Immune evasion protein mK3 defines the physiological importance of non-lysine ubiquitination of MHCI substrates (38.8)

Abstract

Abstract Ubiquitin modification of proteins plays a prominent role in regulation of multiple cell processes including ER-associated degradation. Until recently ubiquitination of substrates was thought to occur only via isopeptide bonds, typically to lysine residues or less frequently to the N-terminus of a substrate. We and others recently found that certain viral MARCH E3 ligases can promote ubiquitin conjugation to non-lysine residues via ester or thiolester bonds. The molecular basis for these novel modifications, however, remained undetermined. To probe the mechanism and importance of non-lysine ubiquitination, we have studied the viral immune evasion protein mK3, a RING-CH type E3 ligase encoded by γHV68. We recently identified Ube2j2 as the primary cellular E2 required for mK3-induced MHCI heavy chain ubiquitination and degradation in the proteasome. Surprisingly, Ube2j2-mK3 preferentially ubiquitinates serine residues via ester bonds even when lysine residues are present on wildtype substrates. Furthermore, Ube2j2-mK3 not only adds the initial ubiquitin on the substrates but also promotes assembly of a lysine 48-linked polyubiquitin chain. These findings demonstrate for the first time the physiological important of this novel ubiquitination mechanism and that the same E2-E3 complex can catalyze the coupling of ubiquitin to substrates via an ester bond and assemble a specific polyubiquitin chain via isopeptide bonds.