Characterization of the pH-dependent Interaction between the Gap Junction Protein Connexin43 Carboxyl Terminus and Cytoplasmic Loop Domains*

A prevailing view regarding the regulation of connexin43 (Cx43) gap junction channels is that, upon intracellular acidification, the carboxyl-terminal domain (Cx43CT) moves toward the channel opening to interact with specific residues acting as a receptor site. Previous studies have demonstrated a direct, pH-dependent interaction between the Cx43CT and a Cx43 cytoplasmic loop (Cx43CL) peptide. This interaction was dependent on α-helical formation for the peptide in response to acidification; more recent studies have shown that acidification also induces Cx43CT dimerization. Whether Cx43CT dimerization is an important structural component in Cx43 regulation remains to be determined. Here we used an assortment of complimentary biophysical techniques to characterize the binding of Cx43CT or its mutants to itself and/or to a more native-like Cx43CL construct (Cx43CL100-155, residues 100-155). Our studies expand the observation that specific Cx43CT domains are important for dimerization. We further show that properties of the Cx43CL100-155 are different from those of the Cx43CL peptide; solvent acidification leads to Cx43CL100-155 oligomerization and a change in the stoichiometry and binding affinity for the Cx43CT. Homo-Cx43CT and Cx43CL100-155 oligomerization as well as the Cx43CT/Cx43CL100-155 interaction can occur under in vivo conditions; moreover, we show that Cx43CL100-155 strongly affects resonance peaks corresponding to Cx43CT residues Arg-376—Asp-379 and Asn-343—Lys-346. Overall, our data indicate that many of the sites involved in Cx43CT dimerization are also involved in the Cx43CT/Cx43CL interaction; we further propose that chemically induced Cx43CT and Cx43CL oligomerization is important for the interaction between these cytoplasmic domains, which leads to chemically induced gating of Cx43 channels.