PSD-95 assembles a ternary complex with the N-methyl-D-aspartic acid receptor and a bivalent neuronal NO synthase PDZ domain

Nitric oxide (NO) biosynthesis in cerebellum is preferentially activated by calcium influx through N-methyl-D-aspartate (NMDA)-type glutamate receptor s, suggesting that there is a specific link between these receptors and neu ronal NO synthase (nNOS). Here, we find that PSD-95 assembles a postsynapti c protein complex containing nNOS and NMDA receptors. Formation of this com plex is mediated by the PDZ domains of PSD-95, which bind to the COOH termi ni of specific NMDA receptor subunits. In contrast, nNOS is recruited to th is complex by a novel PDZ-PDZ interaction in which PSD-95 recognizes an int ernal motif adjacent to the consensus nNOS PDZ domain. This internal motif is a structured "pseudo-peptide" extension of the nNOS PDZ that interacts w ith the peptide-binding pocket of PSD-95 PDZ2. This asymmetric interaction leaves the peptide-binding pocket of the nNOS PDZ domain available to inter act with additional COOH-terminal PDZ ligands. Accordingly, we find that th e nNOS PDZ domain can bind PSD-95 PDZ2 and a COOH-terminal peptide simultan eously. This bivalent nature of the nNOS PDZ domain further expands the sco pe for assembly of protein networks by PDZ domains.