The N-methyl-D-aspartate receptor (NMDAR) plays a critical role in synaptic plasticity. Post-translational modifications of NMDARs, such as phosphorylation, alter both the activity and trafficking properties of NMDARs. Ubiquitination is increasingly being recognized as another post-translational modification that can alter synaptic protein composition and function. We identified Mind bomb-2 as an E3 ubiquitin ligase that interacts with and ubiquitinates the NR2B subunit of the NMDAR in mammalian cells. The protein-protein interaction and the ubiquitination of the NR2B subunit were found to be enhanced in a Fyn phosphorylation-dependent manner. Immunocytochemical studies reveal that Mind bomb-2 is localized to postsynaptic sites and colocalizes with the NMDAR in apical dendrites of hippocampal neurons. Furthermore, we show that NMDAR activity is down-regulated by Mind bomb-2. These results identify a specific E3 ubiquitin ligase as a novel interactant with the NR2B subunit and suggest a possible mechanism for the regulation of NMDAR function involving both phosphorylation and ubiquitination.The N-methyl-D-aspartate receptors (NMDARs) 4 are glutamate-gated ion channels that are important in synaptic plasticity events in the mammalian brain, and are comprised of an obligatory NR1 subunit and modulatory NR2 (A-D) or NR3 subunits (1). While the NR1 subunit has a short intracellular tail, the NR2 subunits have large intracellular C-terminal tails that directly interact with proteins that play essential roles in the regulation of NMDAR function (2, 3).The intracellular domains of NMDAR subunits are phosphorylated by a number of protein kinases. For example, Src-family protein-tyrosine kinases (PTKs), such as Src and Fyn, phosphorylate specific tyrosine sites within the NR2A and NR2B intracellular tails (4 -7), and this positively modulates channel function (5, 7-12). Tyrosine phosphorylation of the NR2 subunits of the NMDAR is enhanced under a number of synaptic plasticityrelated events, including long-term potentiation (LTP) (13-15), fear-related learning (16), exposure to alcohol (17-19), and ischemia (20).Tyrosine phosphorylation also positively regulates the trafficking of NMDARs from intracellular compartments to the postsynaptic density (PSD) (10, 12), the stability of NMDARs at the synaptic membrane (21, 22) and controls the association of the NMDAR with spectrin (23). Tyrosine phosphorylation can also influence which proteins bind to the receptor. For example, phosphatidylinositol 3-kinase (PI 3-kinase), and phospholipase C-␥ (PLC-␥) bind to NR2 subunits in a tyrosine phosphorylation-dependent manner (24,25), and the increased association of PI 3-kinase with phosphorylated NMDARs has been suggested to contribute to altered signaling in the hippocampus after ischemia (20). Thus, tyrosine phosphorylation plays an essential role in both the regulation of NMDAR channel activity and NMDAR-mediated downstream signaling cascades.In addition to phosphorylation, ubiquitination is another post-translational modificatio...