Mannan-binding lectin

From Wikipedia, the free encyclopedia

mannose-binding lectin (protein C) 2, soluble (opsonic defect)
Identifiers
Symbol	MBL2 MBL
HUGO	6922
Entrez	4153
OMIM	154545
RefSeq	NM_000242
UniProt	P11226
Other data
Locus	Chr. 10 q11.2

Mannose binding lectin (MBL), also named mannose- or mannan-binding protein MBP, is an important factor in the innate immunity. 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. 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 oligomer forms, there are indications that dimers and trimers are not biologically active and at least a tetramer form is needed for activation of complement.

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 found on the surface of many pathogens. For example, MBL has been show to bind to yeasts such as Candida albicans , to viruses such as HIV and influenza A, to many bacteria including Salmonella and Streptococci , and to parasites like Leishmania.

In order to active the complement system, MBL in the blood complexes with (binds to) another protein, a serine protease called MASPs (MBL-associated serine proteases). When MBL binds to its target (for example, mannose on the surface of a bacterium), the MASP protein functions like a convertase to clip C3 into C3a and C3b. C3 is abundant in the blood, so this happens very efficiently. The C3b fragments can then bind to the surface of the bacterium. C3b can cause the complement cascade can proceed in combining with other complement proteins to make a membrane attack complex, which causes lysis of pathogens and cells. C3b can also bind to complement receptors on phagocytes causing opsonization of pathogen.