Intramolecular loop/tail interactions are essential for connexin 43-hemichannel activity.

Connexin-assembled gap junctions (GJs) and hemichannels coordinate intercellular signaling processes. Although the regulation of connexins in GJs has been well characterized, the molecular determinants controlling connexin-hemichannel activity are unresolved. Here we investigated the regulation of Cx43-hemichannel activity by actomyosin contractility and intracellular [Ca(2+)] (Ca(2+)) using plasma membrane-permeable TAT peptides (100 μM) designed to interfere with interactions between the cytoplasmic loop (CL) and carboxy-terminal (CT) in primary bovine corneal endothelial cells and HeLa, C6 glioma, and Xenopus oocytes ectopically expressing Cx43. Peptides corresponding to the last 10 CT aa (TAT-Cx43CT) prevented the inhibition of Cx43-hemichannel activity by contractility/high Ca(2+), whereas a reverse peptide (TAT-Cx43CTrev) did not. These effects were independent of zonula occludens-1, a cytoskeletal-associated Cx43-binding protein. In contrast, peptides corresponding to CL (TAT-L2) inhibited Cx43-hemichannel responses, whereas a mutant peptide (TAT-L2(H126K/I130N)) did not inhibit. In these assays, TAT-Cx43CT acted as a scaffold for TAT-L2 and vice versa, a finding supported by surface plasmon resonance measurements. Loop/tail interactions appeared essential for Cx43-hemichannel activity, because TAT-Cx43CT restored the activity of nonfunctional hemichannels, consisting of either Cx43 lacking the C-terminal tail (Cx43(M239)) or intact Cx43 ectopically expressed in Xenopus oocytes. We conclude that intramolecular loop/tail interactions control Cx43-hemichannel activity, laying the basis for developing hemichannel-specific blockers.