Binding of synGAP to PDZ Domains of PSD-95 is Regulated by Phosphorylation and Shapes the Composition of the Postsynaptic Density

1 SynGAP is a Ras/Rap GTPase-activating protein (GAP) present in high concentration in postsynaptic 2 densities (PSDs) from mammalian forebrain where it binds to all three PDZ (PSD-95, Discs-large, ZO-1) 3 domains of PSD-95. We show that phosphorylation of synGAP by Ca 2+ /calmodulin-dependent protein 4 kinase II (CaMKII) decreases its affinity for the PDZ domains as much as 10-fold, measured by surface 5 plasmon resonance. SynGAP is abundant enough in postsynaptic densities (PSDs) to occupy about one 6 third of the PDZ domains of PSD-95. Therefore, we hypothesize that phosphorylation by CaMKII reduces 7 synGAP's ability to restrict binding of other proteins to the PDZ domains of PSD-95. We support this 8 hypothesis by showing that three critical postsynaptic signaling proteins that bind to the PDZ domains of 9 PSD-95 are present at a higher ratio to PSD-95 in PSDs isolated from synGAP heterozygous mice.