Determinants of coupled transport and uncoupled current by the electrogenic SLC26 transporters

Abstract

Members of the SLC26 family of anion transporters mediate the transport of diverse molecules and can function as coupled transporters or as channels. A unique feature of two members of the family, Slc26a3 and Slc26a6, is that they can function as both coupled and uncoupled channel-like modes. To identify features that control these modes of transport we performed in silico modeling of Slc26a6, which revealed high fold similarity of the Slc26a6 and ClC transporters, despite their low sequence identity. Examining the predicted Slc26a6 fold identified a highly conserved Glu- (E357) with a predicted spatial orientation similar to that of ClCec E148, which determines coupled or uncoupled transport by ClCec. This raised the question of whether the conserved Glu- in Slc26a6(E357) and Slc26a3(E367) play a role in the two transport modes of these transporters. Reversing the Glu- charge in a3 and a6 to Lys resulted in the inhibition of all modes of transport. However, neutralizing the charge in Slc26a6(E357A) eliminated all forms of coupled transport and augmented the uncoupled channel-like current activity. The a3(E367A) mutation markedly reduced the coupled transport and converted the stoichiometry of the residual exchange from 2Cl-/1HCO3- to 1:1, while completely sparing the channel-like activity. These findings suggest that a similar structural motif may determine multiple functional modes of these transporters.