Structural Studies on the Recognition of Calmodulin with Its Target Enzymes
Kao-Chao Liu1, Sue-Wei Wu1, Yi-Chen Chen2, Ta-Hsien Lin3, Chia-Lin Chyan1
1Department of Chemistry, National Dong Hwa University, Hualien, Taiwan 974, ROC
2Department of Medical Technology, Tzu-Chi University, Hualien, Taiwan 974, ROC
3Department of Biochemistry, National Yang-Ming University, Taipei, Taiwan 112, ROC
Calmodulin (CaM) is an ubiquitous protein in all eukaryotes. It transmits the Ca2+ signal and activates a large number of key enzymes, controlling a wide spectrum of important biological functions. The structural and mechanistic natures underlying the recognition of CaM with its target enzymes are important for the diverse roles of CaM. The recognition of CaM with the target sequence of a nucleotide gated ion channel (OFCp) was investigated by circular dichroism (CD), fluorescence, and NMR. Our fluorescence and far UV CD data showed that OFCp tightly bound to CaM with stoichiometric ration 1:1 and the dissociation constant in the nM range. However, the NMR data showed that most of the crosspeaks of CaM in the 15N-1H HSQC spectra not only shifted but also split into two sets of peaks with nearly equal intensities when excess OFCp was added. Our data strongly suggest that there are two binding modes of CaM and OFCp and that both CaM/OFCp complexes are slowly inter-exchangeable.