Molecular cloning and characterization of mouse ficolin-A

Characterization of porcine plasma ficolins that bind Actinobacillus pleuropneumoniae serotype 5B

Ficolins are collagenous lectins implicated in resistance to infection by some micro-organisms with surfaces containing N-acetylglucosamine residues. Pigs have two known ficolin genes, alpha and beta, but ficolins in pig plasma appear in various electrophoretic forms, the origin and function of which are unclear. In this investigation the forms of ficolin in pig plasma that bind to the pneumonic pathogen Actinobacillus pleuropneumoniae serotype 5b (APP5) were compared with affinity-purified porcine plasma ficolins. APP5-bound ficolins consisted of two distinct multimeric (non-denatured) forms composed of subunits that migrated as multiple 38,40 and 42 kDa forms (apparent MW) with pI range 5.2-6.0. The APPS-binding forms of ficolin were consistent with ficolin alpha and were indistinguishable from affinity-purified plasma ficolins by peptide mass fingerprint analysis. Subunit bands separated by 2D-PAGE were consistent with porcine ficolin alpha. Affinity-purified plasma ficolins had a major subunit mass of 35081 Da by MALDI-TOF MS. Affinity-purified ficolins digested with N-glycosidase F retained APP5-binding activity and migrated faster as a single band of 38 kDa. These studies indicate that ficolin alpha is the major APPS-binding form in porcine plasma and suggest that N-glycosylation contributes to the apparent electrophoretic heterogeneity of porcine ficolins but is not required for APP5-binding activity.

Evolution of the lectin-complement pathway and its role in innate immunity

Discrimination between self and non-self by lectins (carbohydrate-binding proteins) is a strategy of innate immunity that is found in both vertebrates and invertebrates. In vertebrates, immune recognition mediated by ficolins (lectins that consist of a fibrinogen-like and a collagen-like domain), as well as by mannose-binding lectins, triggers the activation of the complement system, which results in the activation of novel serine proteases. The presence of a similar lectin-based complement system in ascidians, our closest invertebrate relatives, indicates that the complement system probably had a pivotal role in innate immunity before the evolution of an adaptive immune system in jawed vertebrates.

Cloning and characterization of novel ficolins from the solitary ascidian, Halocynthia roretzi

Ficolins are animal lectins with collagen-like and fibrinogen-like domains. They are involved in the first line of host defense against pathogens. Human ficolin/P35 as well as mannose-binding lectin (MBL) activates the complement lectin pathway in association with MBL-associated serine proteases. To elucidate the origin and evolution of ficolins, we separated approximately 40 kDa (p40) and approximately 50 kDa (p50) N-acetylglucosamine-binding lectins from hemolymph plasma of the solitary ascidian. Binding assays revealed that p40 recognizes N-acetyl groups in association with a pyranose ring and that p50 recognizes N-acetylglucosamine alone. Based on the amino acid sequences of the proteins, we isolated two clones each of p40 and p50 from the ascidian hepatopancreas cDNA and determined the entire coding sequences of these clones. Because all of the clones contained both collagen-like and fibrinogen-like domains, we concluded that these were homologs of the mammalian ficolin family and designated ascidian ficolins (AsFCNs). The fibrinogen-like domain of the AsFCNs shows 45.4-52.4% amino acid sequence identity with the mammalian ficolin family. A phylogenetic tree of the fibrinogen-like sequences shows that all the fibrinogen-like domains may have evolved from a common ancestor that branched off an authentic fibrinogen. These results suggest that AsFCNs play an important role with respect to ascidian hemolymph lectin activity and the correlation of different functions with binding specificity.

Hakata antigen, a new member of the ficolin/opsonin p35 family, is a novel human lectin secreted into bronchus/alveolus and bile

Hakata antigen was first reported as a serum protein that reacted with an autoantibody from patients with systemic lupus erythematosus. Recently, it has been found that Hakata antigen is a new member of the ficolin/opsonin p35 family, which is a distinct lectin family, on the basis of homology of structures and the common characteristic of possessing lectin activity. In this study we analyzed the tissue distribution of Hakata antigen. Hakata antigen mRNA and protein were generated in the lung and liver. In the lung, Hakata antigen was produced by both ciliated bronchial epithelial cells and Type II alveolar epithelial cells and was secreted into the bronchus and alveolus. In the liver, Hakata antigen was produced by bile duct epithelial cells and hepatocytes and was also secreted into the bile duct. These results demonstrate that Hakata antigen is a unique lectin protein that exists not only in serum but also in bronchus/alveolus and bile, and indicate that Hakata antigen plays a role in bronchus/alveolus and bile under physiological conditions.

Oligomeric structure and tissue distribution of ficolins from mouse, pig and human

Mouse plasma ficolin was purified by GlcNAc affinity and anion-exchange chromatography. Gel-filtration chromatography and gradient sedimentation indicated that mouse plasma ficolin is a 12-mer of approximately 35 kDa subunits, and electron microscopy showed the same parachute-like structure previously characterized for the pig ficolin 12-mer. Whereas the predominant form in pig plasma is a 24-mer, mouse and human plasma ficolin showed only the 12-mer form. We conclude that mouse plasma ficolin corresponds to the recently described ficolin A. We have identified a second mouse ficolin gene, ficolin B, which means that pig and mouse each have two ficolin genes, and human has three. One ficolin gene in all species is expressed in liver and is the primary source of plasma ficolin. Expression of this gene in other tissues, and expression of the second ficolin gene, appears to vary in different species.