Mannan-binding lectin

Mannose-binding lectin (protein C) 2, soluble (opsonic defect)
File:PBB Protein MBL2 image.jpg PDB rendering based on 1hup.
Available structures PDB Ortholog search: Template:Homologene2PDBe PDBe, Template:Homologene2uniprot RCSB List of PDB id codes 1hup​
Identifiers
Symbols	MBL2 ; COLEC1; HSMBPC; MBL; MBP; MBP1; MGC116832; MGC116833
External IDs	Template:OMIM5 Template:MGI HomoloGene: 88328
Gene ontology Molecular function Template:GNF GO Template:GNF GO Template:GNF GO Template:GNF GO Cellular component Template:GNF GO Template:GNF GO Template:GNF GO Biological process Template:GNF GO Template:GNF GO Template:GNF GO Template:GNF GO
RNA expression pattern
File:PBB GE MBL2 207256 at tn.png
More reference expression data
Orthologs
Template:GNF Ortholog box
Species	Human	Mouse
Entrez	n/a	n/a
Ensembl	n/a	n/a
UniProt	n/a	n/a
RefSeq (mRNA)	n/a	n/a
RefSeq (protein)	n/a	n/a
Location (UCSC)	n/a	n/a
PubMed search	n/a	n/a

Mannose binding lectin (MBL), also named mannose- or mannan-binding protein (MBP), is an important factor in innate immunity.

Function

MBL belongs to the class of collectins in the C-type lectin superfamily, whose function appears to be pattern recognition in the first line of defense in the pre-immune host.

MBL recognizes carbohydrate patterns, found on the surface of a large number of pathogenic micro-organisms, including bacteria, viruses, protozoa and fungi.

Binding of MBL to a micro-organism results in activation of the lectin pathway of the complement system.

Structure

MBL has an oligomeric structure (400-700 kDa), built of subunits that contain three identical peptide chains of 32 kDa each.

Although MBL can form several oligomeric forms, there are indications that dimers and trimers are not biologically active and at least a tetramer form is needed for activation of complement.

Activation

The complement system can be activated through three pathways the classical pathway, the alternative pathway, and the mannose-binding (MB) lectin pathway. The most-recently discovered mannose-binding lectin pathway activates complement through the mannose-binding lectin protein. MBL binds to carbohydrates (specifically Mannose and Fucose residues) found on the surface of many pathogens.

For example, MBL has been show to bind to:

• yeasts such as Candida albicans
• viruses such as HIV and influenza A
• many bacteria including Salmonella and Streptococci
• parasites like Leishmania

Complexes

MBL in the blood is complexed with (bound to) another protein, a serine protease called MASP-2 (MBL-associated serine protease).

In order to activate the complement system when MBL binds to its target (for example, mannose on the surface of a bacterium), the MASP protein functions to cleave the blood protein C4 into C4a and C4b. The C4b fragments can then bind to the surface of the bacterium, and initiate the formation of a C3 convertase.

The subsequent complement cascade catalyzed by C3 convertase results in creating a membrane attack complex, which causes lysis of the pathogen that MBL bound to.

Clinical significance

It is produced in the liver as a response to infection, and is part of many other factors termed acute phase proteins.

External links

• Mannan-Binding+Lectin at the US National Library of Medicine Medical Subject Headings (MeSH)

Further reading

• Sheriff, S., Chang, C.Y., Ezekowitz, R.A. (1994) Human mannose-binding protein carbohydrate recognition domain trimerizes through a triple alpha-helical coiled-coil. Nat.Struct.Biol. 1: 789-794
• Fraser IP, Koziel H, Ezekowitz RA (1998). "The serum mannose-binding protein and the macrophage mannose receptor are pattern recognition molecules that link innate and adaptive immunity". Semin. Immunol. 10 (5): 363–72. doi:10.1006/smim.1998.0141. PMID 9799711.CS1 maint: Multiple names: authors list (link)
• Ji X, Gewurz H, Spear GT (2005). "Mannose binding lectin (MBL) and HIV". Mol. Immunol. 42 (2): 145–52. doi:10.1016/j.molimm.2004.06.015. PMID 15488604.CS1 maint: Multiple names: authors list (link)
• Worthley DL, Bardy PG, Mullighan CG (2005). "Mannose-binding lectin: biology and clinical implications". Internal medicine journal. 35 (9): 548–55. doi:10.1111/j.1445-5994.2005.00908.x. PMID 16105157.CS1 maint: Multiple names: authors list (link)
• Worthley DL, Bardy PG, Gordon DL, Mullighan CG (2007). "Mannose-binding lectin and maladies of the bowel and liver". World J. Gastroenterol. 12 (40): 6420–8. PMID 17072973.CS1 maint: Multiple names: authors list (link)