Molecular basis for positive and negative signaling by the natural killer cell receptor 2B4 (CD244)

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Abstract
Triggering of 2B4 (CD244) can induce natural killer (NK)-cell activation, costimulation, or even inhibition of NK-cell activity. Here, we investigate the molecular basis for the different signals generated by 2B4. We show that the first immunoreceptor tyrosine-based switch motif (ITSM) within the cytoplasmic tail of 2B4 is sufficient for 2B4-mediated NK-cell activation, whereas the third ITSM can negatively influence 2B4 signaling. We further identify signaling molecules that associate with 2B4. Signaling lymphocyte activation molecule-associated protein (SAP) can bind to all 4 ITSMs of 2B4 in a phosphorylation-dependent manner. The phosphorylated third ITSM can additionally recruit the phosphatases SHP-1, SHP-2, SHIP, and the inhibitory kinase Csk. SAP acts as an inhibitor of interactions between 2B4 and these negative regulatory molecules, explaining how 2B4 inhibits NK-cell activation in the absence of functional SAP, as occurs in cells from patients with X-linked lymphoproliferative syndrome (XLP). Recently, another function for SAP was proposed: SAP can recruit the kinase Fyn to the SLAM (CD150) immune receptor. We now show that Fyn can also associate with phosphorylated 2B4. Finally, we demonstrate that Fyn and Csk can both phosphorylate 2B4, suggesting a possible mechanism of 2B4 phosphorylation.