Proline-directed phosphorylation and prolyl isomerization oppose each other to regulate PSD-95 ubiquitination and excitatory synaptic plasticity

PSD-95 is a key scaffolding protein in excitatory synapses, playing a critical role in stabilizing synaptic receptors. Post-translational modifications such as phosphorylation and ubiquitination have been shown to regulate PSD-95’s localization and stability. Threonine 19 (T19) phosphorylation has emerged as a key phosphorylation site involved in NMDAR-dependent LTD; however, its role in PSD-95 ubiquitination is unknown. This study aims to elucidate the role of T19 phosphorylation in regulating PSD-95 ubiquitination, and whether the peptidyl prolyl isomerase 1 (Pin1), a known phospho-T19 binding protein, modifies this process. Our results support the idea that T19 and S25 phosphorylation positively regulate PSD-95 ubiquitination. Furthermore, we found that the Pin1 binding to S25 lowered the phosphorylation state of T19 and prevented PSD-95 ubiquitination. Finally, whole-cell recordings show that GSK3 overexpression depressed baseline synaptic transmission and partially blocked NMDAR-LTD, and both effects were rescued by Pin1 overexpression. Together, these findings provide insights into the molecular mechanisms regulating PSD-95 stability at synapses.