Chimeric evidence for a role of the connexin cytoplasmic loop in gap junction channel gating

Gap junction channels are regulated by gates that close upon exposure to 100% CO2, probably via an increase in intracellular Ca2+ concentration, [Ca2+]i. For defining connexin (Cx) domain(s) involved in gating, we have studied chemical and voltage gating sensitivities of channels made of Cx38, Cx32 or chimeras of the above, expressed in Xenopus oocytes. Cx38 channels are more sensitive to CO2 and voltage than those of Cx32. A 3-min exposure to 100% CO2 reduces Cx38 junctional conductance (Gj) to 0% of initial values at a maximum rate of 25%/min, whereas even a 15-min exposure to 100% CO2 reduces Cx32 Gj by approximately 50% at the slow rate of 9%/min. Of the various Cx32 mutants and Cx32/38 chimeras constructed, two chimeras (Cx32/38I and Cx32/38N) expressed functional channels. Upon exposure to CO2, channels made of Cx32/38I (Cx32 inner loop replaced with that of Cx38) reproduced precisely the uncoupling behavior of Cx38 channels in uncoupling magnitude and in both uncoupling and recoupling rates, whereas channels made of Cx32/38N (N-terminus replaced) behaved closer to Cx32 than to Cx38 channels. Cx38 channels were more voltage sensitive than those of Cx32, with V0, i.e., the transjunctional voltage at which voltage-sensitive conductance is half maximal = 35.3 and 59.5 mV, and n, i.e., equivalent gating charge = 3.3 and 2.1, respectively. Of the two chimeras, Cx32/38I channels were similar to Cx38 channels, with V0 = 40.6 mV, Gj min, i.e., the theoretical minimal normalized junctional conductance = 0.35 and n = 3.0, whereas Cx32/38 N channels displayed very low voltage sensitivity, with V0 = 84.8 mV, Gj min = 0.5 and n = 1.1. The data suggest that the inner loop plays a major role in pH and voltage gating sensitivity, but whether other domains also participate in the gating mechanism cannot be excluded.