title: Dynamics of the antigen-binding grooves in CD1 proteins: reversible hydrophobic collapse in the lipid-free state.
abstract: CD1 proteins mediate the presentation of endogenous and foreign lipids on the cell surface for recognition by T cell receptors. To sample a diverse antigen pool, CD1 proteins are repeatedly internalized and recycled, assisted, in some cases, by lipid transfer proteins such as saposins. The specificity of each CD1 isoform is, therefore, conferred in part by its intracellular pathway but also by distinct structural features of the antigen-binding domain. Crystal structures of CD1-lipid complexes reveal hydrophobic grooves and pockets within these binding domains that appear to be specialized for different lipids. However, the mechanism of lipid loading and release remains to be characterized. Here we gain insights into this mechanism through a meta-analysis of the five human CD1 isoforms, in the lipid-bound and lipid-free states, using all-atom molecular dynamics simulations. Strikingly, for isoforms CD1b through CD1e, our simulations show the near-complete collapse of the hydrophobic cavities in the absence of the antigen. This event results from the spontaneous closure of the binding domain entrance, flanked by two α-helices. Accordingly, we show that the anatomy of the binding cavities is restored if these α-helices are repositioned extrinsically, suggesting that helper proteins encountered during recycling facilitate lipid exchange allosterically. By contrast, we show that the binding cavity of CD1a is largely preserved in the unliganded state because of persistent electrostatic interactions that keep the portal α-helices at a constant separation. The robustness of this binding groove is consistent with the observation that lipid exchange in CD1a is not dependent on cellular internalization.
authors.affiliation: Theoretical Molecular Biophysics Group, Max Planck Institute of Biophysics, Max von Laue Strasse 3, 60438 Frankfurt am Main, Germany.
authors.forename: Diana
authors.initials: D
authors.lastname: Garzón
authors.forename: Claudio
authors.initials: C
authors.lastname: Anselmi
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Bond
authors.forename: José D
authors.initials: JD
authors.lastname: Faraldo-Gómez
mesh_terms: D000941:Antigens / Q000737:chemistry* / Q000276:immunology; D018949:Antigens, CD1 / Q000737:chemistry* / Q000276:immunology; D001665:Binding Sites / Q000276:immunology; D018360:Crystallography, X-Ray; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D008055:Lipids / Q000737:chemistry* / Q000276:immunology; D056004:Molecular Dynamics Simulation; D020033:Protein Isoforms / Q000737:chemistry / Q000276:immunology; D017433:Protein Structure, Secondary; D055672:Static Electricity; D013329:Structure-Activity Relationship
chemical_list: D000941:Antigens; D018949:Antigens, CD1; D008055:Lipids; D020033:Protein Isoforms
keywords: Adaptive Immunity; Allosteric Regulation; Antigen Presentation; Cellular Immune Response; Hydrophobic Collapse; Lipid Binding Protein; Molecular Dynamics