These considerations led Pos et al to a model whereby the ability of a peptide to make strong interactions in the HLA-DR P1 pocket is the crucial factor for resistance to DM editing [23]. (generically referred to here PD-159020 as DM and DO) are non-peptide binding class II major histocompatibility (MHC-II) homologs. Unlike the large family of class I MHC homologs, which have varied functions in many cell types as endocytic receptors, NK ligands, T cell decoys, and presenters of peptides, lipids, and vitamin derivatives [1], for the non-classical MHC-II proteins DM and DO known functions are only in antigen-presenting cells, where they regulate loading of peptides derived from self and foreign antigens. DM functions as a peptide exchange factor required for efficient loading of endosomal peptides onto MHC-II molecules. DO functions as a modulator of DM. The molecular mechanism by which DM promotes peptide exchange and the functions of DM and DO in the overall immune response are outstanding fundamental questions in MHC biology. In the period covered by this review, significant progress has been made towards understanding the structural basis for DM conversation with MHC-II, and new work strengthens the conclusion that DM plays a key role in immunodominance. However, important mechanistic questions about DM action still remain unanswered, and this constrains our ability to integrate these improvements into deeper understanding PD-159020 of how DM functions in development, maintenance, and activation of the CD4+T cell response. For DO, the mechanism of action has been established: DO functions as substrate mimic to competitively inhibit HLA-DM-mediated catalysis of MHC-II peptide exchange. A key role for DO in regulating autoimmunity has been established through studies of H-2O knockout mice. However, the relationship of the molecular mechanism of DO action to its biological role still is not clear. == Insight into DM function from crystal structures of DM-DO and DM-DR == Two crystal structures of caught DM-MHC complexes provided long-awaited insight into how DM engages MHC-II to promote peptide exchange [2,3]. DM functions as an enzyme to catalyze peptide exchange [4,5], and like other enzymes it binds only transiently to its substrate(s) before inducing conversion and releasing product(s). Thus, DM does not bind stably to MHC-peptide complexes [6,7]. DM does not appear PD-159020 to bind to recombinant peptide-free vacant MHC molecules [7], although DM binding to apparently empty MHC molecules produced in their normal cellular context has been reported [8,9]. The discrepancy may be due to differences between metastable peptide-receptive species generated during peptide dissociation [911] and stable peptide-averse species produced in the absence of peptide [11,12]. Previously, a few mutated HLA-DR-peptide complexes with weakened MHC-peptide conversation have been shown to bind to DM sufficiently tightly to be observed biochemically [6,7,13], but until recently all of these have resisted crystallization and detailed structural analysis. In one of the recent structure reports, Pos et al crystallized a PD-159020 DM-MHCII complex after covalent attachment of DM to HLA-DR1 via sortase-A mediated coupling of the DM beta-subunit C-terminus to the HLA-DR1 beta-subunit C-terminus, with the HLA-DR1 PD-159020 transporting a truncated peptide attached via a disulfide bond engineered into the P6 pocket [2]. The peptide was designed to bind only to the C-terminal side of the binding site, leaving the N-terminal side empty; usually such peptides bind weakly if at all, but here the conversation was stabilized through covalent bonding to the MHC. Crucially, leaving the N-terminal side of the site open allows MHC conformational alteration and stable conversation with DM. In the second of the crystal structure reports, Guce et al crystallized DM with HLA-DO [3]. In the Rabbit Polyclonal to ACOT2 complex, DO adopts an overall conformaton highly much like classical MHCII proteins with an open groove, but with conformational alterations at the N-terminal side. The DO structure provides insight into the nature of chain association in the MHCII family and constrains possible functional functions for DO in antigen presentation. DO was shown through enzymatic and mutagenesis studies to act as a substrate mimic, binding tightly to DM and competitively inhibiting the conversation with MHC-peptide. In the crystal structure DO was observed to bind to the same lateral face of DM as does DR, with essentially all interface residues conserved. The DM-MHC conversation in the two structures is virtually identical (Physique 1), alleviating issues that this protein engineering necessary to trap DM with HLA-DR might have induced a non-physiologically relevant conformation, or that DOs mimicry of an MHC-peptide complex might not extend to structural details of its interaction with DM. A FRET study revealed a similar side-by-side arrangement for DM bound to DO in solution [14], further supporting the physiological relevance of the complex visually by X-ray crystallography. == Figure 1. == DM engages DR1 and DO similarly. A. Structure of DM bound.