28zb | pdb_000028zb
Crystal Structure of HLA class II major histocompatibility complex bound to HCMV-IE1 peptide
Structural highlights
FunctionDRA_HUMAN Binds peptides derived from antigens that access the endocytic route of antigen presenting cells (APC) and presents them on the cell surface for recognition by the CD4 T-cells. The peptide binding cleft accommodates peptides of 10-30 residues. The peptides presented by MHC class II molecules are generated mostly by degradation of proteins that access the endocytic route, where they are processed by lysosomal proteases and other hydrolases. Exogenous antigens that have been endocytosed by the APC are thus readily available for presentation via MHC II molecules, and for this reason this antigen presentation pathway is usually referred to as exogenous. As membrane proteins on their way to degradation in lysosomes as part of their normal turn-over are also contained in the endosomal/lysosomal compartments, exogenous antigens must compete with those derived from endogenous components. Autophagy is also a source of endogenous peptides, autophagosomes constitutively fuse with MHC class II loading compartments. In addition to APCs, other cells of the gastrointestinal tract, such as epithelial cells, express MHC class II molecules and CD74 and act as APCs, which is an unusual trait of the GI tract. To produce a MHC class II molecule that presents an antigen, three MHC class II molecules (heterodimers of an alpha and a beta chain) associate with a CD74 trimer in the ER to form a heterononamer. Soon after the entry of this complex into the endosomal/lysosomal system where antigen processing occurs, CD74 undergoes a sequential degradation by various proteases, including CTSS and CTSL, leaving a small fragment termed CLIP (class-II-associated invariant chain peptide). The removal of CLIP is facilitated by HLA-DM via direct binding to the alpha-beta-CLIP complex so that CLIP is released. HLA-DM stabilizes MHC class II molecules until primary high affinity antigenic peptides are bound. The MHC II molecule bound to a peptide is then transported to the cell membrane surface. In B-cells, the interaction between HLA-DM and MHC class II molecules is regulated by HLA-DO. Primary dendritic cells (DCs) also to express HLA-DO. Lysosomal miroenvironment has been implicated in the regulation of antigen loading into MHC II molecules, increased acidification produces increased proteolysis and efficient peptide loading. Publication Abstract from PubMedEfficient production of milligram quantities of properly folded, functional eukaryotic proteins remains a significant challenge for structural and biochemical research, especially for membrane proteins and complex glycoproteins. The Drosophila Schneider 2 (S2) expression system provides a scalable, cost-effective alternative to baculovirus and mammalian platforms. However, its broader use is hampered by lengthy cell line development and variable expression levels. In this study, we present an optimized S2 expression workflow that reduces the time required to create stable cell lines from approximately 4 weeks to about 2 weeks without compromising protein yield. To further enhance expression, especially for low-expressing constructs, we used fluorescence-activated cell sorting (FACS) to select for high-expressing oligoclonal populations. Using a soluble human leucocyte antigen class II (HLA) and the transmembrane protein hCD81 as model proteins, FACS enrichment increased yields by approximately 124-fold for the soluble protein and by approximately 4.6-fold for the membrane protein. Protein functionality was confirmed through receptor-binding assays, SEC-MALS, and high-resolution crystallography for HLA, and ligand-binding assays for hCD81. Overall, this faster, scalable workflow enables rapid, high-yield production of structurally intact, functionally active soluble and membrane proteins from small culture volumes, establishing S2 cells as a fast and reliable platform for eukaryotic protein expression. Rapid generation of Drosophila Schneider 2 (S2) cell lines and FACS-based isolation of high-yield soluble or membrane proteins.,Kohler S, Boning H, Prinz I, Krey T, Nagarathinam K, Ssebyatika G Protein Expr Purif. 2026 Jun 23;242:106966. doi: 10.1016/j.pep.2026.106966. PMID:42331269[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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