Epithelial Signaling and Transport Section, Molecular Physiology and Therapeutics Branch, NIDCR, National Institutes of Health, Bethesda, Maryland 20892, USA.
J Biol Chem. 2012 Feb 10;287(7):5122-32. doi: 10.1074/jbc.M111.297192. Epub 2011 Dec 21.
Slc26a2 is a ubiquitously expressed SO(4)(2-) transporter with high expression levels in cartilage and several epithelia. Mutations in SLC26A2 are associated with diastrophic dysplasia. The mechanism by which Slc26a2 transports SO(4)(2-) and the ion gradients that mediate SO(4)(2-) uptake are poorly understood. We report here that Slc26a2 functions as an SO(4)(2-)/2OH(-), SO(4)(2-)/2Cl(-), and SO(4)(2-)/OH(-)/Cl(-) exchanger, depending on the Cl(-) and OH(-) gradients. At inward Cl(-) and outward pH gradients (high Cl(-)(o) and low pH(o)) Slc26a2 functions primarily as an SO(4)(2-)(o)/2OH(-)(i) exchanger. At low Cl(-)(o) and high pH(o) Slc26a2 functions increasingly as an SO(4)(2-)(o)/2Cl(-)(i) exchanger. The reverse is observed for SO(4)(2-)(i)/2OH(-)(o) and SO(4)(2-)(i)/2Cl(-)(o) exchange. Slc26a2 also exchanges Cl(-) for I(-), Br(-), and NO(3)(-) and Cl(-)(o) competes with SO(4)(2-) on the transport site. Interestingly, Slc26a2 is regulated by an extracellular anion site, required to activate SO(4)(2-)(i)/2OH(-)(o) exchange. Slc26a2 can transport oxalate in exchange for OH(-) and/or Cl(-) with properties similar to SO(4)(2-) transport. Modeling of the Slc26a2 transmembrane domain (TMD) structure identified a conserved extracellular sequence (367)GFXXP(371) between TMD7 and TMD8 close to the conserved Glu(417) in the permeation pathway. Mutation of Glu(417) eliminated transport by Slc26a2, whereas mutation of Phe(368) increased the affinity for SO(4)(2-)(o) 8-fold while reducing the affinity for Cl(-)(o) 2 fold, but without affecting regulation by Cl(-)(o). These findings clarify the mechanism of net SO(4)(2-) transport and describe a novel regulation of Slc26a2 by an extracellular anion binding site and should help in further understanding aberrant SLC26A2 function in diastrophic dysplasia.
Slc26a2 是一种普遍表达的 SO(4)(2-)转运体，在软骨和几种上皮组织中表达水平较高。SLC26A2 的突变与 diastrophic 发育不良有关。Slc26a2 转运 SO(4)(2-)的机制以及介导 SO(4)(2-)摄取的离子梯度知之甚少。我们在这里报告 Slc26a2 作为 SO(4)(2-)/2OH(-)、SO(4)(2-)/2Cl(-)和 SO(4)(2-)/OH(-)/Cl(-)交换体发挥作用，这取决于 Cl(-)和 OH(-)梯度。在向内 Cl(-)和向外 pH 梯度（高 Cl(-)(o)和低 pH(o))下，Slc26a2 主要作为 SO(4)(2-)(o)/2OH(-)(i)交换体发挥作用。在低 Cl(-)(o)和高 pH(o)下，Slc26a2 越来越多地作为 SO(4)(2-)(o)/2Cl(-)(i)交换体发挥作用。SO(4)(2-)(i)/2OH(-)(o)和 SO(4)(2-)(i)/2Cl(-)(o)交换则相反。Slc26a2 还可以交换 Cl(-)为 I(-)、Br(-)和 NO(3)(-)，并且 Cl(-)(o)在转运部位与 SO(4)(2-)竞争。有趣的是，Slc26a2 受到细胞外阴离子结合位点的调节，该位点对于激活 SO(4)(2-)(i)/2OH(-)(o)交换是必需的。Slc26a2 可以通过与 OH(-)和/或 Cl(-)交换转运草酸盐，其性质与 SO(4)(2-)转运相似。Slc26a2 跨膜结构域 (TMD)结构的建模确定了 TMD7 和 TMD8 之间靠近渗透途径中保守的 Glu(417)的细胞外保守序列 (367)GFXXP(371)。Glu(417)的突变消除了 Slc26a2 的转运，而 Phe(368)的突变将 SO(4)(2-)(o)的亲和力增加了 8 倍，同时将 Cl(-)(o)的亲和力降低了 2 倍，但不影响 Cl(-)(o)的调节。这些发现阐明了净 SO(4)(2-)转运的机制，并描述了 Slc26a2 受细胞外阴离子结合位点的新型调节，这应该有助于进一步了解 diastrophic 发育不良中异常 SLC26A2 功能。
