Opening of the human epithelial calcium channel TRPV6

The cryo-electron microscopy structure of the calcium channel TRPV6 in its open and closed states demonstrates a novel gating mechanism involving an alanine hinge. Calcium is an important signalling molecule in biology and its regulation has a key role in human physiology. The transient receptor potential vanilloid subfamily member 6 (TRPV6) channel is constitutively active and highly Ca-selective, mediating Ca(II) uptake in epithelial tissues, and its expression is related to several cancers. However, the structural basis for its constitutive activity and ion permeation is unknown. Here, Alexander Sobolevsky and colleagues report structures of human TRPV6 by cryo-electron microscopy in the open and closed states. These data reveal a novel gating mechanism for tetrameric ion channels, identifying an alanine hinge that is probably important in the role of TRPV6 in physiology and disease. Calcium-selective transient receptor potential vanilloid subfamily member 6 (TRPV6) channels play a critical role in calcium uptake in epithelial tissues1,2,3,4. Altered TRPV6 expression is associated with a variety of human diseases5, including cancers6. TRPV6 channels are constitutively active1,7,8 and their open probability depends on the lipidic composition of the membrane in which they reside; it increases substantially in the presence of phosphatidylinositol 4,5-bisphosphate7,9. Crystal structures of detergent-solubilized rat TRPV6 in the closed state have previously been solved10,11. Corroborating electrophysiological results3, these structures demonstrated that the Ca2+ selectivity of TRPV6 arises from a ring of aspartate side chains in the selectivity filter that binds Ca2+ tightly. However, how TRPV6 channels open and close their pores for ion permeation has remained unclear. Here we present cryo-electron microscopy structures of human TRPV6 in the open and closed states. The channel selectivity filter adopts similar conformations in both states, consistent with its explicit role in ion permeation. The iris-like channel opening is accompanied by an α-to-π-helical transition in the pore-lining transmembrane helix S6 at an alanine hinge just below the selectivity filter. As a result of this transition, the S6 helices bend and rotate, exposing different residues to the ion channel pore in the open and closed states. This gating mechanism, which defines the constitutive activity of TRPV6, is, to our knowledge, unique among tetrameric ion channels and provides structural insights for understanding their diverse roles in physiology and disease.