Voltage-sensing and Substate Rectification

The article by Bukauskas et al. (2002) in this issue of The Journal addresses several key issues in the structure– function of connexin channels, and suggests a structural link between voltage-sensitive gating and the charge in the pore. Previous work on connexin32 (Cx32) noted a correlation between the sign of voltage-sensing charges and the rectification of the subconduc-tance state induced by movement of those charges. The present work investigates in detail the permeability properties of the subconductance state induced by voltage in channels composed of connexin43 (Cx43). The results impart generality to an intriguing structural relationship between voltage-dependent gating and modulation of pore charge selectivity. They also show how the single-channel gating physiology may relate to the molecular signaling function of connexin channels. The importance of electrical and molecular signaling through connexin channels is widely recognized. An increasingly detailed functional and structural picture of connexin channels has been emerging recently, but the relation between this picture and the signaling properties of connexin channels has remained elusive. There are ‫ف‬ 20 connexin isoforms, each producing channels with distinct unitary conductances and gating sensitivities. The maximal unitary conductances range from 15 to over 300 pS, and the fully open states of channels formed by the different isoforms have widely diverse ionic and molecular selectivities (Elfgang et al., 1995; Veenstra et al., 1995; for review see Harris, 2001). Con-nexin channels typically exhibit complex voltage-dependent gating with substantial occupancy of states with less than maximal conductance (substates). The gating of connexin channels can be very sensitive to voltage, as much as e-fold for 2–3 mV, yet the sequences contain no S4-like domain. Therefore, the voltage sensitivity likely arises from a molecular mechanism distinct from that of most voltage-sensitive channels. Each hemichannel contains at least two distinct voltage-sensitive gating mechanisms. One, called " Vj gating " (or " fast Vj gating " in Bukauskas et al., 2002), governs rapid transitions between conducting states, typically between the fully open state and the smallest substate, or " residual " state. The conductance of this substate can be from 5 to 40% of the fully open state, depending on the connexin. A junctional channel can be in a substate due to Vj gating of either hemichannel. For Cx32 and connexin26 (Cx26), the Vj-gating voltage sensor includes charges in the first several positions of the cytoplasmic NH 2-terminal domain and possibly two positions in the NH 2-terminal part of …