F-Type Lectins: Structure, Function, and Evolution

F-type lectins (FTLs) are characterized by a fucose recognition domain (F-type lectin domain; FTLD) that displays a novel jellyroll fold ("F-type" fold) and unique carbohydrate- and calcium-binding sequence motifs. This novel lectin family comprises widely distributed proteins exhibiting single, double, or greater multiples of the FTLD, either tandemly arrayed or combined with other structurally and functionally distinct domains. Further, differences in carbohydrate specificity among tandemly arrayed FTLDs present in any FTL polypeptide subunit, together with the expression of multiple FTL isoforms in a single individual supports a striking diversity in ligand recognition. Functions of FTLs in self/nonself recognition include innate immunity, fertilization, microbial adhesion, and pathogenesis, among others, revealing an extensive structural/functional diversification. The taxonomic distribution of FTLDs is surprisingly discontinuous, suggesting that this lectin family has been subject to secondary loss, lateral transfer, and functional co-option along evolutionary lineages.

Divergent immune roles of two fucolectin isoforms in Apostichopus japonicus

The F-type lectin (fucolectin) family represents a new group with innate immunity. In this study, two fucolectin isoforms (designated as AjFTL-1 and AjFTL-2) were identified in sea cucumber (Apostichopus japonicus) through rapid amplification of cDNA ends. Full-length cDNAs of AjFTL-1 and AjFTL-2 measured 2134 and 1286 bp, encoding two secreted proteins comprising 317 and 181 amino acid residues, respectively. The signal peptide, l-fucose binding motif ("HX(26)RXDX(4)R/K") and cation binding sequence motif ("h2DGx") were conserved in AjFTL-1 and AjFTL-2. However, AjFTL-1 contains an additional complement control protein domain. Multiple sequence alignments supported that AjFTL-1 and AjFTL-2 are new members of the F-type lectin family. Tissues distribution analysis indicated that both AjFTL-1 and AjFTL-2 were widely expressed in all tested tissues, featuring differential expression patterns. Vibrio splendidus infection in vivo can significantly upregulate the mRNA transcripts of the two genes, with a larger magnitude observed in AjFTL-1. By contrast, lipopolysaccharide stimulation in vitro can markedly induce the expression level of AjFTL-2 but not that of AjFTL-1. Silencing AjFTL-2 by siRNA can suppress the AjNOS transcript, whereas injection of the recombinant protein of AjFTL-2 can significantly induce AjNOS expression. By contrast, the loss- and gain-of-function of AjFTL-1 caused no effect on the expression of AjNOS. Our present study provides evidence supporting that AjFTL-1 and AjFTL-2 play diverse roles in the innate immune defense of sea cucumbers toward bacterial infection.

A novel fucolectin from Apostichopus japonicus with broad PAMP recognition pattern

F-type lectin (also known as fucolectin) is a newly identified family of fucose binding lectins with the sequence characters of a fucose binding motif and a unique lectin fold (the "F-type" fold). In the present study, a fucolectin was identified from sea cucumber Apostichopus japonicus (designated AjFL-1). The open reading frame (ORF) of AjFL-1 was of 546 bp, encoding a polypeptide of 181 amino acids with a predicted molecular mass of about 20 kDa. The deduced amino acid sequence of AjFL-1 shared 30%-40% similarity with the fucolectins from other animals. There were a typical F-type lectin domain (FLD) (residues 39-180) and a signal peptide (residues 1-24) in AjFL-1. The mRNA transcript of AjFL-1 could be detected by qRT-PCR in various tissues, such as intestinum, coelomocytes, respiratory tree, tentacle, and body wall, while undetectable in the gonads and longitudinal muscle. The mRNA expression level of AjFL-1 in coelomocytes was significantly up-regulated (47.06-fold to that in control group, p < 0.05) at 12 h after Vibrio splendidus challenge. Immunofluorescence assay showed that AjFL-1 protein was mainly distributed on the membrane, while few in cytoplasm of coelomocytes in sea cucumber. The recombinant AjFL-1 (rAjFL-1) could bind lipopolysaccharide (LPS), peptidoglycan (PGN), mannan (MAN) and fucose (FUC), and exhibited a broader binding activities towards Gram-negative bacterium Escherichia coli, Gram-positive bacterium Micrococcus luteus, as well fungus Pichia pastoris. In addition, rAjFL-1 could strongly promote the agglutination of fungus P. pastoris. These results indicated that AjFL-1 was a novel member of fucose-binding lectin family, which functioned as a pattern recognition receptor with broad spectrum of microbial recognition, and involved in innate immune response of sea cucumber.