Structural and functional properties of the transporter SLC26A6 reveal mechanism of coupled anion exchange

Abstract

Members of the SLC26 family constitute a conserved class of anion transport proteins, which encompasses uncoupled transporters with channel-like properties, coupled exchangers and motor proteins. Among the 10 functional paralogs in humans, several participate in the secretion of bicarbonate in exchange with chloride and thus play an important role in maintaining pH homeostasis. Previously, we have elucidated the structure of murine SLC26A9 and defined its function as an uncoupled chloride transporter (Walter et al., 2019). Here we have determined the structure of the closely related human transporter SLC26A6 and characterized it as a coupled exchanger of chloride with bicarbonate and presumably also oxalate. The structure defines an inward-facing conformation of the protein that generally resembles known structures of SLC26A9. The altered anion selectivity between both paralogs is a consequence of a remodeled ion binding site located in the center of a mobile unit of the membrane-inserted domain, which also accounts for differences in the coupling mechanism.

The molecular principles underlying diverse functions of the SLC26 family of proteins

“…This observation is in line with a recent report that R404V conferred a small, uncoupled chloride transport function on SLC26A6 (Tippett et al, 2023).…”

“…Slightly hyperpolarized Erev with statistical significance was found in cells expressing mA5-R399V, suggesting that the elimination of the basic residue from the anion binding site conferred small uncoupled chloride transport function on SLC26A5. This observation is in line with a recent report that R404V conferred a small, uncoupled chloride transport function on SLC26A6 (Tippett et al, 2023).…”

“…1 , left), SLC26A5 (Bavi et al, 2021; Butan et al, 2022; Futamata et al, 2022; Ge et al, 2021) ( Fig. 1 , middle), SLC26A6 (Tippett et al, 2023), and SLC26A9 (Chi et al, 2020; Walter et al, 2019) ( Fig. 1 , right).…”

“…The importance of this highly conserved basic residue in SLC26A4 (Arg 409 ) is reflected by the presence of multiple disease-associated missense variants, i.e., R409C (c.1225C>T) (Chen et al, 2007), R409H (c.1226G>A) (Van Hauwe et al, 1998), R409L (c.1226G>T) (Chai et al, 2013), and R409P (c.1226G>C) (Park et al, 2003) and has been experimentally demonstrated (Dossena et al, 2009;Gillam et al, 2005;Wasano et al, 2020;Yuan et al, 2012;Zhang et al, 2022). In SLC26A6, Arg 404 is experimentally demonstrated to be important for its anion antiport function (Tippett et al, 2023). Interestingly, SLC26A9, which mediates channel-like fast uncoupled electrogenic chloride transport (Chang et al, 2009b;Dorwart et al, 2007;Loriol et al, 2008), has a nonpolar residue, Val 393 , at the equivalent position (Fig.…”

“…Increasing number of genetic variants are associated with various human diseases such as nephrocalcinosis (SLC26A1), hyperoxalemia (SLC26A1), diastrophic dysplasia (SLC26A2), achondrogenesis (SLC26A2), atelosteogenesis (SLC26A2), multiple epiphyseal dysplasia (SLC26A2), congenital chloride diarrhea (SLC26A3), deafness (SLC26A4 and SLC26A5), bicarbonate metabolism-related diseases (SLC26A6), hypothyroidism (SLC26A7), asthenozoospermia (SLC26A8), bronchiectasis (SLC26A9), and dysregulation of chloride homeostasis and neuroactivity (SLC26A11) (Alper and Sharma, 2013;Stenson et al, 2020). The amino acid sequences among the SLC26 family members are quite similar, and recent structural studies revealed that their overall molecular architectures are also similar for mammalian SLC26A4 , SLC26A5 (Bavi et al, 2021;Butan et al, 2022;Futamata et al, 2022;Ge et al, 2021), SLC26A6 (Tippett et al, 2023), and SLC26A9 (Chi et al, 2020;Walter et al, 2019). Thus, it is conceivable that a common molecular mechanism underlies their diverse physiological roles, and the pathogenic variants found in one SLC26 protein may affect the functions of the other family members in similar manners.…”