Structural Mechanisms in Retrograde Protein Traffic to the Golgi

酸水解酶对溶酶体的分选、聚合Ig受体的转胞作用、Wnt梯度的形成、铁转运蛋白的再循环以及淀粉样前体蛋白的加工都需要逆转录复合物。人类逆转录酶由两个较小的复合物组成，即货物识别Vps26:Vps29:Vps35异源三聚体，以及SNX1和SNX2的膜靶向异源二聚体或同源二聚体。后转录物识别复合体由38-kDa Vps26、20-kDa Vps29和92-kDa Vps35亚基组成。2006年，该实验室使用x射线晶体学证明Vps26是阻滞蛋白的结构表亲，阻滞蛋白是一种直接结合细胞表面受体并指导其内化的转运蛋白家族。与我们小组合作的Juan Bonifacinos实验室发现，Vps26通过c端lobe中的一个保守环与Vps35结合，并发现该环是Vps26在体内正确定位所必需的。另外两个实验室证明Vps29具有金属磷酸酯酶折叠，可以结合两个金属离子。与功能性金属磷酸酯酶相比，在金属磷酸酯酶活性位点作为催化碱的关键His被Phe 63取代。因此，Vps29对一般磷酸酶底物是完全无活性的。然而，已经有报道称，金属依赖性活性在体外对来自主要逆转录物货物的磷酸化肽，阳离子非依赖性甘露糖6-磷酸受体（CI-MPR）。尽管retromer在多种贩运途径中处于中心地位，但它的确切功能一直是个谜。各种各样的建议都强调了作为外壳、适配器或货物蛋白磷酸酶的潜在作用。目前的研究正在采取结构方法来解决这个问题。

The retromer complex is required for the sorting of acid hydrolases to lysosomes, transcytosis of the polymeric Ig receptor, Wnt gradient formation, iron transporter recycling, and processing of the amyloid precursor protein. Human retromer consists of two smaller complexes, the cargo recognition Vps26:Vps29:Vps35 heterotrimer, and a membrane-targeting heterodimer or homodimer of SNX1 andor SNX2. The retromer cargo recognition complex consists of the 38-kDa Vps26, 20-kDa Vps29, and 92-kDa Vps35 subunits. In 2006 this laboratory used x-ray crystallography to show that Vps26 is a structural cousin of the arrestins, a family of trafficking proteins that directly bind to cell surface receptors and direct their internalization. Juan Bonifacinos laboratory, working in collaboration with our group, showed that Vps26 binds to Vps35 through a conserved loop in the C-terminal lobe, and found that this loop is required for the correct localization of Vps26 in vivo. Vps29 was shown by two other labs to have a metallophosphoesterase fold that can bind two metal ions. Compared to functional metallophosphoesterases, a key His that serves as a catalytic base in the metallophosphoesterase active site is replaced by Phe 63. Thus Vps29 is completely inactive with respect to generic phosphatase substrates. However, metal-dependent activity in vitro against a phosphorylated peptide from a major retromer cargo, the cation-independent mannose 6-phosphate receptor (CI-MPR), has been reported. Despite its centrality to multiple trafficking pathways, the precise function of retromer has been enigmatic. Various proposals have emphasized potential roles as a coat, adaptor, or cargo protein phosphatase. Current studies are taking a structural approach to resolving this question.