Structural basis for anion conduction in the calcium-activated chloride channel TMEM16A

Abstract

The calcium-activated chloride channel TMEM16A is a member of a conserved protein family that comprises ion channels and lipid scramblases. Although the structure of the scramblase nhTMEM16 has defined the architecture of the family, it was unknown how a channel has adapted to cope with its distinct functional properties. Here we have addressed this question by the structure determination of mouse TMEM16A by cryo-electron microscopy and a complementary functional characterization. The protein shows a similar organization to nhTMEM16, except for changes at the site of catalysis. There, the conformation of transmembrane helices constituting a membrane-spanning furrow that provides a path for lipids in scramblases has changed to form an enclosed aqueous pore that is largely shielded from the membrane. Our study thus reveals the structural basis of anion conduction in a TMEM16 channel and it defines the foundation for the diverse functional behavior in the TMEM16 family.DOI: http://dx.doi.org/10.7554/eLife.26232.001

Cryo-EM structures of the TMEM16A calcium-activated chloride channel

“…This pore is not exposed to the lipid bilayer as proposed 21–24 . Similar to a previous study 12 , our cryo-EM studies revealed a pore too narrow for the passage of permeant anions. It remains possible that some open conformation of the pore is partly lined by lipids.…”

“…2g, 3g). The nanodisc-reconstituted TMEM16A is structurally similar to the subnanometer-resolution structure of digitonin-solubilized TMEM16A 12 but missing part of the C-terminal cytoplasmic domain owing to the truncation construct used in our study (Extended Data Fig. 5g–i).…”

“…TMEM16A forms a dimer with two pores 8,9 . Thus far, CaCC structural analyses relied on homology modeling of a fungal homolog nhTMEM16 that functions primarily as a lipid scramblase 10–12 and subnanometer-resolution electron cryo-microscopy (cryo-EM) 12 . Here, we present de novo atomic structures of the transmembrane domains of mouse TMEM16A in nanodiscs and in lauryl maltose neopentyl glycol (LMNG) as determined by single-particle cryo-EM.…”

Lipids and Ions Traverse the Membrane by the Same Physical Pathway in the nhTMEM16 Scramblase

“…We initially expected that replacing TMEM16F S514 with V from TMEM16A would abrogate scrambling, but the S514V mutation had little effect on scrambling. Recently a low-resolution structure of TMEM16A was published (Paulino et al, 2017) that provides additional insight into these results (see Discussion).…”

“…It is likely that ion conduction by the Cl -channel TMEM16A occurs via a similar pathway to the one that conducts lipids in the TMEM16 scramblases Jeng et al, 2016;Lim et al, 2016). Recently, a low-resolution cryo-EM structure of the TMEM16A Cl -channel was solved (Paulino et al, 2017) which showed that the TMEM16A ion-conducting pore, instead of being an open aqueduct as in nhTMEM16, is enclosed by protein along~2/3 of its length due to a structural rearrangement that brings TM4 and TM6 closer together to enclose the pore. Otherwise the structures of TMEM16A and nhTMEM16 are very similar.…”