HLA-DMB

HLA-DMB encodes the beta chain of the non-classical MHC class II heterodimer HLA-DM, which resides in late endosomal/lysosomal MHC class II compartments (MIICs) and is required for efficient peptide loading onto classical MHC class II molecules and normal antigen presentation (PMID:8757605, PMID:7528727). A tyrosine-based tetrapeptide motif in the HLA-DMB cytoplasmic tail is necessary and sufficient for targeting to MIICs; mutation of this tyrosine redirects the protein to the cell surface and impairs antigen-presentation function (PMID:8757605). Transcription of DMB is driven by the MHC class II transactivator CIITA, whose C-terminal 41 amino acids are essential for DMB promoter activation, and loss-of-function mutations in DMB that produce aberrant transcripts abolish MHC class II-restricted antigen presentation (PMID:9300700, PMID:7528727).

1. 1994 Medium

   Defining the genomic architecture of HLA-DMB established it as a bona fide but divergent MHC class II beta gene, providing a structural framework for subsequent functional studies.

   Evidence Genomic nucleotide sequencing and exon-intron boundary analysis of HLA-DMB

   PMID:8034636

   Open questions at the time

   • No functional data on DMB protein activity at this stage
   • Relationship to peptide loading on classical MHC class II not addressed
2. 1995 Medium

   Identification of loss-of-function mutations in DMB in presentation-defective B-cell lines demonstrated that intact DMB is essential for MHC class II antigen presentation, linking the gene to a cellular function rather than mere sequence homology.

   Evidence Sequencing and RT-PCR of DMB mutants in antigen-presentation-defective B-LCL lines

   PMID:7528727

   Open questions at the time

   • Rescue by wild-type DMB re-expression not shown in this study
   • Mechanism by which DMB promotes antigen presentation (e.g., peptide exchange catalysis) not determined
   • Single laboratory observation
3. 1996 High

   Mutagenesis and immuno-EM revealed that a cytoplasmic tyrosine-based motif directs HLA-DMB to late endosomal/lysosomal MIICs, and that correct subcellular targeting is required for antigen-presentation function, establishing the trafficking mechanism.

   Evidence CD8-DMB chimera expression in HeLa cells; Tyr→Ala mutagenesis; immunoelectron microscopy; functional assay in B cells

   PMID:8757605

   Open questions at the time

   • Adaptor protein(s) recognizing the tyrosine motif not identified
   • Whether DMA contributes to targeting independently of DMB not resolved
4. 1997 Medium

   Deletion mutagenesis of CIITA showed that its C-terminal 41 amino acids are required for DMB promoter activation, defining the transcriptional control mechanism upstream of DMB expression.

   Evidence CIITA C-terminal deletion mutants with DMB promoter reporter assays and transdominant-negative suppression

   PMID:9300700

   Open questions at the time

   • Direct binding of CIITA to the DMB promoter not demonstrated
   • Whether CIITA acts through the same promoter elements as for classical MHC II genes not resolved

• The precise catalytic mechanism by which HLA-DM (DMB with DMA) facilitates peptide exchange on classical MHC class II molecules, and the identity of the adaptor(s) that recognize the DMB cytoplasmic tyrosine motif for endosomal sorting, remain to be mechanistically defined by direct biochemical or structural evidence in the timeline.
• No structural model of DM-mediated peptide exchange captured in this timeline
• Adaptor proteins binding the DMB tyrosine motif not identified
• Relative contribution of DMB vs DMA subunit to catalytic peptide-editing activity not dissected