Sequence variability of fibrinogen-related proteins (FREPs) in Crassostrea gigas

Fibrinogen-related proteins(FREPs) are lectins with at least a fibrinogen-like domain.They are widespread in Mollusca and play an important role as immune pattern-recognition receptors.These diverse molecules provide interesting models for the study of the evolution of innate immunity.Although oysters are among the most studied mollusks,there have been few investigations of this gene family in oysters.This paper reports the cloning of the first oyster FREP,named CgFREP,from the Pacific oyster Crassostrea gigas.Five types of FREP(CgFREP-1 to CgFREP-5) were obtained based on a single pair of primers,confirming their high diversity.The proteins deduced from these FREPs contained 253-363 amino acids and were divided into three subfamilies based on sequence identities(cutoff point <86%).The topological structure of the phylogenetic tree showed the five CgFREPs clustered together and were completely separated from the subgroup of FREPs obtained from Biomphalaria glabrata(BgFREPs).This implies that FREP diversity evolved before the BgFREPs branched off.An interesting aspect of the CgFREPs was their individual hypermutation.Ninety-three SNPs and five indels were characterized among CgFREPs.Some mutations(e.g.an insertion in CgFREP-5) could cause premature translation termination.Multiple alignments suggest recombination events might have occurred during exchanges between alleles,or by somatic diversification,or both.Individual sequence variability parallels the extraordinary complexity of the innate immune system,suggesting a potential capacity to recognize and eliminate many kinds of pathogens,even in a primitive system.The diversity of immune pattern-recognition receptors may offer an alternative strategy that functions in the absence of highly specific immune globulins,which only emerged in vertebrates.