Sequence Variations in 185/333 Messages from the Purple Sea Urchin Suggest Posttranscriptional Modifications to Increase Immune Diversity

Abstract

The 185/333 gene family is highly expressed in two subsets of immune cells in the purple sea urchin in response to immune challenges. The genes encode a surprisingly diverse set of transcripts, which is a function of the variable presence or absence of blocks of shared sequences, known as elements that generate element patterns. Diversity is also the result of a significant level of point mutations. Together, variable element patterns and single nucleotide polymorphisms result in many unique transcripts. The 185/333 genes only have two exons, with the variable element patterns encoded entirely within the second exon. The diversity of the gene family may be the result of frequent recombination among the 185/333 genes that generates a mosaic distribution of element sequences among the genes. A comparative analysis of the sequences for the genes and messages from individual sea urchins indicates that these two sequence sets have largely different nucleotide sequences and appear to use different element patterns. Furthermore, the nucleotide substitution patterns between genes and messages reveal a strong bias toward transitions, particularly cytidine to uridine conversions. These data are consistent with cytidine deaminase activity and may represent a novel form of immunological diversification in an invertebrate immune response system.

Highly Variable Immune-Response Proteins (185/333) from the Sea Urchin,Strongylocentrotus purpuratus: Proteomic Analysis Identifies Diversity within and between Individuals

“…No such frameshifts have been identified in the 185/333 genes. However, comparisons of gene and mRNA sequences from individual animals suggest that posttranscriptional modifications may be responsible for these missense proteins (15). If these messages are translated, it would explain why the three different anti-185 antisera used in the present study detected different subsets of 185/333 proteins.…”

“…Another explanation for the discrepancy in molecular masses is that 185/333 proteins are glycosylated and form large complexes covalently linked to carbohydrates. There are numerous conserved sites for N-linked glycosylation within the histidine-rich region of the 185/333 proteins (elements [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25], and there are conserved sites for O-linked glycosylation in the carboxyl-terminal region (8,11). However, deglycosylation of N-linked oligosaccharides failed to decrease the molecular masses of 185/333 proteins to the size of predicted monomers (L. C. Smith, unpublished data).…”

“…Additionally, there are surprisingly high levels of frame shifts and the insertion of early stop codons in the mRNA sequences (11). The processes that generate this diversity among 185/333 mRNAs are not yet well defined, but they may include differences among the estimated fifty 185/333 gene loci (11,13,14), high levels of allelic polymorphism at each locus, RNA editing, and/or low-fidelity RNA polymerases (15), followed by posttranslational modifications. All of these processes may have been driven by positive selection associated with anti-pathogen defense (8,11,12,14).…”

“…All of these processes may have been driven by positive selection associated with anti-pathogen defense (8,11,12,14). Nucleotide sequence variability results in high levels of nonconservative amino acid substitutions among predicted proteins of the type consistent with intense evolutionary selection pressure (8,12,14,15).…”

Role for a somatically diversified lectin in resistance of an invertebrate to parasite infection

“…One of the insights emerging from these studies is that invertebrates, although they do not have adaptive immune systems in the sense of those possessed by jawed vertebrates, nonetheless do have sophisticated innate immune systems that have revealed an unforeseen ability to produce diversified antigen recognition molecules (3,4). Possession of expanded families of antigen recognition molecules (5), the ability to engage in extensive diversification of receptors by alternative splicing (4), somatic diversification achieved by gene conversion and point mutation (3), and the presence of complex genomic architecture conducive to the production of diversified antigen receptors (7) have all been shown in invertebrates, potentially blurring some of the traditional distinctions made between innate and adaptive immune systems in the process (8). It is not surprising that invertebrates have immune systems that are not limited to low numbers of pattern recognition receptors, given the diverse suites of pathogens with which they must cope, and the evident success of invertebrates in persisting abundantly over evolutionary time, in both species and numbers.…”