CO ₂ directly modulates connexin 26 by formation of carbamate bridges between subunits

Abstract

Homeostatic regulation of the partial pressure of CO ₂ (PCO ₂) is vital for life. Sensing of pH has been proposed as a sufficient proxy for determination of PCO ₂ and direct CO ₂-sensing largely discounted. Here we show that connexin 26 (Cx26) hemichannels, causally linked to respiratory chemosensitivity, are directly modulated by CO ₂. A ‘carbamylation motif’, present in CO ₂-sensitive connexins (Cx26, Cx30, Cx32) but absent from a CO ₂-insensitive connexin (Cx31), comprises Lys125 and four further amino acids that orient Lys125 towards Arg104 of the adjacent subunit of the connexin hexamer. Introducing the carbamylation motif into Cx31 created a mutant hemichannel (mCx31) that was opened by increases in PCO ₂. Mutation of the carbamylation motif in Cx26 and mCx31 destroyed CO ₂ sensitivity. Course-grained computational modelling of Cx26 demonstrated that the proposed carbamate bridge between Lys125 and Arg104 biases the hemichannel to the open state. Carbamylation of Cx26 introduces a new transduction principle for physiological sensing of CO ₂.

DOI: http://dx.doi.org/10.7554/eLife.01213.001

eLife digest

A number of gaseous molecules, including nitric oxide and carbon monoxide, play important roles in many cellular processes by acting as signalling molecules. Surprisingly, however, it has long been assumed that carbon dioxide – a gaseous molecule that is produced during cellular metabolism – is not a signalling molecule.

Controlling the concentration of carbon dioxide (CO ₂) in a biological system is essential to sustain life, and it was thought that the body used pH – which is the concentration of hydrogen ions – as a proxy for the level of CO ₂. The concentration of CO ₂ is related to pH because CO ₂ reacts with water to form carbonic acid, which quickly breaks down to form hydrogen ions and bicarbonate ions. This close relationship has led many researchers to equate pH-sensing with CO ₂-sensing, and to suggest that a physiological receptor for CO ₂ does not exist.

Recent research into structures called connexin hemichannels has challenged this view. Researchers found that when pH levels were held constant, increasing the level of CO ₂ caused the structures to open up, suggesting that CO ₂ could be directly detected by the hemichannels. Each hemichannel contains six connexin subunits, but the details of how the CO ₂ molecules interact with the individual connexin subunits to open up the hemichannels remained mysterious.

Now Meigh et al. show that CO ₂ molecules bind to a specific amino acid (lysine) at a particular place (residue 125) in one of the connexin subunits to form a carbamate group. This group then interacts with the amino acid (arginine) at residue 104 in a neighbouring connexin subunit to form a carbamate bridge between the two subunits. This leads to structural changes that cause the gap junction hemichannels to open and release signals that can activate other cells. Since connexin hemichannels are found throughout the human body, these results suggest that CO ₂ might act as a signalling molecule in processes as diverse as the control of blood flow, breathing, hearing and reproduction.

DOI: http://dx.doi.org/10.7554/eLife.01213.002