highest contribution for caldesmon and calpon in binding.