IRBP: Structure and Function

描述（由申请人提供）：光感受器间类维生素A结合蛋白（IRBP）由光感受器分泌到光感受器间基质中，是视网膜变性的候选者。在基质中，IRBP可进入Muller细胞绒毛、外节和顶端RPE。Mata等人提出IRBP在Muller细胞和视锥细胞外节之间携带11-顺式和全反式视黄醇。然而，这种作用或另一种功能知之甚少，因为对IRBP的结构知之甚少。我们的长期目标是确定IRBP的结构和功能之间的关系。IRBP由同源的“模块”组成（四足动物有4个模块，硬骨鱼有2个模块）。每个模块长约300个残基，并结合1至2个类维生素A或脂肪酸分子。单个模块的X射线晶体结构显示：1）疏水性β-β α-螺旋折叠，以及2）含有高度保守的碱性残基的溶剂暴露表面结构域。我们的假设是：β-β-α折叠形成类维生素A结合位点，表面结构域允许IRBP与基质和/或外部片段相互作用。这将通过以下补充具体目标进行评估。 目标一：确定β-β-α-螺旋折叠是否是IRBP的疏水配体结合域，以及它是否可以区分11-顺式和全反式视黄醇。 目标二：为了确定保守的表面结构域是否：1）结合糖胺聚糖，特别是透明质酸，和/或2）促进类维生素A在外段的释放或递送。 目的#3：确定IRBP-配体复合物以及全长天然牛IRBP和斑马鱼IRBP的X射线晶体结构。

DESCRIPTION (provided by applicant): Interphotoreceptor retinoid-binding protein (IRBP), which is secreted by photoreceptors into the interphotoreceptor matrix, is a candidate for retinal degenerations. In the matrix, IRBP has access to Muller cell villi, outer segments, and apical RPE. Mata et al. proposed that IRBP carries 11-cis and all-trans retinol between the Muller cells and cone outer segments. However, this role, or another function is poorly understood because little is known about the structure of IRBP. Our long term goal is to define the relationship between the structure and function of IRBP. IRBP consists of homologous "modules" (4 in tetrapods and 2 in teleost fish). Each module is about 300 residues in length, and binds 1 to 2 molecules of retinoid or fatty acid. The X-ray crystal structure of an individual module shows: 1) a hydrophobic betabetaalpha-spiral fold, and 2) a solvent exposed surface domain containing highly conserved basic residues. Our Hypothesis is: the betabetaalpha fold forms the retinoid-binding site, and the surface domain allows IRBP to interact with the matrix and or outer segment. This will be evaluated through the following complementary Specific Aims. Aim #1: To determine if the betabetaalpha-spiral fold is IRBP's hydrophobic ligand-binding domain, and whether it can discriminate between 11-cis and all-trans retinol. Aim #2: To determine whether the conserved surface domain: 1) binds glycosaminoglycans particularly hyaluronan, and/or 2) facilitates the release or delivery of retinoids at the outer segment. Aim #3: To determine the X-ray crystal structure of the IRBP-ligand complex, and the full-length native bovine IRBP and zebrafish IRBP.