Plays a critical role in catalyzing the release of class II-associated invariant chain peptide (CLIP) from newly synthesized MHC class II molecules and freeing the peptide binding site for acquisition of antigenic peptides. Heterodimer of an alpha chain (DMA) and a beta chain (DMB). Interacts with MHCII; this interaction mediates rapid selection of high-affinity peptides. Localizes to late endocytic compartment. Associates with lysosome membranes. The YXXZ (Tyr-Xaa-Xaa-Zaa, where Zaa is a hydrophobic residue) motif mediates the targeting to the lysosomal compartments. Belongs to the MHC class II family.