Recent advances in structural characterization of volume-regulated anion channels (VRACs)

Volume-regulated anion channels (VRACs) encoded by the LRRC8 gene family play essential roles in diverse and fundamentally important physiological processes in vertebrate cells. The recent determination of high-resolution cryo-electron microscopy (cryo-EM) structures of homomeric and heteromeric LRRC8 channel complexes has created unprecedented opportunities for understanding the molecular basis of VRAC structure, function and pharmacology. Native LRRC8 channels are obligatory heteromers composed of at least one LRRC8A subunit together with one of the other paralogues (LRRC8B-E) with an unknown stoichiometry. This heteromeric nature of endogenously expressed VRACs and the difficulties associated with controlling the composition and stoichiometry of heterologously expressed LRRC8 channels present considerable experimental challenges. The development of LRRC8 chimeras, which exhibit normal functional and regulatory properties and that can be expressed as homomeric channels, circumvents many of these challenges. In this review, we discuss the recent advances in the structural characterization of LRRC8 channels, with a primary focus on the cryo-EM structures of one such chimera, created by swapping 25 residues from LRRC8A subunits to LRRC8C subunits and termed as 8C-8A(IL1²⁵).