The murine γ-herpesvirus-68 K3 (MK3) is a PHD/LAP finger protein that downregulates major histocompatibility complex (MHC) class I expression. In transfected cell lines, MK3 was expressed in the endoplasmic reticulum (ER) membrane, where it bound the cytoplasmic tail of newly synthesized H-2Db glycoproteins and targeted them for degradation. Proteasome inhibitors blocked the degradation and led to an accumulation of ubiquitinated H-2Db. Because this retained its native conformation, ubiquitination preceded any denaturation or dislocation to the cytosol. The PHD/LAP finger of MK3 was not required for H-2Db binding but was essential for its ubiquitination and degradation. Thus, γ-herpesviruses have adapted the cellular PHD/LAP motif to immune evasion, apparently for the catalysis of MHC class I ubiquitination.