Calcium-independent bacterial activator of cyclic nucleotide phosphodiesterase and Ca2+/Mg2+-ATPase.

Ca2+-independent protein-modulator (BacM) was found in the culture medium of Staphylococcus aureus. BacM activated calmodulin-dependent cyclic nucleotide phosphodiesterase and Ca2+/Mg2+-ATPase in the same way as calmodulin. BacM was shown to be a proteolytic fragment of the exotoxin secreted by the S. aureus strain under study. The kinetic analyses of the ATPase activation by BacM and CaM were performed. These studies demonstrated that the enzyme molecule contains at least two activator-sensitive sites. Experiments on the ATPase activation by Ca2+ both in the presence and in the absence of BacM and CaM documented that CaM-ATPase and BacM-ATPase complexes can exist at low concentrations of calcium. Analysis of activation curves of ATPase by Ca2+ revealed three Ca2+-binding sites in the enzyme-activator complex.