Synaptic activity bidirectionally regulates a novel sequence‐specific S‐Q phosphoproteome in neurons

Protein phosphorylation plays a critical role in neuronal transcription, translation, cell viability, and synaptic plasticity. In neurons, phospho‐enzymes and specific substrates directly link glutamate release and post‐synaptic depolarization to these cellular functions; however, many of these enzymes and their protein substrates remain uncharacterized or unidentified. In this article, we identify a novel, synaptically driven neuronal phosphoproteome characterized by a specific motif of serine/threonine‐glutamine ([S/T]‐Q, abbreviated as SQ). These SQ‐containing substrates are predominantly localized to dendrites, synapses, the soma; and activation of this SQ phosphoproteome by bicuculline application is induced via calcium influx through L‐type calcium channels. On the other hand, acute application of NMDA can inactivate this SQ phosphoproteome. We demonstrate that the SQ motif kinase Ataxia‐telangiectasia mutated can also localize to dendrites and dendritic spines, in addition to other subcellular compartments, and is activated by bicuculline application. Pharmacology studies indicate that Ataxia‐telangiectasia mutated and its sister kinase ataxia telangiectasia mutated and Rad3‐related up‐regulate these neuronal SQ substrates. Phosphoproteomics identified over 150 SQ‐containing substrates whose phosphorylation is bidirectionally regulated by synaptic activity.We discovered a novel phosphoproteome in neurons which is characterized by the phosphorylation of a specific motif (serine/threonine‐glutamine, abbreviated as SQ). The SQ phosphoproteome is activated directly by synaptic activity‐mediated Ca 2+ influx from L‐type calcium channels, and is localized to multiple subcellular domains, including dendritic shafts/spines. Substrates identified in this phosphoproteome exhibit diverse subcellular localizations and functions.