Dual palmitoylation of PSD-95 mediates its vesiculotubular sorting, postsynaptic targeting, and ion channel clustering

Postsynaptic density-95 (PSD-95/SAP-90) is a palmitoylated peripheral membr ane protein that scaffolds ion channels at excitatory synapses. To elucidat e mechanisms for postsynaptic ion channel clustering, we analyzed the cellu lar trafficking of PSD-95,We find that PSD-95 transiently associates with a perinuclear membranous compartment and traffics with vesiculotubular struc tures, which migrate in a microtubule-dependent manner. Trafficking of PSD- 95 with these vesiculotubular structures requires dual palmitoylation, whic h is specified by five consecutive hydrophobic residues at the NH2 terminus . Mutations that disrupt dual palmitoylation of PSD-95 block both ion chann el clustering by PSD-95 and its synaptic targeting. Replacing the palmitoyl ated NH2 terminus of PSD-95 with alternative palmitoylation motifs at eithe r the NH2 or COOH termini restores ion channel clustering also induces post synaptic targeting, respectively. In brain, we find that PSD-95 occurs not only at PSDs but also in association with intracellular smooth tubular stru ctures in dendrites and spines. These data imply that PSD-95 is an itineran t vesicular protein; initial targeting of PSD-95 to an intracellular membra ne compartment may participate in postsynaptic ion channel clustering by PS D-95.