Sumoylation plays important roles in the modulation of protein function, 
neurotransmission and plasticity, but the mechanisms regulating this 
post-translational system in neurons remain largely unknown. Here we demonstrate 
that the synaptic diffusion of Ubc9, the sole conjugating enzyme of the 
sumoylation pathway, is regulated by synaptic activity. We use restricted 
photobleaching/photoconversion of individual hippocampal spines to measure the 
diffusion properties of Ubc9 and show that it is regulated through an 
mGlu5R-dependent signalling pathway. Increasing synaptic activity with a GABAA 
receptor antagonist or directly activating mGlu5R increases the synaptic 
residency time of Ubc9 via a Gαq/PLC/Ca(2+)/PKC cascade. This activation 
promotes a transient synaptic trapping of Ubc9 through a PKC 
phosphorylation-dependent increase of Ubc9 recognition to phosphorylated 
substrates and consequently leads to the modulation of synaptic sumoylation. Our 
data demonstrate that Ubc9 diffusion is subject to activity-dependent regulatory 
processes and provide a mechanism for the dynamic changes in sumoylation 
occurring during synaptic transmission.