In the endosomes, HLA-DM functions by promoting the dissociation of the CLIP peptide (a place holder peptide) from MHC class II which allows endosomal peptides to bind. Possibly by its preferential binding of the open, peptide-less conformation of MHC class II, HLA-DM then catalyzes peptide exchange, favoring more stable peptide-MHC complexes (which are resistant to DM action). Thus, HLA-DM is thought to play an important role in "editing" the peptide repertoire.
HLA-DM is a protein involved in the immune system. It is a molecular chaperone that works in lysosomes and endosomes. HLA-DM plays a role in the proper loading of peptides and presentation of the peptides in macrophages, dendritic cells, B cells, and other antigen presenting cells. The protein is unable to bind peptides itself, so it interacts with major histocompatibility protein II (MHCII) to speed up the molecule’s binding and releasing of peptides. HLA-DM regulates the peptides that bind to MHCII and present the antigen on antigen presenting cells, therefore controlling which antigen(s) the body mounts an immune response to.
The genes for HLA-DM are located in the MHCII region of the human chromosome 6. The genes code for the alpha and beta chains that makeup the protein. HLA-DM consists of two polymers and three ligands: BETA-D-MANNOSE (BMA), CHLORIDE ION (CL), and 2-(ACETYLAMINO)-2-DEOXY-A-D-GLUCOPYRANOSE (NDG). The structure of HLA-DM was determined by x-ray diffraction at a resolution of 2.27 Å.
^Busch R, Rinderknecht CH, Roh S, Lee AW, Harding JJ, Burster T, Hornell TM, Mellins ED (October 2005). "Achieving stability through editing and chaperoning: regulation of MHC class II peptide binding and expression". Immunol. Rev.207: 242–60. doi:10.1111/j.0105-2896.2005.00306.x. PMID16181341.
3. Vogt Anne B, Kropshofer Harald (April 1999). "HLA-DM – an endosomal and lysosomal chaperone for the immune system". Trends in Biochemical Sciences. 24 (4): 150-154. doi:10.1016/S0968-0004(99)01364-X