Fig. 6.

The mechanical coupling and dynamic field focusing theories. S2 (green helix) and S4 (red helix) of a wild-type BK_Ca VSD undergo hypothetical motions during activation. Only segments from one subunit are shown for clarity, possessing voltage-sensing residues: D153 (S2, red), R167 (S2, blue) and R213 (S4, blue) (44). The membrane voltage drop is illustrated by equipotential lines (in black, except the portion traversed by R213 in S4, in red lines). (A) According to the mechanical coupling theory (Fig. 5 F–J, scheme III), segment activation causes the displacement, or nudging, of its neighbor, producing a fractional movement of its charge through the membrane field. (B) According to the dynamic field focusing theory, segment activation causes aqueous crevices to form, focusing the field. As a result, in the S2/S4 doubly activated state, these segments have traversed a large portion of the membrane potential, giving rise to q_app, fitted in scheme IV (Fig. 5 K–O).