Abstract
Both inhibitory and activating forms of natural killer (NK) cell receptors are found in mammals. The activating receptors play a direct role in the recognition of virally infected or transformed cells and transduce activating signals into the cell by partnering with an adaptor protein, which contains a cytoplasmic activation motif. Activating NK receptors encoded by the mammalian leukocyte receptor complex (e.g., killer cell immunoglobulin-like receptors) and the natural killer complex (e.g., Ly49s) partner with the adaptor protein DAP12, whereas NK receptors encoded in the CD94/NKG2 complex partner with the adaptor protein DAP10. Novel immune-type receptors (NITRs) found in bony fish share several common features with immunoglobulin-type NK receptors. Nitr9 is a putative activating receptor in zebrafish that induces cytotoxicity within the context of human NK cells. One isoform of Nitr9, Nitr9L, is shown here to preferentially partner with a zebrafish ortholog of Dap12. Cross-linking the Nitr9L-Dap12 complex results in activation of the phosphytidylinositol 3-kinase-->AKT-->extracellular signal-regulated kinase pathway suggesting that the DAP12-based activating pathway is conserved between bony fish and mammals.