Structure of the fungal β‐glucan‐binding immune receptor dectin‐1: Implications for function

Abstract

The murine molecule dectin-1 (known as the b-glucan receptor in humans) is an immune cell surface receptor implicated in the immunological defense against fungal pathogens. Sequence analysis has indicated that the dectin-1 extracellular domain is a C-type lectin-like domain, and functional studies have established that it binds fungal b-glucans. We report several dectin-1 crystal structures, including a high-resolution structure and a 2.8 Å resolution structure in which a short soaked natural b-glucan is trapped in the crystal lattice. In vitro characterization of dectin-1 in the presence of its natural ligand indicates higher-order complex formation between dectin-1 and b-glucans. These combined structural and biophysical data considerably extend the current knowledge of dectin-1 structure and function, and suggest potential mechanisms of defense against fungal pathogens.Keywords: immune recognition; fungal pathogen; b-glucan; protein crystallography; C-type lectin-like domain Dectin-1 is a cell-surface immune receptor for b-glucans, which are major structural cell wall components that are conserved in fungi (Brown and Gordon 2001). Recognition of b-glucans by dectin-1 can trigger phagocytosis of fungal pathogens and protective inflammatory responses. Originally identified as a dendritic cell receptor in the mouse (Ariizumi et al. 2000), dectin-1 is now known to be widely expressed in both mouse and human cells, particularly on monocytes/macrophages and neutrophils (Brown 2006). Primary sequence analysis indicates that dectin-1 is a 28-kDa type II membrane protein. An extracellular Ctype lectin-like domain (CTLD) is connected by a stalk to a transmembrane region, followed by a cytoplasmic tail containing an immunoreceptor tyrosine-based activation (ITAM)-like motif (Ariizumi et al. 2000). First recognized as a calcium-dependent carbohydrate-binding domain, the CTLD fold is also seen in non-calciumdependent protein recognition interactions (Drickamer 1999). Few of the residues required for calcium coordination in classical CTLDs are conserved in dectin-1. The dectin-1 CTLD has two potential N-linked glycosylation sites, whereas its human homolog, the b-glucan receptor (bGR), has none. In addition to its role in b-glucan binding, it has been suggested that dectin-1 recognizes an endogenous T-cell ligand in a carbohydrate-independent manner (Ariizumi et al. 2000), but there is no evidence as to the nature of this potential ligand. Abbreviations: AUC, analytical ultracentrifugation; bGR, human b-glucan receptor; BGC, b-glucan; CTLD, C-type lectin-like domain; DLS, dynamic light scattering; TTX, Triton X-100.Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi

Structural Flexibility of the Macrophage Dengue Virus Receptor CLEC5A

“…Therefore, from a structural perspective, CLEC5A might be expected to bind to a protein ligand but is reported to bind to dengue virus in a manner that is inhibited by sugars (3). However, there is a precedent for non-canonical sugar binding in Dectin-1, which is a C-type lectin that binds ␤-glucan oligosaccharides in the absence of calcium (43), although the molecular basis for this interaction is incompletely understood (53). We therefore analyzed the structure of CLEC5A for sites with potential for interactions with known dengue virus surface carbohydrate species and investigated whether CLEC5A bound to a wide range of microarrayed glycans.…”

The Pattern Recognition Receptor Dectin-1: From Fungi to Mycobacteria

“…Crystal structural data also indicates that Dectin-1 maintains a metal binding site characteristic of the snake venom coagulation factor IX binding protein. However, metal ion binding by Dectin-1 appears not to be required for proper protein folding or ligand association [19].…”

“…How Dectin-1 binds β-glucans is unclear since as a member of the non-classical C-type lectin receptor family they lack the conserved residues with the CRD required for carbohydrate binding. Nevertheless, mutagenesis studies indicate that Trp 221 and His 223 within the CRD are crucial for β-glucan binding [21,19]. Structural data indicates that these residues are present within a groove that constitutes a potential binding site for β-glucans [19].…”

“…Nevertheless, mutagenesis studies indicate that Trp 221 and His 223 within the CRD are crucial for β-glucan binding [21,19]. Structural data indicates that these residues are present within a groove that constitutes a potential binding site for β-glucans [19]. Charge analysis of this groove suggests β-glucan binding is driven mainly by hydrophobic forces [22].…”

Structural analysis of natural killer cell receptor protein 1 (NKR-P1) extracellular domains suggests a conserved long loop region involved in ligand specificity

“…If some structures had the same identity, the one with the better resolution was used. Templates and identities used were: 1YPQ (C-type lectin-like domain of human oxidized low density lipoprotein receptor 1 (LOX-1) [31]) for mNKR-P1A/C (identity 31 %/32 %), 1XPH (CD209 antigen-like protein 1 [32]) for rNKR-P1B (identity 33 %), 3HUP (early activation antigen CD69 [33]) for mNKR-P1F (identity 33 %), 1E87 (early activation antigen CD69 [34]) for mNKR-P1G (identity 35 %) and 2BPD (beta-glucan receptor Dectin-1 [35]) for hNKR-P1 (identity 32 %). Only in the case of rNKR-P1A was an alternative technique tried.…”