Gap junction hemichannel interactions with zwitterionic lipid, anionic lipid, and cholesterol: molecular simulation studies.

Interactions with membrane lipids can exert dramatic functional consequences on gap junction proteins. Recent experimental work has highlighted the importance of anionic lipids and cholesterol in facilitating channel activity. In this work, we have employed a coarse-grained molecular model in conjunction with molecular dynamics (MD) simulations to study the interactions between a connexin 26 (Cx26) hemichannel and a number of lipid species, including palmitoyloleoylphosphatidylcholine (POPC), anionic palmitoyloleoylphosphatidic acid (POPA), and cholesterol, in order to identify sites at the protein interface which may exhibit preferential, specific binding to these lipids, as well as determine the characteristics of these interactions. We have also employed an atomistic model of Cx26 embedded in a mixed PA/PC bilayer as a comparison and to elucidate further lipid-protein interactions. Our simulation results suggest enrichment of interfacial PA at the intracellular leaflet at high bulk PA concentrations. PC can form tight binding interactions with the hemichannel, particularly at intersubunit crevices (classical nonannular sites). In mixed bilayers, however, POPA competes with POPC for these sites, displacing the latter in some cases. While the residues responsible for interactions with PC and PA are similar, the latter exhibits a unique property of being capable of forming stable hydrophilic contacts with multiple residues spanning two different adjacent subunits at both leaflets of the bilayer, as opposed to POPC which can only do so at the extracellular side. These results suggest that POPA may be essential to channel function by acting as an intersubunit lipid bridge. Additionally, we propose that the compositional enrichment of POPA at the Cx26 interface may serve important roles in voltage gating. Simulation of a mixed POPC:cholesterol bilayer suggests that the hemichannel enhances the transbilayer mobility of vicinal cholesterols, increasing the likelihood of site-hopping and interleaflet flip-flop transitions.