Homology modeling identifies C-terminal residues that contribute to the Ca2+ sensitivity of a BKCa channel

Activation of BKCa channels by direct Ca2+ binding and membrane depolarization occur via independent and additive molecular processes. The calcium bowl domain is critically involved in Ca2+ -dependent gating, and we have hypothesized that a sequence within this domain may resemble an EF hand motif. Using a homology modeling strategy, it was observed that a single Ca2+ ion may be coordinated by the oxygen-containing side chains of residues within the calcium bowl (i.e., (912)ELVNDTNVQFLD(923)). To examine these predictions directly, alanine-substituted BKCa channel mutants were expressed in HEK 293 cells and the voltage and Ca2+ dependence of macroscopic currents were examined in inside-out membrane patches. Over the range of 1 - 10 mu M free Ca2+, single point mutations (i.e., E912A and D923A) produced rightward shifts in the steady-state conductance-voltage relations, whereas the mutants N918A or Q920A had no effect on Ca2+-dependent gating. The double mutant E912A/D923A displayed a synergistic shift in Ca2+ -sensitive gating, as well as altered kinetics of current activation/deactivation. In the presence of 1, 10, and 80 mM cytosolic Mg2+, this double mutation significantly reduced the Ca2+ -induced free energy change associated with channel activation. Finally, mutations that altered sensitivity of the holo-channel to Ca2+ also reduced direct Ca-45 binding to the calcium bowl domain expressed as a bacterial fusion protein. These findings, along with other recent data, are considered in the context of the calcium bowl's high affinity Ca2+ sensor and the known properties of EF hands.