Structural Mechanisms in Retrograde Protein Traffic to the Golgi

反转录复合体是酸性水解酶分选到溶酶体、多聚体Ig受体的跨细胞作用、Wnt梯度形成、铁转运体循环和淀粉样前体蛋白加工所必需的。人类反转录聚体由两个较小的复合体组成，即货物识别Vps26：Vps29：VPS35杂三聚体和SNX1和或SNX2的膜靶向异二聚体或同源二聚体。逆转录货物识别复合体由38 kDa Vps 26、20 kDa Vps 29和92 kDa VPS35亚基组成。2006年，该实验室使用X射线结晶学证明，Vps26是阻滞素的结构近亲，阻滞素是一类直接与细胞表面受体结合并指导其内化的运输蛋白家族。Juan Bonifacinos实验室与我们的团队合作，发现Vps26通过C-末端叶中的一个保守环与VPS35结合，并发现这个环是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，作为金属磷酸酯酶活性部位催化碱基的关键组氨酸被Phe 63取代。因此，Vps29对于普通的磷酸酶底物来说是完全无效的。然而，金属依赖的活性在体外对一个主要的逆转录物的磷酸化肽，阳离子非依赖性甘露糖6-磷酸受体(CI-MPR)，已有报道。尽管逆转录病毒在多条贩运途径中处于中心地位，但它的确切作用一直是个谜。各种提案都强调了作为外壳、接头或货物蛋白磷酸酶的潜在作用。目前的研究正在采取结构性的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.