In vivo dimeric association of class I MHC heavy chains. Possible relationship to class I MHC heavy chain-beta 2-microglobulin dissociation.

Abstract

Class I MHC molecules have been thought to occur in vivo both as class I MHC heavy chain-beta 2-m heterodimers, which are or are not associated with antigenic peptide, and as free class I MHC heavy chains. Class I MHC molecules are now found also to occur in another type of structure: a heavy chain-heavy chain dimer. Biochemical studies show that heavy chain dimers are disulfide-linked via a conserved cytoplasmic domain cysteine. H-2Ld, H-2Db, and H-2Dd class I dimers fail to react with certain alpha 1 and alpha 2 domain-specific antibodies. Furthermore, although beta 2-m-specific antibodies coprecipitate class I MHC heavy chains, they do not coprecipitate class I MHC heavy chain dimers. Pulse-chase studies show that heavy chain dimer formation occurs at different points in the biosynthesis of class I MHC molecules in beta 2-m+ and beta 2-m- cells: in beta 2-m+ cells, heavy chain dimers form after the class I molecules have traversed the medial Golgi cisternae, whereas in beta 2-m- cells they form immediately. Culturing of beta 2-m+ cells with exogenous beta 2-m prevents the formation of H-2Ld/Db heavy chain dimers. We conclude that dimer formation occurs as a consequence of loss or unavailability of beta 2-m. Class I MHC heavy chain dimerization may provide a mechanism for removal of immunologically dysfunctional molecules.

Dissociation of β2m from MHC class I triggers formation of noncovalent transient heavy chain dimers

“…Several findings in the literature are consistent with the formation of complexes of MHC class I HCs in the absence of β 2 m and peptide; this applies both to the 'classical', or class Ia, proteins HLA-A/B/C (and in mouse: H-2D/K/L) as well as to the 'non-classical', or class Ib, protein HLA-F (Armony et al, 2021;Bodnar et al, 2003;Chakrabarti et al, 1992;Matko et al, 1994;Triantafilou et al, 2000). Still, it is important to differentiate these HC/HC dimers from class I associations described elsewhere (partially reviewed in (Arosa et al, 2021(Arosa et al, , 2007Campbell et al, 2012)), namely homo-and heterotypic HC/β 2 m/peptide trimers that are covalently dimerized via disulfide bonds in their cytosolic tails (Capps et al, 1993;Makhadiyeva et al, 2012) and that may play a role in binding the LILRB NK cell receptor (Baia et al, 2016); the non-covalent nano-and microscale clusters of HC/β 2 m/peptide trimers (not detected in our system) that may stem from the fusion of exocytic vesicles with the plasma membrane and that may play a role in TCR recognition (Blumenthal et al, 2016;Ferez et al, 2014;Fooksman et al, 2006;Lu et al, 2012); the macroscopic 'clusters' of class I molecules at the signaling interface between cells (Fassett et al, 2001) and the covalent dimers of the HLA-B*27:05 heavy chain that are linked by disulfide bonds through Cys-67 (Chen et al, 2017), though our non-covalent HC/HC dimers may be a precursor to the formation of the latter.…”

“…We next asked which molecular characteristics are required for HC/HC interactions, and we first tested whether the HCs are held together by cytosolic disulfide bonds as previously suggested for certain allotypes under oxidizing conditions (Baía et al, 2016;Capps et al, 1993;Makhadiyeva et al, 2012). We replaced the single cysteine in the cytosolic tail of K b -GFP (residue 332) with a serine and tested for association between this mutant and wild type HA K b (Figure 1E).…”

“…For FHCs present at the cell surface, important regulatory functions mediated by homo-and heteromeric interactions in cis and trans have been proposed (Arosa et al, 2021(Arosa et al, , 2007Campbell et al, 2012), which suggest defined spatiotemporal organization and dynamics of FHC in the plasma membrane. Indeed, clustering and covalent dimerization of MHC-I have been identified using a variety of approaches including recombinant proteins and live cells (Allen et al, 1999;Antoniou et al, 2011;Armony et al, 2021;Baia et al, 2016;Blumenthal et al, 2016;Bodnar et al, 2003;Capps et al, 1993;Chakrabarti et al, 1992;Fassett et al, 2001;Ferez et al, 2014;Lu et al, 2012;Makhadiyeva et al, 2012;Matko et al, 1994;Triantafilou et al, 2000); these represent complexes of different and largely unclear composition, size, and type of intermolecular bonding. zation in vitro and in cells, we propose a molecular model structure of MHC-I HC/HC dimers supported by in silico docking and molecular dynamics (MD) simulation.…”

Cell Surface B2m-Free Human Leukocyte Antigen (HLA) Monomers and Dimers: Are They Neo-HLA Class and Proto-HLA?

“…The dimerization of HLA HCs was investigated on H-2L d and H-2D b MHC class I molecules of the mouse by Capps and Zuniga [ 129 , 130 ]. They generated antiserum against a synthetic peptide corresponding to a portion of the cytoplasmic domains of the HCs of H-2L d and H-2D b .…”

“…B2m-free HC HLA variant molecules may bind to peptides [ 14 , 15 , 16 , 17 , 87 , 94 , 103 , 129 ]. Most importantly, they may serve as ligands for different polypeptides and proteins, which include insulin receptors [ 6 ] and T cell receptor (α/β) molecules [ 29 , 30 ] as well as KIR and Leukocyte Ig-like families, expressed on NK cells, different subsets of T cells, and other immune cells.…”