Ubiquitination of Human Leukocyte Antigen (HLA)-DM by Different Membrane-associated RING-CH (MARCH) Protein Family E3 Ligases Targets Different Endocytic Pathways*

Abstract

Background: HLA-DM plays an essential role in MHC class II antigen presentation.

Results: MARCH family E3 ligases regulate HLA-DM trafficking by direct ubiquitination of DMα and indirectly through a tyrosine-based targeting signal.

Conclusion: Ubiquitination is a common mechanism regulating both classical and nonclassical MHC molecules.

Significance: HLA-DM activity can be controlled post-translationally allowing the immune system to adapt to different conditions.

Abstract

HLA-DM plays an essential role in the peptide loading of classical class II molecules and is present both at the cell surface and in late endosomal peptide-loading compartments. Trafficking of DM within antigen-presenting cells is complex and is, in part, controlled by a tyrosine-based targeting signal present in the cytoplasmic tail of DMβ. Here, we show that DM also undergoes post-translational modification through ubiquitination of a single lysine residue present in the cytoplasmic tail of the α chain, DMα. Ubiquitination of DM by MARCH1 and MARCH9 induced loss of DM molecules from the cell surface by a mechanism that cumulatively involved both direct attachment of ubiquitin chains to DMα and a functional tyrosine-based signal on DMβ. In contrast, MARCH8-induced loss of surface DM was entirely dependent upon the tyrosine signal on DMβ. In the absence of this tyrosine residue, levels of DM remained unchanged irrespective of whether DMα was ubiquitinated by MARCH8. The influence of MARCH8 was indirect and may have resulted from modification of components of the endocytic machinery by ubiquitination.