Spatial map of the extracellular face of the BK VSD obtained from the geometric fit to SE traces. The position of each donor is represented by colored filled circles. The 95% confidence isosurfaces are shown as transparent volumes. For the sake of clarity, the donors from only one subunit are shown. The reference point is the acceptor (black circle) near the fourfold symmetry axis. The crystal structure of the pore section from the Kv1.2/2.1 paddle chimera (transparent gray) and the modeled IbTX-bodipy (purple) are presented in the background as references. The tentative area where the bulk of the BK VSD may be located is encircled by a black line. The space-filled boundary of the Kv1.2/2.1 paddle chimera is shown as a fainter gray line. (A and B) Top and side views, respectively, of the spatial map for BK constructs in the absence of the β1-subunit. (C and D) Top and side views, respectively, of the spatial map for BK constructs in the presence of the β1-subunit. Donors from the β1-LBT constructs TM1 and TM2 are shown as cyan and orange circles, respectively. For completeness, both solution types are shown for S0 + β1, TM1, and TM2 constructs. However, the less likely solutions are rendered transparent to reinforce the fact that they appear nonparsimonious.