Interactions of calmodulin with metal ions and with its target proteins revealed by conformation-sensitive monoclonal antibodies

Two monoclonal antibodies (mAbs) raised against bovine calmodulin (CaM), CAM1 and CAM4, enable one to monitor conformational changes that occur in the molecule. The interaction of CAM1 with CaM depends on the Ca2+ occupancy of its Ca(2+)-binding sites. CAM4, in contrast, interacts with CaM in a Ca(2+)-independent manner, interacting with both holoCaM and EGTA-treated CaM to a similar extent. Their interaction with various CaMs, CaM tryptic fragments and chemically modified CaM, as well as molecular graphics, led to identification of the CAM1 and CAM4 epitopes on the C- and N-terminal lobes of CAM respectively. The two mAbs were used as macromolecular probes to detect conformational changes occurring in the CaM molecule upon binding of metal ions and target proteins and peptides. MAb CAM1 successfully detected changes associated with Al3+ binding even in the presence of Ca2+, indicating that Al3+ and Ca2+ ions may bind to the protein simultaneously, leading to a new conformation of the molecule. MAbs CAM1 and CAM4 were used to follow the interactions of CaM with its target peptides and proteins. Complexes with melittin, mastoparan, calcineurin and phosphodiesterase showed different immunological properties on an immuno-enzyme electrode, indicating unique structural properties for each complex.