The SpTransformer Gene Family (Formerly Sp185/333) in the Purple Sea Urchin and the Functional Diversity of the Anti-Pathogen rSpTransformer-E1 Protein

Abstract

The complex innate immune system of sea urchins is underpinned by several multigene families including the SpTransformer family (SpTrf; formerly Sp185/333) with estimates of ~50 members, although the family size is likely variable among individuals of Strongylocentrotus purpuratus. The genes are small with similar structure, are tightly clustered, and have several types of repeats in the second of two exons and that surround each gene. The density of repeats suggests that the genes are positioned within regions of genomic instability, which may be required to drive sequence diversification. The second exon encodes the mature protein and is composed of blocks of sequence called elements that are present in mosaics of defined element patterns and are the major source of sequence diversity. The SpTrf genes respond swiftly to immune challenge, but only a single gene is expressed per phagocyte. Many of the mRNAs appear to be edited and encode proteins with altered and/or missense sequence that are often truncated, of which some may be functional. The standard SpTrf protein structure is an N-terminal glycine-rich region, a central RGD motif, a histidine-rich region, and a C-terminal region. Function is predicted from a recombinant protein, rSpTransformer-E1 (rSpTrf-E1), which binds to Vibrio and Saccharomyces, but not to Bacillus, and binds tightly to lipopolysaccharide, β-1,3-glucan, and flagellin, but not to peptidoglycan. rSpTrf-E1 is intrinsically disordered but transforms to α helical structure in the presence of binding targets including lipopolysaccharide, which may underpin the characteristics of binding to multiple targets. SpTrf proteins associate with coelomocyte membranes, and rSpTrf-E1 binds specifically to phosphatidic acid (PA). When rSpTrf-E1 is bound to PA in liposome membranes, it induces morphological changes in liposomes that correlate with PA clustering and leakage of luminal contents, and it extracts or removes PA from the bilayer. The multitasking activities of rSpTrf-E1 infer multiple and perhaps overlapping activities for the hundreds of native SpTrf proteins that are produced by individual sea urchins. This likely generates a flexible and highly protective immune system for the sea urchin in its marine habitat that it shares with broad arrays of microbes that may be pathogens and opportunists.

An Organismal Model for Gene Regulatory Networks in the Gut-Associated Immune Response

“…These include phagocytic cell types (a subset of filopodial cells and rarer, motile ovoid cells that appear upon acute immune challenge), highly motile amoeboid cells travel rapidly throughout the blastocoel, interacting with other immune cells and epithelia, and globular cells , a set of motile vesicular cell that are marked by expression of perforin/MPEG-like genes ( 17 , 25 ). The phagocytic filopodial cells express the sea urchin-specific Trf genes in response to bacterial challenge, which parallels similar responses in adult phagocytic coelomocytes ( 34 ). Together, this assemblage of immune cell types dynamically interacts in the course of larval immune response.…”

“…These and other immune innovations within the echinoderm lineage [e.g., the transformer (Trf, 185/333) proteins; reviewed in Ref. ( 34 )] highlight the diversification of proteins that potentially interact directly with pathogens, as has been observed in other systems ( 11 ) and provide a rich platform to study the integration of these quickly evolving proteins with more conserved elements of regulatory circuitry.…”

SpTransformer proteins from the purple sea urchin opsonize bacteria, augment phagocytosis, and retard bacterial growth

“…There are multiple levels at which diversification of the SpTrf system are thought to occur (reviewed in [ 72 ]). i) SpTrf gene sequences may be diversified through genomic instability that is the outcome of tight clusters, segmental duplications, multiple types of repeats, shared sequences among genes, and short tandem repeats surrounding each gene.…”

Echinochrome A Release by Red Spherule Cells Is an Iron-Withholding Strategy of Sea Urchin Innate Immunity

“…Cell-derived immunity in sea urchins is provided by the coelomocytes -a heterogeneous population consisting of 4 distinct morphotypes: phagocytes, vibratile cells, and colourless and red spherule cells. The former can be subdivided into discoidal, polygonal, and small phagocytes, which express a myriad of immune effectors belonging to the (Sp)Transformer gene family [6,7]. The phagocytes are tasked with identifying, ingesting, and destroying invading pathogens, whereas the vibratile cells are said to be involved in hemostasis [8][9][10].…”