Expression of chimeric connexins reveals new properties of the formation and gating behavior of gap junction channels.

Direct intercellular communication occurs through specialized channels, which are formed by the interaction of two half-channels, or connexons, contributed by each of the two participating cells. The ability to establish intercellular communication is specified, in part, by the expression of different structural proteins, termed connexins. Connexins can control the establishment of intercellular communication by selectively pairing with some but not other family members. To characterize the protein domains that allow connexins to recognize and discriminate between alternative partners, we have created chimeras composed of selected regions of rat connexin43, which forms channels with Xenopus connexin38, and rat connexin32, which cannot. Pairs of Xenopus oocytes were used to test the ability of the chimeras to form homotypic channels with themselves, and heterotypic channels with the parent connexins or with endogenous Xenopus connexin38. While all hybrid molecules tested were efficiently expressed by oocytes, most were devoid of functional activity. A chimera consisting of connexin32 from the N terminus to the second transmembrane domain, fused to connexin43 from the middle cytoplasmic loop to the C terminus, designated as 3243H4, was able to pair functionally with Xenopus connexin38 and one of its parent connexins, connexin43. Voltage-dependent closure of heterotypic channels containing 3243H4 was asymmetric, exhibited novel characteristics that were not predicted by the behavior of the parent connexins and was dependent on the type of connexin with which 3243H4 was paired. In contrast, 3243H4 was unable to form functional channels with either itself or the other parent, connexin32. Together, these results suggest that these connexins are not composed of functionally exchangeable regions and that multiple domains, namely the middle cytoplasmic portion and the second extracellular domain, can influence the interactions between connexins present in adjacent cells. Furthermore, they indicate that voltage gating is not strictly intrinsic behavior for a given connexin, but can be modulated by the partner connexins to which they are paired. Finally, the finding that 3243H4 is functional only in heterotypic configurations, and cannot form homotypic channels, suggests the existence of a novel form of selectivity: self-discrimination. The latter property may represent another mechanism that operates to control the extent of communication between cells.