Ashmore Jonathan, Chambard Jean-Marie
Department of Physiology, University College London, Gower Street, WC1E 6BT London, UK.
Novartis Found Symp. 2006;273:59-68; discussion 68-73, 261-4.
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.
转运蛋白SLC26超家族的两个成员在哺乳动物内耳中表达。它们是pendrin（SLC26A4）和prestin（SLC26A5）。任一基因发生突变都会导致耳聋，但原因不同。Prestin构成了耳蜗放大过程中机械反馈步骤的分子基础。外毛细胞（OHC）的运动基底外侧膜含有高密度的prestin（每个细胞超过10^7个拷贝）。因此，OHC可作为研究这种转运蛋白的实验对象。在通过消减cDNA策略鉴定出prestin之前，人们就已经认识到OHC能选择性转运果糖。圆柱形的OHC会可逆地肿胀，因此当用果糖等渗替代浴液中的葡萄糖时会缩短。这种摄取具有饱和性，Km = 16mM。我们在异源表达系统中研究了这种转运。通过电生理学测定的prestin表达水平比在OHC中至少低30倍。放射性通量测量表明果糖被转运到转染细胞中。通过细胞肿胀测量通量表明，每转运一个果糖大约有300个水分子协同转运，可能是通过同一个对水杨酸盐敏感的孔道。Pendrin也具有相同的特性。这一现象揭示了SLC26细胞外表面的特征，而电荷转运则揭示了细胞内表面的阴离子结合特征。
