Unexpected diversity displayed in cDNAs expressed by the immune cells of the purple sea urchin, Strongylocentrotus purpuratus

We recently identified a unique family of transcripts, the 185/333 family, that comprise ∼60% of the mRNAs induced by coelomocytes from the purple sea urchin in response to immunological challenge from lipopolysaccharide. An analysis of 81 full-length cDNAs revealed 67 unique nucleotide sequences encoding 64 different proteins. Diversity of the transcripts was based on 25 sequence blocks, or “elements,” which resulted in 22 different element patterns based on their presence or absence. Furthermore, there was a high level of nucleotide variation within elements, including single nucleotide polymorphisms and insertions/deletions, both of which resulted in amino acid sequence variability. The deduced 185/333 proteins contained an NH2-terminal leader, a glycine-rich region with an RGD motif, a histidine-rich region, and a COOH-terminal region. Two 185/333 genes, identified in the partially assembled Strongylocentrotus purpuratus genome, have two exons. The first encoded the leader, and the second encoded the remainder of the predicted protein. Estimates from quantitative PCR indicated that there were ∼100 alleles in the diploid genome. These results suggested that the purple sea urchin may have mechanisms for generating high levels of diversity in response to immunological challenge that have not been considered previously.