Here we identify a major phosphorylation site on subunit NR2B that is phosphorylated by Ca 2؉ /calmodulindependent protein kinase II (CaM kinase II), an abundant protein kinase located at postsynaptic sites in glutamatergic synapses. For the initial identification of the site, we constructed a recombinant fusion protein containing 334 amino acids of the C terminus of the NR2B subunit and phosphorylated it with CaM kinase II in vitro. By peptide mapping, automated sequencing, and mass spectrometry, we identified the major site of phosphorylation on the fusion protein as Ser-383, corresponding to Ser-1303 of full-length NR2B. The K m for phosphorylation of this site in the fusion protein was ϳ50 nM, much lower than that of other known substrates for CaM kinase II, suggesting that the receptor is a high affinity substrate. We show that serine 1303 in the fulllength NR2B and/or the cognate site in NR2A is a major site of phosphorylation of the receptor both in the postsynaptic density fraction and in living hippocampal neurons.Plasticity in the strength of transmission at synapses is essential for higher order brain functions such as learning and memory (1). In the hippocampus and cortex, activation of the NMDA 1 subtype of glutamate receptors can trigger long lasting changes in synaptic strength. These changes include long term potentiation (2) and long term depression (3). NMDA receptors contain two classes of subunits in hetero-oligomeric associations, the core NR1 subunit and the regulatory NR2 (A-D) subunits (4, 5). The NR2 subunits are equipped with uniquely long carboxyl-terminal tails that are believed to extend into the cytoplasm (6). These tails may participate in transduction mechanisms or in forms of regulation of the NMDA receptor that are not yet fully understood. NR2A and NR2B are the major regulatory subunits of NMDA receptors in the forebrain (4, 5). Previous work from this laboratory has shown that NR2B is the principal NMDA receptor subunit found in the postsynaptic density fraction prepared from forebrain (7), suggesting that this subunit may participate in anchoring the NMDA receptor at postsynaptic sites. Indeed, recent work from our lab and the Seeburg lab (8) has demonstrated a direct association between NR2B and the postsynaptic density protein PSD-95.NMDA receptors are ligand and voltage-gated Ca 2ϩ channels (9, 10). Binding of glutamate released from the presynaptic terminal, coupled with strong depolarization of the postsynaptic membrane produces an influx of Ca 2ϩ into the postsynaptic compartment (11). This Ca 2ϩ influx initiates a wide array of biochemical events in the synapse that can lead to long term potentiation or long term depression (12, 13). One potential target for immediate activation by this Ca 2ϩ influx is CaM kinase II. CaM kinase II has long been known to be concentrated in the postsynaptic density (14 -17) and to be essential for expression of both long term potentiation and long term depression (18 -20). One target for regulation by CaM kinase II at the postsynaptic si...

A novel interfacial route has been developed for the synthesis of a bright-red-emitting new subnanocluster, Au(23), by the core etching of a widely explored and more stable cluster, Au(25)SG(18) (in which SG is glutathione thiolate). A slight modification of this procedure results in the formation of two other known subnanoclusters, Au(22) and Au(33). Whereas Au(22) and Au(23) are water soluble and brightly fluorescent with quantum yields of 2.5 and 1.3 %, respectively, Au(33) is organic soluble and less fluorescent, with a quantum yield of 0.1 %. Au(23) exhibits quenching of fluorescence selectively in the presence of Cu(2+) ions and it can therefore be used as a metal-ion sensor. Aqueous- to organic-phase transfer of Au(23) has been carried out with fluorescence enhancement. Solvent dependency on the fluorescence of Au(23) before and after phase transfer has been studied extensively and the quantum yield of the cluster varies with the solvent used. The temperature response of Au(23) emission has been demonstrated. The inherent fluorescence of Au(23) was used for imaging human hepatoma cells by employing the avidin-biotin interaction.

Gold nanoparticles of 20-100 nm diameter were synthesized within HEK-293 (human embryonic kidney), HeLa (human cervical cancer), SiHa (human cervical cancer), and SKNSH (human neuroblastoma) cells. Incubation of 1 mM tetrachloroaurate solution, prepared in phosphate buffered saline (PBS), pH 7.4, with human cells grown to approximately 80% confluency yielded systematic growth of nanoparticles over a period of 96 h. The cells, stained due to nanoparticle growth, were adherent to the bottom of the wells of the tissue culture plates, with their morphology preserved, indicating that the cell membrane was intact. Transmission electron microscopy of ultrathin sections showed the presence of nanoparticles within the cytoplasm and in the nucleus, the latter being much smaller in dimension. Scanning near field microscopic images confirmed the growth of large particles within the cytoplasm. Normal cells gave UV-visible signatures of higher intensity than the cancer cells. Differences in the cellular metabolism of cancer and noncancer cells were manifested, presumably in their ability to carry out the reduction process.

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