Distinct mechanisms survey the structural integrity of HLA-B*27:05 intracellularly and at the surface

HLA-B*27:05 is associated with the development of autoimmune spondyloarthropathies, but the precise causal relationship between the MHC haplotype and disease pathogenesis is yet to be elucidated. Studies focusing on the structure and cellular trafficking of HLA-B*27:05 implicate several links between the onset of inflammation and the unusual conformations of the molecule inside and at the surface of antigen presenting cells. Several lines of evidence emphasize the emergence of those unnatural protein conformations under conditions where peptide loading onto B*27:05 is impaired. To understand how cellular factors distinguish between poorly loaded molecules from the optimally loaded ones, we have investigated the intracellular transport, folding, and cell surface expression of this particular B27 subtype. Our findings show that B*27:05 is structurally unstable in the absence of peptide, and that an artificially introduced disulfide bond between residues 84 and 139 conferred enhanced conformational stability to the suboptimally loaded molecules. Empty or suboptimally loaded B*27:05 can escape intracellular retention and arrive at the cell surface leading to the appearance of increased number of β2m-free heavy chains. Our study reveals a general mechanism found in the early secretory pathways of murine and human cells that apply to the quality control of MHC class I molecules, and it highlights the allotype-specific structural features of HLA-B*27:05 that can be associated with aberrant antigen presentation and that might contribute to the etiology of disease

Correction: Distinct mechanisms survey the structural integrity of HLA-B*27:05 intracellularly and at the surface

[This corrects the article DOI: 10.1371/journal.pone.0200811.]

at the University of Veterinary Medicine Hannover

Implication of lipid microdomains in regulation of protein trafficking and epithelial cell morpholog

Latent Embeddings for Zero-Shot Classification

We present a novel latent embedding model for learning a compatibility function between image and class embeddings, in the context of zero-shot classification. The proposed method augments the state-of-the-art bilinear compatibility model by incorporating latent variables. Instead of learning a single bilinear map, it learns a collection of maps with the selection, of which map to use, being a latent variable for the current image-class pair. We train the model with a ranking based objective function which penalizes incorrect rankings of the true class for a given image. We empirically demonstrate that our model improves the state-of-the-art for various class embeddings consistently on three challenging publicly available datasets for the zero-shot setting. Moreover, our method leads to visually highly interpretable results with clear clusters of different fine-grained object properties that correspond to different latent variable maps.Comment: 2016 IEEE Conference on Computer Vision and Pattern Recognition (CVPR

Wild type and disulfide mutant of HLA B*27:05 are equally stable at the cell surface.

<p>(A) Brefeldin (BFA) A decay. Cell surface levels of B*27:05 and B*27:05-Y84C were detected by staining STF1 (left) and STF1-TAP2 cells (right) with anti-HA. Cells were harvested and stained at the times indicated, representing the duration of treatment with BFA. For each sample, the mean fluorescence intensity value recorded by the flow cytometer is depicted on the graph. (B) Averaged decay from the cell surface. The graph shows the average cell surface levels normalized to the values detected at time point zero (SEM, n = 4). The value for time point zero was set to 100% and the following values were depicted as its percentage. In peptide-deficient STF1 cells (left), both constructs show similar cell surface lifetimes, whereas in peptide-proficient STF1-TAP2 cells (right), B*27:05-Y84C is slightly less stable at the cell surface than the wild type protein.</p

The disulfide mutant of HLA B*27:05 shows accelerated anterograde transport and a longer lifetime than the wild type form in TAP2-deficient cells.

<p>(A) Pulse-chase analysis and Endo F1 resistance. STF1 (upper panel) and STF1-TAP2 cells (lower panel) expressing HA-tagged wild type B*27:05 and B*27:05-Y84C were labeled with ³⁵S-methionine/-cysteine and were chased for the times indicated. HA-tagged molecules were isolated from lysates by sequential immunoprecipitation using W6/32 and anti-HA antibodies, respectively. Where indicated, immunoisolates were treated with Endoglycosidase F1. (B) Quantification of Endo F1 resistance. The total band intensity was calculated for each lane, and the proportion of the Endo F1-resistant band was depicted as percentage of the total. The disulfide mutant B*27:05-Y84C reaches Endo F1 resistance faster than the wild type form in peptide-deficient STF1 cells (left), whereas in peptide-proficient STF1-TAP2 cells, both constructs show similar glycan maturation rates (right). (C) Folding kinetics calculated by W6/32 reactivity. The total amounts of protein (Endo F1-sensitive plus Endo F1-resistant bands) were calculated for each lane, and the intensities were depicted as percentage of the total intensity of the band from time point zero. In peptide-deficient STF1 cells, the amount of wild type B*27:05 remains stable during the first hour of chase and starts to decline rapidly, whereas the amount of B*27:05-Y84C increases steadily within the first hour, and degradation of this molecule occurs much more slowly (left). In peptide-proficient STF1-TAP2 cells, the production and folding of wild type B*27:05 and B*27:05-Y84C are comparable (right).</p

Tapasin dependence of major histocompatibility complex class I molecules correlates with their conformational flexibility

Major histocompatibility complex (MHC) class I molecules present cell internally derived peptides at the plasma membrane for surveillance by cytotoxic T lymphocytes. The surface expression of most class I molecules at least partially depends on the endoplasmic reticulum protein, tapasin, which helps them to bind peptides of the right length and sequence. To determine what makes a class I molecule dependent on support by tapasin, we have conducted in silico molecular dynamics (MD) studies and laboratory experiments to assess the conformational state of tapasin-dependent and -independent class I molecules. We find that in the absence of peptide, the region around the F pocket of the peptide binding groove of the tapasin-dependent molecule HLA-B*44:02 is in a disordered conformational state and that it is converted to a conformationally stable state by tapasin. This novel chaperone function of tapasin has not been described previously. We demonstrate that the disordered state of class I is caused by the presence of two adjacent acidic residues in the bottom of the F pocket of class I, and we suggest that conformational disorder is a common feature of tapasin-dependent class I molecules, making them essentially unable to bind peptides on their own. MD simulations are a useful tool to predict such conformational disorder of class I molecules