Ficolins
- Model of Ficolin L tetramer of trimers
Each structural unit is a trimer of ficolin polypeptides stabilized by a collagen-like triple helix. The C-terminal fibrinogen-like domains are clustered together at one end of the trimer. Cysteine residues in the N-terminal domains of the polypeptides in a trimer form disulphide bonds with equivalent residues in other trimeric units to create higher-order assemblies such as the tetramer of trimers depicted above.
- Evolution of ficolins
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- Filled circles highlight key branch points in the evolution of mammalian ficolins. The human ficolins L and M are the products of a recent gene duplication whereas ficolin H is more distantly related. Independent ficolin gene expansions have occurred in the mammalian, Xenopus, and sea squirt lineages.
- Evolutionary tree taken from Endo et al, Genomics 84, 737.
Structure of TL-5A CRD with bound GlcNAc
- GlcNAc and Ca2+ are shown in dark blue. GlcNAc is bound primarily through the acetyl moiety with limited contacts to the sugar ring. Protein Data Bank structure ID: 1JC9.
Introduction
The ficolins are a group of soluble animal proteins with roles in innate immunity. The classification of ficolins as lectins is controversial because the ligand binding domain in ficolins is specific for acetyl groups in both carbohydrates (eg GlcNAc, ManNAc, GalNAc) and non-carbohydrates (eg N-acetylglycine, N-acetylcysteine, acetylcholine). Binding of sugars is not primarily dependent on the sugar ring, and sugars which do not contain acetyl groups are generally not ficolin ligands. However, many of the bacterial surface molecules which appear to be natural ligands for the ficolins contain carbohydrate moieties, and ficolins have similar functional properties to lectins.
Structure and function
A ficolin polypeptide consists of a small N-terminal domain, a collagen-like domain, a neck region, and a fibrinogen-like domain, which shows similarity to the C-terminal halves of the beta and gamma chains of fibrinogen. The collagen-like domain mediates the association of ficolin polypeptides into trimers, and the N-terminal domain contains cysteine residues which permit the covalent assembly of trimers into higher oligomers with a 'bouquet-like' appearance. This supramolecular organization resembles that of the collectins, a group of C-type lectins which have a C-type CRD in place of the fibrinogen-like domain found in ficolins. Collectins and ficolins are also functionally similar. The collectin mannose binding protein (MBP) is a serum host defence protein in which the C-type CRDs recognize arrays of GlcNAc and mannose residues on pathogen surfaces. MBP initiates the lectin branch of the complement system via activation of MBP-associated proteases (MASPs), leading to elimination of the target pathogen. Two of the three human ficolins, ficolins L and H, are also serum proteins which bind to pathogen surfaces via interaction with carbohydrates (and probably with other molecules), and trigger complement activation though association with MASPs. Ficolin L also acts as an opsonin, promoting phagocytosis of pathogens by neutrophils. Ficolin L polymorphisms affect serum protein levels and sugar binding and may have pathophysiological implications. The third human ficolin, ficolin M, is found in secretory granules in neutrophils and monocytes, recognizes pathogens in a carbohydrate-dependent manner and activates complement via MASPs. Ficolin M may also act as a phagocytic receptor. Ficolins L and H are produced in the liver, in common with MBP, and ficolins M and H are produced in the lung, like the antimicrobial collectins SP-A and SP-D. Human ficolins and MBP also participate in the recognition and clearance of apoptotic cells. Two ficolins, A and B, are present in mouse. Ficolin B is found in the lysosomes of activated macrophages and is suggested to be the ortholog of ficolin M, but it appears that only ficolin A is associated with MASPs and can activate complement. The mouse ortholog of ficolin H is a pseudogene.
Evolution
Ficolins are present in mammals and in the amphibian Xenopus laevis. A protein containing a fibrinogen-like domain in conjunction with a collagen-like region has been predicted from a bird genome (Gallus gallus), but no such proteins have been predicted from the genomes of fish species. Ficolins are also present in the body fluids of the sea squirt Halocynthia roretzi, a chordate invertebrate organism. The ficolins in sea squirt differ slightly in domain organization from those in higher animals, having a shorter collagen-like region and a longer neck region. With some exceptions, ficolins in non-human species are not true orthologs of any of the human ficolins, although they may perform equivalent functions. Ficolins are not present in the model invertebrates C elegans and Drosophila, but two proteins from the horseshoe crab Tachypleus tridentatus have fibrinogen-like ligand binding domains which display high similarity to the fibrinogen-like domains in mammalian ficolins, although they are not present in conjunction with collagen-like regions. These 'tachylectins', TL-5A and TL-5B, are specific for acetyl groups in both carbohydrate and non-carbohydrate molecules, form propeller-like oligomers, and are a key part of the innate immune system in the horseshoe crab.
Ligand binding
A crystal structure has been solved for TL-5A in complex with GlcNAc. The bound calcium ion in the structure is likely to be present in all ficolin fibrinogen-like domains. In TL-5A, the ligand binding site is connected to the calcium ion site by a disulphide bond, potentially accounting for the calcium dependence of ligand binding. The ligand binding site is a hydrophobic funnel of aromatic residues with an alanine residue at the base. The acetyl group of the GlcNAc molecule is accommodated inside the funnel, and some contacts are also made to the hexose ring. The residues involved are moderately- or well-conserved among ficolins. However, an unpublished structure of human ficolin L in complex with GlcNAc shows the sugar molecule bound at the opposite side of the fibrinogen-like domain than is seen in the TL-5A structure. It may be the case that different mechanisms of ligand binding operate in different fibrinogen-like domains.
- Human ficolin synonyms
- Ficolin M: ficolin 1, ficolin A, p35-related protein
- Ficolin L: ficolin 2, ficolin B, hucolin, EBP-37, p35
- Ficolin H: ficolin 3, Hakata antigen
