The pathway of US11-dependent degradation of MHC class I heavy chains involves a ubiquitin-conjugated intermediate

The human cytomegalovirus protein, US11, initiates the destruction of MHC c lass I heavy chains by targeting them for dislocation from the ER to the cy tosol and subsequent degradation by the proteasome. We report the developme nt of a permeabilized cell system that recapitulates US11-dependent degrada tion of class I heavy chains. We have used this system, in combination with experiments in intact cells, to identify and order intermediates in the US 11-dependent degradation pathway, We find that heavy chains are ubiquitinat ed before they are degraded. Ubiquitination of the cytosolic tail of heavy chain is not required for its dislocation and degradation, suggesting that ubiquitination occurs after at least part of the heavy chain has been dislo cated from the ER. Thus, ubiquitination of the heavy chain does not appear to be the signal to start dislocation, Ubiquitinated heavy chains are assoc iated with membrane fractions, suggesting that ubiquitination occurs while the heavy chain is still bound to the ER membrane. Our results support a mo del in which US11 co-opts the quality control process by which the cell des troys misfolded ER proteins in order to specifically degrade MHC class I he avy chains.