Structural determinants of CO₂-sensitivity in the β connexin family suggested by evolutionary analysis

A subclade of connexins comprising Cx26, Cx30, and Cx32 are directly sensitive to CO₂. CO₂ binds to a carbamylation motif present in these connexins and causes their hemichannels to open. Cx26 may contribute to CO₂-dependent regulation of breathing in mammals. Here, we show that the carbamylation motif occurs in a wide range of non-mammalian vertebrates and was likely present in the ancestor of all gnathostomes. While the carbamylation motif is essential for connexin CO₂-sensitivity, it is not sufficient. In Cx26 of amphibia and lungfish, an extended C-terminal tail prevents CO₂-evoked hemichannel opening despite the presence of the motif. Although Cx32 has a long C-terminal tail, Cx32 hemichannels open to CO₂ because the tail is conformationally restricted by the presence of proline residues. The loss of the C-terminal tail of Cx26 in amniotes was an evolutionary innovation that created a connexin hemichannel with CO₂-sensing properties suitable for the regulation of breathing.