This entry represents the trimerisation domain of the MHC class II-associated invariant chain (Ii). Ii plays a critical role in the assembly of the MHC, as well as in MHC II antigen processing by stabilizing peptide-free class II alpha/beta heterodimers in a complex soon after their synthesis and directing transport of the complex from the endoplasmic reticulum to compartments where peptide loading of class II takes place [ 16337363]. In antigen-presenting cells (APCs), loading of MHC II molecules with peptides is regulated by Ii, which blocks MHC II antigen-binding sites in pre-endosomal compartments [16181341]. Several molecules then act upon MHC II molecules in endosomes to facilitate peptide loading: Ii-degrading proteases, the peptide exchange factor, human leukocyte antigen-DM (HLA-DM), and its modulator, HLA-DO (DO).
The Invariant chain contains a single transmembrane domain. Ii first assembles into a trimer and then associates with three class II alpha/beta MHC heterodimers. Although the membrane-proximal region of the Ii luminal domain is structurally disordered, the C-terminal segment of the luminal domain is largely alpha-helical and contains a major interaction site for the Ii trimer [9843486].
More information about these proteins can be found at Protein of the Month: MHC. | 
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