Figure 4. Two calmodulin molecules bind to a single AQP0 tetramer.

A, ¹H dimension of ¹⁵N-calmodulin HSQC titration series using full-length AQP0 tetramers in the presence of Ca²⁺. The molar stoichiometric ratio of CaM:AQP0 is indicated. B, Plot of signal intensity for several resonances from ¹⁵N-calmodulin during the titration. Standard deviations are indicated. The NMR signal for ¹⁵N-calmodulin is almost completely attenuated at a 1:2 molar ratio, indicating that two calmodulin molecules bind to a single AQP0 tetramer. C, As in panel A, but in the presence of EGTA. No AQP0/CaM interaction is detected in the absence of Ca²⁺.