Liu Shengchun, Bromley-Brits Kelley, Xia Kun, Mittelholtz Jill, Wang Ruitao, Song Weihong
Townsend Family Laboratories, Department of Psychiatry, Brain Research Center, The University of British Columbia, Vancouver, BC, Canada.
Eur J Neurosci. 2008 Nov;28(10):1980-8. doi: 10.1111/j.1460-9568.2008.06497.x.
The presenilin-associated complex regulates two independent intramembranous cleavage activities, i.e. gamma-secretase and epsilon-secretase activity. The gamma-secretase complex requires four critical components for its activity: presenilin 1, anterior pharynx-defective 1, nicastrin 1 and presenilin enhancer 2, all of which are degraded through the ubiquitin-proteasome pathway. Recently, TMP21, a type I transmembrane protein involved in endoplasmic reticulum/Golgi transport, was identified as a member of the presenilin complex. Knockdown of TMP21 selectively regulated pathogenic gamma-secretase activity, resulting in increased amyloid beta protein 40 and 42, without affecting the epsilon-cleavage of Notch. A further understanding of TMP21 degradation is required to examine the biological consequences of TMP21 protein level aberrations and their potential role in the pathogenesis of Alzheimer's disease and drug development. Here we show that human TMP21 has a short half-life of approximately 3 h. Treatment with proteasomal inhibitors can increase TMP21 protein levels in both a time- and dose-dependent manner, and both co-immunoprecipitation and immunofluorescent staining show that TMP21 is ubiquitinated. Inhibition of the lysosomal pathway failed to show a dose-dependent increase in TMP21 protein levels. Taken together, these results indicate that the degradation of TMP21, as with the other presenilin-associated gamma-secretase complex members, is mediated by the ubiquitin-proteasome pathway.
早老素相关复合物调节两种独立的膜内切割活性，即γ-分泌酶和ε-分泌酶活性。γ-分泌酶复合物的活性需要四个关键组分：早老素1、咽前缺陷蛋白1、尼卡斯特林1和早老素增强子2，所有这些组分均通过泛素-蛋白酶体途径降解。最近，参与内质网/高尔基体转运的I型跨膜蛋白TMP21被鉴定为早老素复合物的成员。敲低TMP21可选择性调节致病性γ-分泌酶活性，导致淀粉样β蛋白40和42增加，而不影响Notch的ε-切割。需要进一步了解TMP21的降解情况，以研究TMP21蛋白水平异常的生物学后果及其在阿尔茨海默病发病机制和药物开发中的潜在作用。在此我们表明，人TMP21的半衰期约为3小时。蛋白酶体抑制剂处理可使TMP21蛋白水平呈时间和剂量依赖性增加，免疫共沉淀和免疫荧光染色均显示TMP21被泛素化。抑制溶酶体途径未能使TMP21蛋白水平呈剂量依赖性增加。综上所述，这些结果表明，与其他早老素相关的γ-分泌酶复合物成员一样，TMP21的降解是由泛素-蛋白酶体途径介导的。
