An abundance of free regulatory (19 <i>S</i> ) proteasome particles regulates neuronal synapses

Abstract

The proteasome, the major protein-degradation machine in cells, regulates neuronal synapses and long-term information storage. Here, using super-resolution microscopy, we found that the two essential subcomplexes of the proteasome, the regulatory (19<i>S</i>) and catalytic (20<i>S</i>) particles, are differentially distributed within individual rat cortical neurons. We discovered an unexpected abundance of free 19<i>S</i> particles near synapses. The free neuronal 19<i>S</i> particles bind and deubiquitylate lysine 63-ubiquitin (Lys⁶³-ub), a non-proteasome-targeting ubiquitin linkage. Pull-down assays revealed a significant overrepresentation of synaptic molecules as Lys⁶³-ub interactors. Inhibition of the 19<i>S</i> deubiquitylase activity significantly altered excitatory synaptic transmission and reduced the synaptic availability of AMPA receptors at multiple trafficking points in a proteasome-independent manner. Together, these results reveal a moonlighting function of the regulatory proteasomal subcomplex near synapses.