Oresic Kristina, Ng Caroline L, Tortorella Domenico
Department of Microbiology, Mount Sinai School of Medicine, New York, New York 10029, USA.
J Biol Chem. 2009 Feb 27;284(9):5905-14. doi: 10.1074/jbc.M807568200. Epub 2009 Jan 2.
The human cytomegalovirus proteins US2 and US11 have co-opted endoplasmic reticulum (ER) quality control to facilitate the destruction of major histocompatibility complex class I heavy chains. The class I heavy chains are dislocated from the ER to the cytosol, where they are deglycosylated and subsequently degraded by the proteasome. We examined the role of TRAM1 (translocating chain-associated membrane protein-1) in the dislocation of class I molecules using US2- and US11-expressing cells. TRAM1 is an ER protein initially characterized for its role in processing nascent polypeptides. Co-immunoprecipitation studies demonstrated that TRAM1 can complex with the wild type US2 and US11 proteins as well as deglycosylated and polyubiquitinated class I degradation intermediates. In studies using US2- and US11-TRAM1 knockdown cells, we observed an increase in levels of class I heavy chains. Strikingly, increased levels of glycosylated heavy chains were observed in TRAM1 knockdown cells when compared with control cells in a pulse-chase experiment. In fact, US11-mediated class I dislocation was more sensitive to the lack of TRAM1 than US2. These results provide further evidence that these viral proteins may utilize distinct complexes to facilitate class I dislocation. For example, US11-mediated class I heavy chain degradation requires Derlin-1 and SEL1L, whereas signal peptide peptidase is critical for US2-induced class I destabilization. In addition, TRAM1 can complex with the dislocation factors Derlin-1 and signal peptide peptidase. Collectively, the data support a model in which TRAM1 functions as a cofactor to promote efficient US2- and US11-dependent dislocation of major histocompatibility complex class I heavy chains.
人类巨细胞病毒蛋白US2和US11利用内质网（ER）质量控制机制来促进主要组织相容性复合体I类重链的降解。I类重链从内质网错位到细胞质中，在那里它们被去糖基化，随后被蛋白酶体降解。我们使用表达US2和US11的细胞研究了TRAM1（转运链相关膜蛋白-1）在I类分子错位中的作用。TRAM1是一种内质网蛋白，最初因其在新生多肽加工中的作用而被鉴定。免疫共沉淀研究表明，TRAM1可以与野生型US2和US11蛋白以及去糖基化和多聚泛素化的I类降解中间体形成复合物。在使用US2和US11-TRAM1敲低细胞的研究中，我们观察到I类重链水平增加。引人注目的是，在脉冲追踪实验中，与对照细胞相比，TRAM1敲低细胞中糖基化重链水平增加。事实上，US11介导的I类错位比US2对TRAM1的缺乏更敏感。这些结果进一步证明，这些病毒蛋白可能利用不同的复合物来促进I类错位。例如，US11介导的I类重链降解需要Derlin-1和SEL1L，而信号肽肽酶对US2诱导的I类不稳定至关重要。此外，TRAM1可以与错位因子Derlin-1和信号肽肽酶形成复合物。总体而言，这些数据支持一个模型，即TRAM1作为辅助因子发挥作用，以促进主要组织相容性复合体I类重链高效地依赖于US2和US11的错位。
