The HLA-DRα Chain Is Modified by Polyubiquitination

Ubiquitination plays a major role in regulating cell surface and intracellular localization of major histocompatibility complex class II molecules. Two E3 ligases, MARCH I and MARCH VIII, have been shown to polyubiquitinate lysine residue 225 in the cytoplasmic tail of I-Aβ and HLA-DRβ. We show that lysine residue 219 in the cytoplasmic tail of DRα is also subject to polyubiquitination. Each chain of the HLA-DR heterodimer is independently recognized and ubiquitinated, but DRβ is more extensively modified. In the cytoplasmic tail of DRβ lysine, residue 225 is the only residue that is absolutely required for ubiquitination; all other residues can be deleted or substituted without loss of function. In contrast, although lysine 219 is absolutely required for modification of DRα, other features of the DRα tail act to limit the extent of ubiquitination.