The NMDA receptors (NMDARs) are ion channels through which Ca2+ influx triggers various intracellular responses. Tyrosine phosphorylation of NMDARs regulates NMDA channel activities, which may be important in neuronal plasticity. The biological significance of the tyrosine phosphorylation events, however, differs among NMDAR subunits: tyrosine phosphorylation of NMDARε1 increases NMDA channel activities, but that of NMDARε2 does not. Since signal transductions from various cell surface receptors are mediated by protein–protein interaction through phosphotyrosine and the Src homology 2 (SH2) domain, we examined the possibility that phosphotyrosines in NMDARε2 contribute to the intracellular signalling events.
We first show that Fyn is deeply involved in the phosphorylation of NMDARε2 and second that a phosphotyrosine in NMDARε2 interacts with the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3-kinase). Both the level of tyrosine phosphorylation on NMDARε2 and the amounts of the p85 subunit (p85) bound to NMDARε2 are decreased in Fyn-deficient mice. Moreover, we show that ischaemia stimulates the binding of p85 to phosphorylated NMDARε2, suggesting a physiological role of the phosphotyrosine/SH2–based interaction between NMDARε2 and p85 in the brain.
The tyrosine phosphorylation event on NMDARs is important in not only the regulation of its channel activity but also intracellular signalling mediated through the interaction of the NMDAR with SH2 domain-containing molecules.