The Mannan-Binding Lectin-Associated Serine Proteases (MASPs) and MAp19: Four Components of the Lectin Pathway Activation Complex Encoded by Two Genes

The lectin pathway of complement is considered to be the most ancient complement pathway as inferred from identification of ancient homologs of mannose-binding lectin (MBL) and MBL-associated serine proteases (MASPs) in some invertebrates. MBL homologs with galactose selectivity and an MASP3-like sequence also occur in bony fish, linking the evolution of the lectin complement pathway from invertebrates to higher vertebrates. However, these cannot be considered authentic complement components until confirmatory functional evidence is obtained. Here, we report the isolation and characterization of two MBL homologs from a cyprinid teleost, the common carp, Cyprinus carpio. One, designated GalBL, corresponds to the MBL-like molecule with the galactose specificity. The other is an authentic MBL with mannose specificity. Both were found to associate with a serine protease that cleaves native human C4 into C4b but not C4i with a hydrolyzed thioester. Molecular cloning and phylogenetic analysis revealed this C4-activating protease to be carp MASP2, indicating that MASP2 arose before the emergence of bony fish. Database mining of MBL-like genes reveals that MBL and GalBL genes are arranged in tandem in the zebrafish genome and that both lectins are conserved in the distantly related puffer fish. These results imply that bony fish have developed a diverged set of MBL homologs that function in the lectin complement pathway.

Section: Lectin Pathway Of Bony Fish Complement: Identification Of Twmentioning

confidence: 99%

Lectin Pathway of Bony Fish Complement: Identification of Two Homologs of the Mannose-Binding Lectin Associated with MASP2 in the Common Carp (Cyprinus carpio)

Nakao

Kajiya

Sato

et al. 2006

“…The complement activation following upon MBL binding to pathogens is dependent on MBL-associated serine proteases (MASPs) (11,12). After MASP binding to MBL, the complement cascade is activated via the formation of the C4b2a complex, which is able to generate and bind the opsonins C3b and iC3b, thereby facilitating opsonophagocytosis.…”

mentioning

confidence: 99%

Mannose-Binding Lectin (MBL) Facilitates Opsonophagocytosis of Yeasts but Not of Bacteria despite MBL Binding

Brouwer

Dolman

Houdt

et al. 2008

Mannose-binding lectin (MBL) is a serum protein of the innate immune system. After binding to a microorganism, MBL in complex with MBL-associated serine proteases activates the complement system, resulting in cleavage of complement factor C3. Cleaved C3 on the surface of the microorganism mediates opsonization for clearance, but the impact of MBL on subsequent phagocytosis has not been widely studied. We investigated the role of MBL in complement activation and phagocytosis of various bacteria and yeast species by flow cytometry. We measured both the C3 deposition during serum opsonization of fluorescent-labeled microorganisms as well as subsequent uptake of these microorganisms by human neutrophils. In MBL-deficient sera, a consistently decreased C3 deposition on both zymosan and Candida albicans was found and a reduced phagocytosis by neutrophils that was restored by exogenous MBL. This indicates that the lectin pathway of complement activation is important for the opsonophagocytosis of yeasts. In contrast, the C1q-dependent classical pathway dominated in the opsonization and phagocytosis of Staphylococcus aureus, Streptococcus pneumoniae, and Escherichia coli, whereas no effect of MBL was found. Both the lectin and the classical pathway of complement activation were highly amplified by the alternative route for opsonophagocytosis by neutrophils of yeast as well as microbial species. In summary, our data demonstrate that yeast species are preferentially opsonized and subsequently phagocytosed via activation of the lectin pathway of complement, whereas the uptake of bacterial strains was found to be largely MBL independent.

“…MASP-2 and sMAP are generated by alternative splicing from a single structural gene, and sMAP consists of the first CUB (CUB1) domain, the EGF-like domain, and an extra 4 aas at the C-terminal end encoded by a sMAPspecific exon. MASP-1 and MASP-3 are also generated from a single gene by alternative splicing (22). When MBL and ficolins bind to carbohydrates on the surface of microbes, the proenzyme form of MASP is cleaved between the second CCP and the protease domain, resulting in the active form that consists of two polypeptides called H and L chains, and thus acquiring proteolytic activities against complement components.…”

mentioning

confidence: 99%

Small Mannose-Binding Lectin-Associated Protein Plays a Regulatory Role in the Lectin Complement Pathway

Iwaki

Kanno

Takahashi

et al. 2006

Mannose-binding lectin (MBL) and ficolins are pattern recognition proteins acting in innate immunity, and they trigger the activation of the lectin complement pathway through MBL-associated serine proteases (MASPs). Upon activation of the lectin pathway, MASP-2 cleaves C4 and C2. A truncated form of MASP-2, named small MBL-associated protein (sMAP), is also associated with MBL/ficolin-MASP complexes. To clarify the role of sMAP, we have generated sMAP-deficient (sMAP−/−) mice by targeted disruption of the sMAP-specific exon. Because of the gene disruption, the expression level of MASP-2 was also decreased in sMAP−/− mice. When recombinant sMAP (rsMAP) and recombinant MASP-2 (rMASP-2) reconstituted the MBL-MASP-sMAP complex in deficient serum, the binding of these recombinant proteins to MBL was competitive, and the C4 cleavage activity of the MBL-MASP-sMAP complex was restored by the addition of rMASP-2, whereas the addition of rsMAP attenuated the activity. Therefore, MASP-2 is essential for the activation of C4 and sMAP plays a regulatory role in the activation of the lectin pathway.