Mapping the HLA-DO/HLA-DM complex by FRET and mutagenesis

Abstract

HLA-DO (DO) is a nonclassic class II heterodimer that inhibits the action of the class II peptide exchange catalyst, HLA-DM (DM), and influences DM localization within late endosomes and exosomes. In addition, DM acts as a chaperone for DO and is required for its egress from the endoplasmic reticulum (ER). These reciprocal functions are based on direct DO/DM binding, but the topology of DO/DM complexes is not known, in part, because of technical limitations stemming from DO instability. We generated two variants of recombinant soluble DO with increased stability [zippered DOαP11A (szDOv) and chimeric sDO-Fc] and confirmed their conformational integrity and ability to inhibit DM. Notably, we found that our constructs, as well as wild-type sDO, are inhibitory in the full pH range where DM is active (4.7 to ∼6.0). To probe the nature of DO/DM complexes, we used intermolecular fluorescence resonance energy transfer (FRET) and mutagenesis and identified a lateral surface spanning the α1 and α2 domains of szDO as the apparent binding site for sDM. We also analyzed several sDM mutants for binding to szDOv and susceptibility to DO inhibition. Results of these assays identified a region of DM important for interaction with DO. Collectively, our data define a putative binding surface and an overall orientation of the szDOv/ sDM complex and have implications for the mechanism of DO inhibition of DM.antigen processing | antigen presentation | MHC class II M ajor histocompatibility complex (MHC) class II molecules present peptides on the antigen-presenting cell (APC) surface to sensitize CD4 + lymphocytes. The class II presentation pathway is well-characterized and includes roles for three accessory molecules: invariant chain (Ii), HLA-DM (DM), and HLA-DO (DO) (1). Newly synthesized MHC class II molecules bind to Ii in the ER. A trafficking signal in the cytoplasmic domain of invariant chain directs the complex to late endosomal compartments (2, 3), called MIIC (MHC II containing compartments). The invariant chain (Ii) is proteolytically degraded in this low-pH environment, leaving a nested set of invariant chain fragments, CLIPs (class II-associated invariant chain peptides), in the class II peptide-binding groove (4). CLIP is ultimately exchanged for antigenic peptides by DM, a nonclassic MHC class II molecule, which further influences peptide selection in a process termed "peptide editing" (5-10). DM also stabilizes empty MHC class II to maintain a peptide receptive structure; otherwise, empty MHC class II proteins become peptide averse (11).DO, another nonclassic MHC class II αβ heterodimer, first was described as an inhibitor of DM function, because overexpression of DO in human cells increased levels of CLIP-bound class II molecules (12, 13). In vitro evidence most often corroborated the view of DO as a negative modulator of DM (12,14). Results suggesting that DO inhibition of DM is robust at early endosomal pH (6.0-6.5) and attenuated at the lower pH (4.5-5.0) of late endosomal/lysosomal MHC II containing compartments...

Structural basis of mouse cytomegalovirus m152/gp40 interaction with RAE1γ reveals a paradigm for MHC/MHC interaction in immune evasion

“…Although the structural details of these interactions have not yet been reported, efforts using mutagenesis, FRET, and molecular modeling (40,41) suggest that a large lateral interface of DO with DM or of DM with DR is involved. This interface contrasts sharply with the interaction surface of m152, which exploits its α1α2 pd and its α3 Ig-like domains.…”

Pathways of Antigen Processing

“…DO αβ dimers associate tightly with DM molecules, and are retained in the ER in the absence of DM, suggesting that in DO-positive cells DM and DO move in concert to endosomes (Figure 4) (95). Studies using Förster (Fluorescence) Resonance Energy Transfer (FRET) and mutational analysis that defined the DM/DR interface suggested that that DO and DR bind to the same region of DM (96). Recently the crystal structure of the DO/DM complex confirmed this, and demonstrated an apparent displacement of a segment of the DO α-chain α-helix compared with that of this α-helix in MHC-II-peptide complexes, which may reflect the conformational alteration that DM imparts to induce the dissociation of low affinity peptides (97).…”

The MHC Class II Cofactor HLA-DM Interacts with Ig in B Cells

“…To determine whether this interaction is pH-sensitive, we examined binding in real time, using biolayer interferometry (Octet). We immobilized sDM on Octet sensors via DMα C-terminal biotin (27) and incubated the sensors with mAb b12 Fab at varying pH (4.7-7.2). The Fab/DM interaction rapidly achieved equilibrium, allowing comparison of equilibrium signal magnitude at each pH (Supplemental Fig.…”

“…For biotinylation, an AviTag sequence (Avidity, Aurora, CO) was added to the C-terminus of the sDMα chain (27), and biotinylation was performed according to manufacturer's instructions. Human mAb b12 and mAb b6 (both IgG1-κ, anti-HIV-1 gp120) were obtained from Dr. Dennis Burton, Scripps Research Institute, La Jolla, CA (28).…”