Hunt, D. F. | bioRxiv Science

bioRxiv2017

OGT (O-GlcNAc Transferase) selectively modifies multiple residues unique to lamin A

The LMNA gene encodes lamins A and C with key roles in nuclear structure, signaling, chromatin organization, and genome integrity. Mutations in LMNA cause >12 diseases, termed laminopathies. Lamins A and C are identical for their first 566 residues. However, they form distinct filaments in vivo with apparently distinct roles. We report that lamin A is O-GlcNAc modified in human hepatoma (Huh7) cells and in mouse liver. In vitro assays with purified OGT enzyme showed robust O-GlcNAcylation of recombinant mature lamin A tails (residues 385-646), with no detectable modification of lamin B1, lamin C, or progerin ({Delta}50) tails. Using mass spectrometry, we identified 11 O-GlcNAc sites in a sweet spot unique to lamin A, with up to seven sugars per peptide. Most sites were unpredicted by current algorithms. Double-mutant (S612A/T643A) lamin A tails were still robustly O-GlcNAc-modified at seven sites. By contrast, O-GlcNAcylation was undetectable on tails bearing deletion {Delta}50, which causes Hutchinson-Gilford progeria syndrome, and greatly reduced by deletion {Delta}35, suggesting this region is required for substrate recognition or modification by OGT in vitro. These results suggest OGT, an essential protein and master regulator, regulates partners or function(s) unique to lamin A that are lost in progeria.

biochemistry↗

bioRxiv2016

Canonical and cross-reactive binding of NK cell inhibitory receptors to HLA-C allotypes is dictated by peptides bound to HLA-C

BackgroundHuman natural killer (NK) cell activity is regulated by a family of killer-cell Ig-like receptors (KIR) that bind human leucocyte antigen (HLA) class I. Combinations of KIR and HLA genotypes are associated with disease, including susceptibility to viral infection and disorders of pregnancy. KIR2DL1 binds HLA-C alleles of group C2 (Lys80) and KIR2DL2 and KIR2DL3 bind HLA-C alleles of group C1 (Asn80). However, this model does not capture allelic diversity in HLA-C or the impact of HLA-bound peptides. The goal of this study was to determine the extent to which the endogenous HLA-C peptide repertoire can influence the specific binding of inhibitory KIR to HLA-C allotypes.\n\nResultsThe impact of HLA-C bound peptide on inhibitory KIR binding was investigated taking advantage of the fact that HLA-C*05:01 (HLA-C group 2, C2) and HLA-C*08:02 (HLA-C group 1, C1) have identical sequences apart from the key KIR specificity determining epitope at residues 77 and 80. Endogenous peptides were eluted from HLA-C*05:01 and used to test the peptide dependence of KIR2DL1 and KIR2DL2/3 binding to HLA-C*05:01 and HLA-C*08:02 and subsequent impact on NK cell function. Specific binding of KIR2DL1 to the C2 allotype occurred with the majority of peptides tested. In contrast, KIR2DL2/3 binding to the C1 allotype occurred with only a subset of peptides. Cross-reactive binding of KIR2DL2/3 with the C2 allotype was restricted to even fewer peptides. Unexpectedly, two peptides promoted binding of the C2 allotype-specific KIR2DL1 to the C1 allotype. We showed that presentation of endogenous peptides, or predicted HIV Gag peptides, by HLA-C can promote KIR cross-reactive binding.\n\nConclusionsKIR2DL2/3 binding to C1 is more peptide selective than that of KIR2DL1 binding to C2, which provides an explanation for why KIR2DL3-C1 interactions appear weaker than KIR2DL1-C2. In addition, cross-reactive binding of KIR is characterized by even higher peptide selectivity. We demonstrate a hierarchy of functional peptide selectivity of KIR-HLA-C interactions with relevance to NK cell biology and human disease associations. This selective peptide sequence-driven binding of KIR provides a potential mechanism for pathogen as well as self-peptide to modulate NK cell activation through altering levels of inhibition.

immunology↗