F-TYPE LECTINS: BIOCHEMICAL, GENETIC, AND TOPOLOGICAL CHARACTERIZATION OF A NOVEL LECTIN FAMILY IN LOWER VERTEBRATES

Abstract

In vertebrates, immunoglobulins have long been considered the primary means for recognition of potential pathogens since they are the product of an adaptive immune system capable of generating and selecting for the most efficient antibody when adequately primed. However, initiating a naive system requires signals from cells that employ invariant receptors rather than antibodies. These innate receptors appear to recognize repetitive polymers commonly found only in microbes. Frequently, these receptors are lectins specific for polysaccharides ubiquitous of the microbial surface. Lectins in the blood or lymph are widespread among metazoans while antibodies are a vertebrate innovation suggesting that lectins may be evolutionarily their functional precursors. Even in primitive jawed vertebrates, there is a complete adaptive immune system, but it is relatively inefficient in comparison to mammals. Therefore, lectins might have a prominent immune function in lower vertebrates comparable to antibodies. To test this, a teleost was surveyed for humoral and hepatic lectins. A fucose-specific lectin of 32 kDa (FBP32) was initially purified from the palmetto bass and upon sequencing indicated it was unlike other reported lectins. The primary structure is characterized by a tandem polypeptide motif (FBPL) with partial homology to a long pentraxin from a frog. An inflammatory challenge of bass to test if FBP32 behaved like a mammalian pentraxin indicated that the FBP32 transcript level increases, but protein levels appear constitutive. An extensive search using both molecular cloning and gene database queries revealed that FBP32 is a member of a diverse protein family reflecting varying concatenations of the FBPL and even present in a cell surface receptor, but of sporadic phylogenetic distribution most notably being absent in mammals. Analysis of FBP32s genic structure reveals that it is flanked by phase 1 introns, which may explain the domains ability to concatenate and shuffle to form mosaic proteins. In collaboration with experts, the tertiary structure of an FBPL including its fucose-binding site was elucidated revealing a novel lectin fold, the F-type lectin fold (FTL), that is shared by unrelated proteins. Characterization of FBPLs demonstrates that the study of mammals alone may not reveal the full extent of immune system innovation.