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. In B-cells, the interaction between HLA-DM and MHC class II molecules is regulated by HLA-DO. Heterodimer of an alpha chain (DMA) and a beta chain (DMB) (PubMed:16547258, PubMed:9768757). Interacts with MHCII; this interaction mediates rapid selection of high-affinity peptides in a pH-dependent manner, with an optimum at pH 5.5 (PubMed:23260142). Localizes to late endocytic compartment. Associates with lysosome membranes. The following alleles of DMA are known: DMA*01:01, DMA*01:02, DMA*01:03 (DMA3.2) and DMA*01:04 (DMA3.4). The sequence shown is that of DMA*01:01. Belongs to the MHC class II family.