Topology of human Cx26 and Cx43 indicating crucial domains as well as peptides that affect protein and channel functions. Topology of human Cx26 and Cx43 indicating crucial domains as well as peptides that affect protein and channel functions. (Left) Illustrates the extracellular loops (EL1, EL2) of Cx26 (intracellular protein parts not shown), indicating the position of highly conserved Cys residues (three per loop, yellow-filled circles) that act to stabilize the loops. Crucial residues involved in interactions between the loops upon docking of two opposed hemichannels are also indicated; for EL1, Asn-54 (green-filled circle) forms a hydrogen bond with Leu-56 of EL1 from the opposite connexin (orange), whereas Gln-57 (green) forms a hydrogen bond with Gln-57 from the opposite connexin. For EL2, salt bridges are formed between Lys-168, Asp-179, and Thr-177 from one side (green) with Asn-176 from the opposite connexin (orange). (Right) Illustrates the Cx43 topology indicating the location of conserved EL1/EL2 Cys residues (yellow). Several important domains are illustrated including the VCYD and FPISH motifs on EL1, SRPTEK on EL2 and the L2, Gap19, and CaM (calmodulin interaction site) sequences on the cytoplasmic loop (CL). Domains on the C-terminal tail (CT) include the tubulin-binding JM domain (juxtamembrane, crucial for microtubule binding), the drebrin-binding domain (Drb; crucial domain indicated; links Cx43 to F-actin), the Nedd4 domain (ubiquitin ligase), SH3 domain, and the ZO-1─binding domain (links Cx43 to F-actin). Peptides mimicking some of these domains are illustrated in the color of the corresponding domain. RRNY peptide (RRNYRRNY) is not a mimetic peptide; like L2 and Gap19, it interacts with the Cx43 CT and prevents GJ closure while inhibiting HC opening. JM2 peptide is a mouse version that differs in residue 243 from the human. The two yellow-marked CT sequences (315–326 and 340–348) are α-helical domains, whereas the rest of the CT is intrinsically disordered. The light green-marked domains are important Tyr-based sorting sequences involved in Cx43/GJ internalization (Y265AYF, Y286KLV). Blue-filled circles with white amino acid letter codes indicate mutations characterized by increased HC function relative to the function of GJs. Black-filled circles with white letter codes indicate ODDD mutations with reduced GJ and HC function. Red-filled circles with black Tyr or Ser indicate major phosphorylation sites, including CK1-targeted Ser-325,328,330, MAPK-targeted Ser-255,262,279,282, PKC-targeted Ser-368, Akt/PKB-targeted Ser-369,373, and Src/Tyk2-targeted Tyr-247,265,313. Ser-364,365 is phosphorylated by PKA but in an indirect manner involving other kinases. A detailed account on the role of the various amino acids and domains illustrated here can be found in sections II and III. Luc Leybaert et al. Pharmacol Rev 2017;69:396-478 Copyright © 2017 by The American Society for Pharmacology and Experimental Therapeutics