Polyubiquitination is required for US11-dependent movement of MHC class I heavy chain from endoplasmic reticulum into cytosol

The human cytomegalovirus protein US11 induces the dislocation of MHC class I heavy chains from the endoplasmic reticulum (ER) into the cytosol for de gradation by the proteasome. With the use of a fractionated, permeabilized cell system, we find that US11 activity is needed only in the cell membrane s and that additional cytosolic factors are required for heavy chain disloc ation. We identify ubiquitin as one of the required cytosolic factors. Cyto sol depleted of ubiquitin does not support heavy chain dislocation from the ER, and activity can be restored by adding back purified ubiquitin. Methyl ated-ubiquitin or a ubiquitin mutant lacking all lysine residues does not s ubstitute for wild-type ubiquitin, suggesting that polyubiquitination is re quired for US11-dependent dislocation. We propose a new function for ubiqui tin in which polyubiquitination prevents the lumenal domain of the MHC clas s I heavy chain from moving back into the ER lumen. A similar mechanism may be operating in the dislocation of misfolded proteins from the ER in the c ellular quality control pathway.