NMDA receptors (NMDARs) are composed of NR1, NR2, and NR3 subunits. We previously identified p16 as a protein whose expression is upregulated in the NR3A KO brain compared to WT. Interestingly, forced expression of p16 in cultured cortical and hippocampal neurons results in an increase in NMDA-evoked currents, but not in AMPA- or GABA-evoked currents (Tu et al., 2003, SfN abstract 796.2). Since proteins that modulate NMDAR activity are often directly or indirectly associated with the NMDAR, we decided to examine this possibility. Based on the analysis of protein motifs using the Scansite 2.0 program (Obenauer et al., 2003), the C-terminal end of p16 was predicted to bind class I PDZ domains. We thus hypothesized that p16 might interact with PSD-95, which binds to NR2 and contains PDZ domains. We co-expressed p16-EGFP (a fusion protein of p16 and EGFP) and PSD-95 in COS-7 cells, and performed co-immunoprecipitation experiments. The experiments showed that these two proteins associate each other in these cel...