Synaptic Targeting and Function of SAPAPs Mediated by Phosphorylation-Dependent Binding to PSD-95 MAGUKs

Abstract

The PSD-95/SAPAP/Shank complex functions as the major scaffold in orchestrating the formation and plasticity of the post-synaptic densities (PSDs). We previously demonstrated that the exquisitely specific SAPAP/Shank interaction is critical for Shank synaptic targeting and Shank-mediated synaptogenesis. Here, we show that the PSD-95/SAPAP interaction, SAPAP synaptic targeting, and SAPAP-mediated synaptogenesis require phosphorylation of the N-terminal repeat sequences of SAPAPs. The atomic structure of the PSD-95 guanylate kinase (GK) in complex with a phosphor-SAPAP repeat peptide, together with biochemical studies, reveals the molecular mechanism underlying the phosphorylation-dependent PSD-95/SAPAP interaction, and it also provides an explanation of a PSD-95 mutation found in patients with intellectual disabilities. Guided by the structural data, we developed potent non-phosphorylated GK inhibitory peptides capable of blocking the PSD-95/SAPAP interaction and interfering with PSD-95/SAPAP-mediated synaptic maturation and strength. These peptides are genetically encodable for investigating the functions of the PSD-95/SAPAP interaction in vivo. Using structural biology, cell biology, and electrophysiology approaches, Zhu et al. demonstrate that phosphorylation of the N-terminal repeating sequences of SAPAPs is required for the SAPAP/PSD-95 complex formation and SAPAP's synaptic targeting and maturation functions. They also developed a potent non-phosphorylated PSD-95 GK inhibitory peptide that can effectively disrupt the SAPAP/PSD-95 complex formation and thus inhibit excitatory synaptic activities. Keywords: GK domain; PSD-95; SAPAP; MAGUK; postsynaptic density; synaptic scaffold proteins; synaptogenesis; synaptic plasticity