Ubiquitination is essential for human cytomegalovirus US11-mediated dislocation of MHC class I molecules from the endoplasmic reticulum to the cytosol

Human cytomegalovirus encodes two glycoproteins, US2 and US11, which cause rapid degradation of MHC class I molecules, thus preventing recognition of virus-infected cells by the immune system. This degradation process involve s retrograde transport or 'dislocation' of MHC class I molecules from the e ndoplasmic reticulum (ER) to the cytosol, where they are deglycosylated by an N-glycanase and degraded by the proteasome. At present it is unknown whe ther ubiquitination is required for US2- and US11-mediated dislocation and degradation of MHC class I molecules. Here, we show that in E36ts20 hamster cells, which contain a temperature-sensitive mutation in the E1 ubiquitin- activating enzyme, US11-mediated degradation of MHC class I molecules is st rongly impaired at the non-permissive temperature, indicating the necessity for ubiquitination in this process. We next addressed the question of whet her ubiquitination is a condition for the retrograde movement of MHC class I molecules from the ER to the cytosol, or whether ubiquitination is merely required for recognition of dislocated MHC class I molecules by the protea some. In the absence of a functional ubiquitin system, complexes of US11 an d MHC class I molecules accumulate in the ER. In this state the membrane to pology of MHC class I molecules does not significantly change, as judged fr om proteinase K digestions. Thus the results indicate that a functional ubi quitin system is essential for dislocation of MHC class I molecules from th e ER to the cytosol.