Voltage-gating mechanisms of connexin channels

Channels formed by the connexin family of proteins display multiple forms of voltage dependence that have different sensitivities and time courses. Intercellular (junctional) channels are sensitive to two orientations of applied voltage: inside-out or transmembrane voltage (V_i-o or V_m), and transjunctional voltage (V_j). At least two voltage-gating mechanisms operate in both intercellular channels and in unapposed hemichannels. The V_j or fast-gating mechanism is sensitive to V_j. The loop or slow-gating mechanism is also sensitive to V_j and may also be sensitive to V_i-o/V_m and to channel closure by chemical agents. Both types of voltage-gating are intrinsic to hemichannels; in an intercellular channel, each apposed hemichannel contains separate gating structures arranged in series. The molecular determinants and the mechanism of polarity determination of V_j/fast-gating have been studied extensively for homomeric Cx26 and Cx32 hemichannels. These studies have shown that difference in gating polarity of Cx26 and Cx32 hemichannels results from a difference in the charge of the second amino acid residue. The voltage polarity to which Cx32 hemichannels close can be reversed by negative charge substitutions up to the tenth but not the eleventh residue. This has led to a structural model in which the first ten amino acid residues of the amino-terminal domain form the entry of the channel pore by virtue of a turn initiated by the flexibility of a glycine residue at the twelveth position. This model is supported by nuclear magnetic resonance (NMR)-derived structures of a peptide of the amino-terminal domain of Cx26 and permeation studies demonstrating that charged residues in the amino-terminal domain are determinants of unitary conductance and contribute to the rectification of current through the open channel. Initially, it was proposed that the inward movement of the charges in the amino-terminal domain initiates V_j/fast-gating. This view was complicated by the discovery of homomeric channels that display bipolar V_j/fast-gating, leading to the possibility that the voltage sensor functions as a center-open toggle switch. The subsequent conformational changes that result in channel closure to a substate may involve the actions of a proline kink in the second transmembrane domain, an interaction between the cytoplasmic loop and carboxyl-terminal domain, or could result from the movement of the amino-terminal domain as a gating particle. Less is known about the molecular determinants and mechanisms of loop/slow-gating. The process is mechanistically distinct from V_j/fast-gating, in at least its initiation. The two processes may share structural elements, although it is unlikely that the conformational changes with gating will be identical.