Structure of a LRRC8 chimera with physiologically relevant properties reveals heptameric assembly and pore-blocking lipids

Volume-regulated anion channels (VRACs) mediate Cl- and organic solute efflux from vertebrate cells and are essential for cell volume homeostasis. VRACs are heteromeric assemblies of LRRC8A-E proteins with unknown stoichiometries. Homomeric LRRC8A and LRRC8D channels have a hexameric structure. However, these channels are either non-functional or exhibit abnormal functional properties limiting their utility for structure-function analyses. We circumvented these limitations by developing novel homomeric LRRC8 chimeric channels with physiologically relevant functional properties. We demonstrate here that the LRRC8C-LRRC8A(IL125) chimera comprising LRRC8C and 25 amino acids unique to the first intracellular loop (IL1) of LRRC8A has a heptameric structure like that of homologous pannexin channels. Membrane lipids are a key structural element of the channel and are located between subunits and occluding the channel pore. Our results suggest that native VRAC/LRRC8 channels are heptamers and that associated lipids are likely essential for normal channel gating and regulation.