Zhang D, Pan Z H, Zhang X, Brideau A D, Lipton S A
Laboratory of Cellular and Molecular Neuroscience, Children's Hospital, Boston, MA 02115, USA.
Proc Natl Acad Sci U S A. 1995 Dec 5;92(25):11756-60. doi: 10.1073/pnas.92.25.11756.
Ionotropic receptors for gamma-aminobutyric acid (GABA) are important to inhibitory neurotransmission in the mammalian retina, mediating GABAA and GABAC responses. In many species, these responses are blocked by the convulsant picrotoxinin (PTX), although the mechanism of block is not fully understood. In contrast, GABAC responses in the rat retina are extremely resistant to PTX. We hypothesized that this difference could be explained by molecular characterization of the receptors underlying the GABAC response. Here we report the cloning of two rat GABA receptor subunits, designated r rho 1 and r rho 2 after their previously identified human homologues. When coexpressed in Xenopus oocytes, r rho 1/r rho 2 heteromeric receptors mimicked PTX-resistant GABAC responses of the rat retina. PTX resistance is apparently conferred in native heteromeric receptors by r rho 2 subunits since homomeric r rho 1 receptors were sensitive to PTX; r rho 2 subunits alone were unable to form functional homomeric receptors. Site-directed mutagenesis confirmed that a single amino acid residue in the second membrane-spanning region (a methionine in r rho 2 in place of a threonine in r rho 1) is the predominant determinant of PTX resistance in the rat receptor. This study reveals not only the molecular mechanism underlying PTX blockade of GABA receptors but also the heteromeric nature of native receptors in the rat retina that underlie the PTX-resistant GABAC response.
γ-氨基丁酸（GABA）的离子型受体对哺乳动物视网膜中的抑制性神经传递很重要，介导GABAA和GABAC反应。在许多物种中，这些反应会被惊厥剂印防己毒素（PTX）阻断，尽管阻断机制尚未完全了解。相比之下，大鼠视网膜中的GABAC反应对PTX具有极强的抗性。我们推测这种差异可以通过GABAC反应背后受体的分子特征来解释。在此，我们报告了两个大鼠GABA受体亚基的克隆，根据它们先前鉴定出的人类同源物命名为r rho 1和r rho 2。当在非洲爪蟾卵母细胞中共表达时，r rho 1/r rho 2异源受体模拟了大鼠视网膜对PTX抗性的GABAC反应。PTX抗性显然是由r rho 2亚基赋予天然异源受体的，因为同源的r rho 1受体对PTX敏感；单独的r rho 2亚基无法形成有功能的同源受体。定点诱变证实，第二个跨膜区域中的单个氨基酸残基（r rho 2中的甲硫氨酸代替r rho 1中的苏氨酸）是大鼠受体中PTX抗性的主要决定因素。这项研究不仅揭示了PTX阻断GABA受体的分子机制，还揭示了大鼠视网膜中天然受体的异源性质，这些受体是对PTX抗性的GABAC反应的基础。
