Fig. 6. A ball and receptor model of UNC-7/UNC-9 heterotypic GJ gating.

(A) Homotypic UNC-7b GJs are conductive because of the absence of a receptor side for the N terminus ball. (B) A receptor site for the ball (represented by an indentation facing the channel pore) is induced in the UNC-7 hemichannel by UNC-9 hemichannel docking. Interactions between the ball and the receptor depend on putative salt bridges between HKK and DDD (represented by horizontal red lines) and V_j polarity. The ball binds to the receptor site to block the channel pore when UNC-7b cell is more depolarized than UNC-9 cell but detach from the receptor site when UNC-9 cell is more depolarized than UNC-7b cell. The K¹³ and K¹⁴ charged residues in UNC-7b’s unique N terminus sequence (represented by red crosses) impact GJ gating in a membrane voltage-dependent manner. Specifically, a more hyperpolarized membrane potential of UNC-7b cell enhances GJ conductance due to a stronger pulling effect on the N-terminal ball from a negative electric field produced by hyperpolarization of the UNC-7b cell. (C) The N termini of the other UNC-7 isoforms (except for UNC-7c) are too big to fit in the receptor site. (D) UNC-7c does not confer rectification because it lacks the necessary residues to form the HKK-DDD salt bridges. The solid blue and black spheres represent the N termini of UNC-7c and UNC-9, respectively. The plus and minus symbols indicate whether the membrane potential of one cell is more depolarized or hyperpolarized compared with that of the contacting cell.