In vivo composition of NMDA receptor signaling complexes differs between membrane subdomains and is modulated by PSD-95 and PSD-93.

[gm1 gangliosidosis]

Lipid rafts are dynamic membrane microdomains enriched in cholesterol and sphingolipids involved in the compartmentalization of signaling pathways , trafficking and sorting of proteins . At synapses , the glutamatergic NMDA receptor and its cytoplasmic scaffold protein PSD -95 move between postsynaptic density (PSD ) and rafts following learning or ischemia . However it is not known whether the signaling complexes formed by these proteins are different in rafts nor the molecular mechanisms that govern their localization . To examine these issues in vivo we used mice carrying genetically encoded tags for purification of protein complexes and specific mutations in NMDA receptors , PSD -95 and other postsynaptic scaffold proteins . Isolation of PSD -95 complexes from mice carrying tandem affinity purification tags showed differential composition in lipid rafts , postsynaptic density and detergent-soluble fractions . Raft PSD -95 complexes showed less CaMKIIalpha and SynGAP and enrichment in Src and Arc /Arg 3 .1 compared with PSD complexes . Mice carrying knock-outs of PSD -95 or PSD -93 show a key role for PSD -95 in localizing NR 2 A -containing NMDA receptor complexes to rafts . Deletion of the NR 2 A C terminus or the C-terminal valine residue of NR 2 B , which prevents all PDZ interactions , reduced the NR 1 association with rafts . Interestingly , the deletion of the NR 2 B valine residue increased the total amount of lipid rafts . These data show critical roles for scaffold proteins and their interactions with NMDA receptor subunits in organizing the differential expression in rafts and postsynaptic densities of synaptic signaling complexes .