Visualizing Phosphorylation

Abstract

Treanor et al. explored the organization of natural killer (NK) cell inhibitory receptors, such as killer Ig-like receptors (KIRs), which serve to prevent cytotoxic action toward cells displaying self antigens. NK cells, like T cells, form an immunological synapse at the site of contact between the target cell and the NK cell. Engagement of major histocompatibility complex (MHC) class I with self peptides triggers inhibitory signals mediated through phosphorylation of immunoreceptor tyrosine-based inhibition motifs (ITIMs) on KIRs, which recruit phosphatases preventing activation of the cytotoxic response. Treanor et al. used fluorescence lifetime imaging (FLIM) of Förster resonance energy transfer between KIR fused to green fluorescent protein (GFP) and a generic antibody against phosphotyrosine labeled with a fluorescence acceptor to show that phosphorylation of KIR was restricted to microclusters in the immunological synapse and that KIR phosphorylation did not spread outside the immunological synapse. When in contact with two cells, KIR phosphorylation was restricted to the immunological synapses between each cell. Thus, NK cell inhibition can be restricted to the site of contact with one target cell and leave the NK cell free to continue surveillance of additional cells, allowing the NK cell to respond appropriately if it encounters a nonself-presenting cell. Beyond the importance of these results for understanding immune responses, the authors describe a method that can be applied to the study of local activation of any tyrosine-phosphorylated protein. B. Treanor, P. M. P. Lanigan, S. Kumar, C. Dunsby, I. Munro, E. Auksorius, F. J. Culley, M. A. Purbhoo, D. Phillips, M. A. A. Neil, D. N. Burshtyn, P. M. W. French, D. M. Davis, Microclusters of inhibitor killer immunoglobulin-like receptor signaling at natural killer cell immunological synapses. J. Cell. Biol. 174 , 153-161 (2006). [Abstract] [Full Text]