Crystallization and preliminary X-ray analysis of a trimeric form of human mannose binding protein

Residual dipolar coupling derived orientational constraints on ligand geometry in a 53 kDa protein-ligand complex

The geometric relationships between ligands and the functional groups that bind ligands in soluble ligand-protein complexes have traditionally been deduced from distance constraints between pairs of NMR active nuclei spanning the ligand-protein interface. Frequently, the steep inverse distance dependence of the nuclear Overhauser effect (NOE), from which the distance constraints are derived, makes identification of sufficient numbers of constraints difficult. In these cases the ability to supplement NOE-derived information with distance-independent angular information can be very important. Here, the observation of residual dipolar couplings from alpha-methyl mannose bound to mannose binding-protein in a dilute liquid crystalline medium has allowed the determination of a bound ligand's average orientation. The 3-fold rotational symmetry of mannose-binding protein defines its orientational tensor and obviates the need to determine experimentally the protein's average orientation. Through superimposition of ligand and protein orientational tensors we describe the binding geometry of alpha-methyl mannose bound to mannose-binding protein. This new method is of general applicability to the study of ligands bound to proteins, and it is of particular interest when neither X-ray crystallography nor NOE techniques can provide sufficient information to describe binding geometries.

The mannose receptor is a pattern recognition receptor involved in host defense

The mannose receptor recognizes the patterns of carbohydrates that decorate the surfaces and cell walls of infectious agents. This macrophage and dendritic cell pattern-recognition receptor mediates endocytosis and phagocytosis. The mannose receptor is the prototype of a new family of multilectin receptor proteins (membrane-spanning receptors containing eight-ten lectin-like domains, which appear to play a key role in host defense) and provides a link between innate and adaptive immunity. Recent advances include the identification of three new members of the mannose receptor family, additional work on defining the molecular requirements for sugar binding, a role for the mannose receptor in antigen presentation of lipoglycan antigens and evidence that the mannose receptor is associated with a signal transduction pathway leading to cytokine production.

Trimeric structure of a C-type mannose-binding protein

BACKGROUND

Mannose-binding proteins (MBPs) are C-type (Ca(2+)-dependent) animal lectins found in serum. They recognize cell-surface oligosaccharide structures characteristic of pathogenic bacteria and fungi, and trigger the neutralization of these organisms. Like most lectins, MBPs display weak intrinsic affinity for monovalent sugar ligands, but bind avidly to multivalent ligands.

RESULTS

We report physical studies in solution and the crystal structure determined at 1.8 A Bragg spacings of a trimeric fragment of MBP-A, containing the carbohydrate-recognition domain (CRD) and the neck domain that links the carboxy-terminal CRD to the collagen-like portion of the intact molecule. The neck consists of a parallel triple-stranded coiled coil of alpha-helices linked by four residues to the CRD. The isolated neck peptide does not form stable helices in aqueous solution. The previously characterized carbohydrate-binding sites lie at the distal end of the trimer and are separated from each other by 53 A.

CONCLUSIONS

The carbohydrate-binding sites in MBP-A are too far apart for a single trimer to bind multivalently to a typical mammalian high-mannose oligosaccharide. Thus MBPs can recognize pathogens selectively by binding avidly only to the widely spaced, repetitive sugar arrays on pathogenic cell surfaces. Sequence alignments reveal that other C-type lectins are likely to have a similar oligomeric structure, but differences in their detailed organization will have an important role in determining their interactions with oligosaccharides.

Human mannose-binding protein carbohydrate recognition domain trimerizes through a triple alpha-helical coiled-coil

Human mannose-binding protein is a hexamer of trimers with each subunit consisting of an amino-terminal region rich in cysteine, 19 collagen repeats, a 'neck', and a carbohydrate recognition domain that requires calcium to bind ligand. A 148-residue peptide, consisting of the 'neck' and carbohydrate recognition domains forms trimers in solution and in crystals. The structure of this trimeric peptide has been determined in two different crystal forms. The 'neck' forms a triple alpha-helical coiled-coil. Each alpha-helix interacts with a neighbouring carbohydrate recognition domain. The spatial arrangement of the carbohydrate recognition domains suggest how MBP trimers form the basic recognition unit for branched oligosaccharides on microorganisms.