Sugar transport by mammalian members of the SLC26 superfamily of anion-bicarbonate exchangers

The mammalian cochlea contains a population of outer hair cells (OHCs) whose electromotility depends on an assembly of 'motor' molecules in the basolateral membrane of the cell. Named 'prestin', the molecule is a member of the SLC26 anion transporter superfamily. We show both directly and indirectly that SLC26A5, rat prestin, takes up hexoses when expressed in several cell lines. Direct measurements of labelled fructose transport into COS-7 cells expessing prestin are reported here. Indirect measurements, using imaging techniques, show that transfected HEK-293 or CHO-K1 cells undergo reversible volume changes when exposed to isosmotic glucose-fructose exchange. The observations are consistent with the sugar transport. A similar transport was observed using a C-terminal green fluorescent protein (GFP)-tagged pendrin (SLC26A4) construct. Cells transfected with GFP alone did not respond to sugars. The data are consistent with fructose being transported by prestin with an apparent K-m = 24 mM. From the voltage-dependent capacitance of transfected cells, we estimate that 250 000 prestin molecules were present and hence that the single transport rate is not more than 3000 fructose molecules s(-1). Comparison of the transfected cell swelling rates induced by fructose and by osmotic steps indicates that water was co-transported with sugar. We suggest that the structure of SLC26 family members allows them to act as neutral substrate transporters and may explain observed properties of cochlear hair cells.