Activity-Dependent Ubiquitination of GluA1 and GluA2 Regulates AMPA Receptor Intracellular Sorting and Degradation

AMPA receptors (AMPARs) have recently been shown to undergo post-translational ubiquitination in mammalian neurons. However, the underlying molecular mechanisms are poorly understood and remain controversial. Here, we report that all four AMPAR subunits (GluA1-4) are rapidly ubiquitinated upon brief application of AMPA or bicuculline in cultured neurons. This process is Ca²⁺ dependent and requires the activity of L-type voltage-gated Ca²⁺ channels and Ca²⁺/calmodulin-dependent kinase II. The ubiquitination of all subunits occurs exclusively on AMPARs located on the plasma membrane post-endocytosis. The sites of ubiquitination were mapped to Lys-868 in GluA1 and Lys-870/Lys-882 in GluA2 C-terminals. Mutation of these lysines did not affect basal surface expression or AMPA-induced internalization of GluA1 and GluA2 subunits. Instead, it reduced the intracellular trafficking of AMPARs to the late endosomes and thus protein degradation. These data indicate that ubiquitination is an important regulatory signal for controlling AMPAR function, which may be crucial for synaptic plasticity.