Liu Yan, Wang Yin, Wu Cindy, Liu Yang, Zheng Pan
Division of Immunotherapy, Department of Surgery, Program of Molecular Medicine and Cancer Center, University of Michigan, Ann Arbor, Michigan 48109, USA.
J Biol Chem. 2006 Nov 17;281(46):34768-74. doi: 10.1074/jbc.M607778200. Epub 2006 Sep 12.
Epilepsy of progressive myoclonus type 2 gene A (EPM2A) encodes a dual specificity protein phosphatase called Laforin. Laforin is also a tumor suppressor that dephosphorylates GSK3beta at the critical Ser9 position and regulates Wnt signaling. The epilepsy-causing mutations have a deleterious effect on phosphatase activity, regardless of whether they locate in the carbohydrate-binding domain (CBD) at the N terminus or the dual specificity phosphatase domain (DSPD) at the C terminus. How mutations outside the DSPD reduce the phosphatase activity of Laforin remains unexplained. Here we report that Laforin expressed in mammalian cells forms dimers that are highly resistant to SDS treatment. Deleting CBD completely abolished the dimerization and phosphatase activity of Laforin. Moreover, all of the naturally occurring Laforin mutations tested impaired laforin GSK3beta dephosphorylation at Ser9 dimerization, and beta-catenin accumulation in nucleus. Our results demonstrate a critical role of dimerization in Laforin function and suggest an important new dimension in protein phosphatase function and in molecular pathogenesis of Lafora's disease.
进行性肌阵挛性癫痫2型基因A（EPM2A）编码一种名为拉福林的双特异性蛋白磷酸酶。拉福林也是一种肿瘤抑制因子，它在关键的丝氨酸9位点使糖原合成酶激酶3β（GSK3β）去磷酸化，并调节Wnt信号通路。导致癫痫的突变对磷酸酶活性有有害影响，无论它们位于N端的碳水化合物结合结构域（CBD）还是C端的双特异性磷酸酶结构域（DSPD）。DSPD之外的突变如何降低拉福林的磷酸酶活性仍不清楚。在这里我们报告，在哺乳动物细胞中表达的拉福林形成对SDS处理具有高度抗性的二聚体。删除CBD完全消除了拉福林的二聚化和磷酸酶活性。此外，所有测试的天然存在的拉福林突变均损害了拉福林在丝氨酸9二聚化时对GSK3β的去磷酸化作用，以及β-连环蛋白在细胞核中的积累。我们的结果证明了二聚化在拉福林功能中的关键作用，并提示了蛋白磷酸酶功能和拉福拉病分子发病机制中的一个重要新层面。
