The group studied the BK channel from mice. This channel is opened by changes in both voltage and intracellular Ca2+ concentration. Depolarization drives a positively charged section of the protein outwards and thus drags apart the S6 channel domains that form the channel's gate. Addition of Ca2+, by contrast, widens the diameter of an intracellular gating ring, once again pulling apart the S6 gates.
The two control mechanisms converge on the same target—positioning of the S6 gates—so one control mechanism can be used to probe the mechanism used by the other. The authors changed the length of the linkers that run from gating ring to gates, and tested how much voltage was needed to open the new channel variants.
Shortening the linkers made it easier to open the channel with voltage changes, probably because the diameter of the gating ring is fixed, so shortening the linker pulls the S6 gates outwards toward the gating ring. Lengthening the linker made it more difficult to open the channel with a voltage change. There was a linear relationship between the changes in linker length and required voltage, indicating that the linker-gating ring complex is acting as a perfect, Hookean spring.
The same linear relationship did not hold up when channels with different linker lengths were probed with varying Ca2+ levels. “With calcium the gating ring becomes an active machine and changes in shape,” says Magleby.
Magleby hopes to confirm the proposed movements with fluorescence proximity probes. But already, says Niu, “you don't have to look at the channels as blobs of protein. You can have mechanical models that really work.” ▪