The FREP gene family in the snail Biomphalaria glabrata: additional members, and evidence consistent with alternative splicing and FREP retrosequences

Abstract

Fibrinogen-related proteins (FREPs) found in hemolymph of the snail Biomphalara glabrata are hypothesized to be involved in non-self recognition. Among 150 cloned FREP cDNAs examined, we have identified three additional FREP members, FREPs 3.3, 12.1 and 13.1, bringing the total of FREP subfamilies to 13. The new FREPs each encode two immunoglobulin superfamily domains and a fibrinogen domain. Additionally, five truncated cDNAs with >99% nucleotide identity in coding regions to FREPs 3.2, 12.1 or 13.1 were identified. The truncated forms, the first reported for FREPs, lack a partial exon, one complete exon, or two complete exons plus the 3′UTR. Our preferred hypothesis is that all five truncated cDNAs observed arise from alternative splicing of full-length FREP genes. Genomic sequences lacking at least two introns and corresponding to the 3′ ends of the cDNAs of FREP12.1 and its two truncated forms were also recovered. Although these could be the source of the truncated cDNAs, they are believed to be retrosequences.