Contribution of lysine 11-linked ubiquitination to MIR2-mediated major histocompatibility complex class I internalization.

The polyubiquitin chain is generated by the sequential addition of ubiquitin moieties to target molecules, a reaction between specific lysine residues that is catalyzed by E3 ubiquitin ligase. The Lys(48)-linked and Lys(63)-linked polyubiquitin chains are well established inducers of proteasome-dependent degradation and signal transduction, respectively. The concept has recently emerged that polyubiquitin chain-mediated regulation is even more complex because various types of atypical polyubiquitin chains have been discovered in vivo. Here, we demonstrate that a novel complex ubiquitin chain functions as an internalization signal for major histocompatibility complex class I (MHC I) membrane proteins in vivo. Using a tetracycline-inducible expression system and quantitative mass spectrometry, we show that the polyubiquitin chain generated by the viral E3 ubiquitin ligase of Kaposi sarcoma-associated herpesvirus, MIR2, is a Lys(11) and Lys(63) mixed-linkage chain. This novel ubiquitin chain can function as an internalization signal for MHC I through its association with epsin1, an adaptor molecule containing ubiquitin-interacting motifs.