We also demonstrate that recombinant forms of zebrafish DICP Ig domains bind lipids, which is a shared characteristic with members of the mammalian CD300 and TREM families of innate immune receptors. We describe herein the genomic organization, sequence complexity and predicted protein structures of the DICPs in zebrafish, which likely are unique to bony fish. Through genome scanning utilizing MDIR and NITR Ig domain sequences, an additional multigene family encoding diverse Ig domain-containing proteins (DICPs) was identified. A direct cloning strategy identified a distantly related multigene family (modular domain immune-type receptors ). NITRs function in allogeneic recognition in a manner akin to activating/inhibitory NK receptors. NITRs are the most complex family of V region-containing immune-type receptors described thus far outside of immunoglobulin (Ig) and T cell antigen receptors (TCRs). We have described variable (V) region-containing transmembrane receptors (novel immune-type receptors ) in zebrafish and other bony fish. Of the various nonmammalian animal models in which these molecules have been identified, the zebrafish ( Danio rerio) offers many unique methodological advantages. The presence of such multigene families in modern representatives of phylogenetically important species emphasizes their significance. It is likely that at least some receptors encoded by these genes are elements of unrecognized receptor-signaling networks and function through novel mechanisms. Many of these genes are encoded in multigene families and exhibit patterns of structural variation that are predicted to be associated with functional differences. It is more difficult to recognize common features of other receptors that are classified as immune-type because of their structural domain composition and signaling properties. These findings suggest that novel multigene families encoding diversified immune receptors have arisen in different vertebrate lineages and effect parallel patterns of ligand recognition that potentially impact species-specific advantages.Īs the phylogenetically widely divergent species in which immune receptors have been characterized increases, several major trends can be recognized: 1) innate immune receptors have a long evolutionary history with marked similarities in receptor structure and function across wide phylogenetic boundaries, 2) primary mediators of adaptive immunity have undergone many changes during the evolution of vertebrates but share remarkable similarities in basic aspects of genetic recombination (rearrangement) and clonal selection and 3) structures of receptors that mediate natural killer (NK)-type function can vary markedly even within members of a single class of vertebrate species (mammals). Recombinant DICP Ig domains bind lipids, a property shared by mammalian CD300 and TREM family members. Sequence differences between D1 domains are concentrated in hypervariable regions on the front sheet strands of the Ig fold. Molecular models indicate that most D1 domains are of the variable (V) type D2 domains are Ig-like. Interindividual polymorphism and alternative RNA processing contribute to DICP diversity. The sequence and number of Ig domains, transmembrane regions and signaling motifs varies between DICPs. Twenty-nine distinct loci mapping to three chromosomal regions encode receptor-type structures possessing two classes of Ig ectodomains (D1 and D2). A heretofore-unrecognized multigene family encoding diverse immunoglobulin (Ig) domain-containing proteins (DICPs) was identified in the zebrafish genome.
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