Abstract
The F-type lectin (FTL) family is one of the most recent to be identified and structurally characterized. Members of the FTL family 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, yielding lectin subunits of pleiotropic properties even within a single species. Furthermore, the extraordinary variability of FTL sequences (isoforms) that are expressed in a single individual has revealed genetic mechanisms of diversification in ligand recognition that are unique to FTLs. Functions of FTLs in self/non-self-recognition include innate immunity, fertilization, microbial adhesion, and pathogenesis, among others. In addition, although the F-type fold is distinctive for FTLs, a structure-based search revealed apparently unrelated proteins with minor sequence similarity to FTLs that displayed the FTLD fold. In general, the phylogenetic analysis of FTLD sequences from viruses to mammals reveals clades that are consistent with the currently accepted taxonomy of extant species. However, the surprisingly discontinuous distribution of FTLDs within each taxonomic category suggests not only an extensive structural/functional diversification of the FTLs along evolutionary lineages but also that this intriguing lectin family has been subject to frequent gene duplication, secondary loss, lateral transfer, and functional co-option.
Highlights
Recognition of glycans exposed on the surface of microbial pathogens and parasites by the host’s cell-associated and soluble lectins is considered the initial key step in the innate immune response of both invertebrates and vertebrates (1–5)
The structure of the MsaFBP32/l-Fuc complex revealed that the overall structure of the N-F-type lectin domain (FTLD) is similar to that of the C-terminal FTLD (C-FTLD) and that recognition of l-Fuc by each FTLD is mediated by a repertoire of polar and apolar interactions similar to those observed in anguilla agglutinin (AAA) (Figure 3)
The structural and functional analyses of the FTLD, together with its distribution in extant viral, prokaryotic, and eukaryotic species reveal an intriguing evolutionary history of this lectin domain with key adaptations to a diverse array of functions carried out by the FTLD itself, either as single units or as tandemly arrayed domains. This functional diversity is further expanded for FTLDs associated with structurally and functionally distinct associated domains, either belonging to other lectin families (CTLs and PXNs), enzymes, or other proteins
Summary
Members of the FTL family 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, yielding lectin subunits of pleiotropic properties even within a single species. The F-type fold is distinctive for FTLs, a structure-based search revealed apparently unrelated proteins with minor sequence similarity to FTLs that displayed the FTLD fold.
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