pH-dependent intramolecular binding and structure involving Cx43 cytoplasmic domains.

pH-induced closure of connexin43 (Cx43) channels involves interaction of the Cx43 carboxyl-terminal (Cx43CT) with a separate "receptor" domain. The receptor location and structure and whether the interaction is directly intramolecular are unknown. Here we show resonant mirror technology, enzyme-linked sorbent assays, and nuclear magnetic resonance (NMR) experiments demonstrating pH-dependent binding of Cx43CT to region 119-144 of Cx43 (Cx43L2), which we propose is the receptor. NMR showed that acidification induced alpha-helical order in Cx43L2, whereas only a minor modification in Cx43CT structure was detected. These data provide the first demonstration of chemically induced structural order and binding between cytoplasmic connexin domains.