Department of Molecular Pharmacology and Therapeutics, Loyola University Chicago, Maywood, IL 60153 Department of Chemistry, State University of New York at Albany, Albany, NY 12222 Department of Physiology, University of Pennsylvania, Philadelphia, PA 19104 Department of Oncology, University of Wisconsin-Madison, Madison, WI 53705.
Mol Biol Cell. 2014 Apr;25(8):1355-65. doi: 10.1091/mbc.E13-11-0705. Epub 2014 Feb 12.
Troyer syndrome is an autosomal recessive hereditary spastic paraplegia (HSP) caused by frameshift mutations in the SPG20 gene that results in a lack of expression of the truncated protein. Spartin is a multifunctional protein, yet only two conserved domains--a microtubule-interacting and trafficking domain and a plant-related senescence domain involved in cytokinesis and mitochondrial physiology, respectively--have been defined. We have shown that overexpressed spartin binds to the Ile44 hydrophobic pocket of ubiquitin, suggesting spartin might contain a ubiquitin-binding domain. In the present study, we demonstrate that spartin contributes to the formation of dendritic aggresome-like induced structures (DALIS) through a unique ubiquitin-binding region (UBR). Using short hairpin RNA, we knocked down spartin in RAW264.7 cells and found that DALIS frequency decreased; conversely, overexpression of spartin increased the percentage of cells containing DALIS. Using nuclear magnetic resonance spectroscopy, we characterized spartin's UBR and defined the UBR's amino acids that are key for ubiquitin binding. We also found that spartin, via the UBR, binds Lys-63-linked ubiquitin chains but does not bind Lys-48-linked ubiquitin chains. Finally, we demonstrate that spartin's role in DALIS formation depends on key residues within its UBR.
特罗耶综合征是一种常染色体隐性遗传性痉挛性截瘫（HSP），由 SPG20 基因的移码突变引起，导致截短蛋白表达缺失。Spartin 是一种多功能蛋白，但只有两个保守结构域——一个微管相互作用和运输结构域，以及一个与植物衰老相关的结构域，分别参与细胞分裂和线粒体生理学——已经被定义。我们已经表明，过表达的 Spartin 与泛素的 Ile44 疏水口袋结合，这表明 Spartin 可能含有一个泛素结合结构域。在本研究中，我们通过一个独特的泛素结合区域（UBR）证明 Spartin 有助于形成树突聚集体样诱导结构（DALIS）。通过短发夹 RNA，我们在 RAW264.7 细胞中敲低 Spartin，发现 DALIS 频率降低；相反，过表达 Spartin 增加了含有 DALIS 的细胞百分比。通过核磁共振波谱学，我们对 Spartin 的 UBR 进行了表征，并定义了 UBR 中关键的泛素结合氨基酸。我们还发现 Spartin 通过 UBR 结合 Lys-63 连接的泛素链，但不结合 Lys-48 连接的泛素链。最后，我们证明 Spartin 在 DALIS 形成中的作用取决于其 UBR 中的关键残基。
