THE CYTOPLASMIC DOMAIN OF RAT NKR-P1 RECEPTOR INTERACTS WITH THE N-TERMINAL DOMAIN OF P56(LCK) VIA CYSTEINE RESIDUES

NKR-P1 is a type II transmembrane protein which acts as an activation receptor on natural killer (NK) cells. The cytoplasmic domains of the CD4, CD8 and 4-1BB receptors contain the sequence Cys-X-Cys-Pro which is directly involved in coupling to another pair of cysteines in the N -terminal domain of the src family tyrosine kinase p56(lck). The cytop lasmic domain of NKR-P1 in rodents also contains the Cys-X-Cys-Pro seq uence, but the capacity of the receptor to bind p56(lck) is presently unknown. We tested for direct coupling between these proteins using bo th protein biochemistry and the yeast two-hybrid technique. Immunoprec ipitation studies showed that p56(lck) can be co-immunoprecipitated wi th NKR-P1 from a rat NK tumor cell line. In addition, the cytoplasmic domain of NKR-P1 interacted with the N-terminal domain of p56(lck) in yeast as assessed by reporter gene activation. Integrity of the cystei ne pairs in both proteins was critical in mediating the interaction. T he experiments suggest that the association of p56(lck) with NKR-P1 is somewhat weaker than the p56(lck) association with CD8 alpha, but of much lower avidity than between CD4 and p56(lck). This could reflect a higher activation threshold for the NKR-P1 and CD8 receptors, which a re involved in cytolytic responses, compared to CD4 which is involved in T cell helper function.