TRAM1 Participates in Human Cytomegalovirus US2- and US11-mediated Dislocation of an Endoplasmic Reticulum Membrane Glycoprotein

• Published in: The Journal of biological chemistry 2009-02, Vol.284 (9), p.5905-5914
• Main Authors: Oresic, Kristina, Ng, Caroline L., Tortorella, Domenico
• Format: Article
• Language: English
• Subjects: Aspartic Acid Endopeptidases - metabolism; Astrocytoma - metabolism; Endoplasmic Reticulum - metabolism; Glycosylation; Histocompatibility Antigens Class I - immunology; Histocompatibility Antigens Class I - metabolism; Human cytomegalovirus; Humans; Immunoprecipitation; Membrane Glycoproteins - antagonists & inhibitors; Membrane Glycoproteins - genetics; Membrane Glycoproteins - metabolism; Membrane Proteins - metabolism; Membrane Transport Proteins - genetics; Membrane Transport Proteins - metabolism; Membrane Transport, Structure, Function, and Biogenesis; Protein Transport; Proteins - metabolism; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger - genetics; RNA, Messenger - metabolism; RNA, Small Interfering - pharmacology; RNA-Binding Proteins - genetics; RNA-Binding Proteins - metabolism; Subcellular Fractions; Tumor Cells, Cultured; Ubiquitin - metabolism; Viral Envelope Proteins - genetics; Viral Envelope Proteins - metabolism; Viral Proteins - genetics; Viral Proteins - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M807568200

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.