Sugar transport by members of the SLC26 superfamily.
Two members of the SLC26 superfamily of transport proteins are expressed in the mammalian inner ear. These are pendrin (SLC26A4) and prestin (SLC26A5). Mutations in either lead to deafness but for different reasons. Prestin forms the molecular underpinning of the mechanical feedback step in cochlear amplification. The outer hair cell (OHC) motor basolateral membrane contains a high density of prestin (more than 10(7) copies per cell). OHCs can thus act as a laboratory to study this transporter. Before prestin was identified by a subtractive cDNA strategy, it was recognized that OHCs selectively transport fructose. Cylindrical OHCs swell reversibly and therefore shorten on exposure to iso-osmotic replacement of bath glucose with fructose. The uptake is saturable with Km = 16mM. We studied such transport in a heterologous expression system. Prestin expression levels, assayed by electrophysiology, were at least 30 times lower than in OHCs. Radioactive flux measurements show that fructose was transported into transfected cells. Measuring fluxes by cell swelling suggested that about 300 water molecules are co-transported per fructose, probably through the same salicylate-sensitive pore. Pendrin shares the same properties. The phenomenon reveals features of the extracellular surface of SLC26 while charge translocation reveals anion binding features of intracellular surface.