An abundance of free regulatory (19 S ) proteasome particles regulates neuronal synapses

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 S ) and catalytic (20 S ) particles, are differentially distributed within individual rat cortical neurons. We discovered an unexpected abundance of free 19 S particles near synapses. The free neuronal 19 S particles bind and deubiquitylate lysine 63–ubiquitin (Lys 63 -ub), a non–proteasome-targeting ubiquitin linkage. Pull-down assays revealed a significant overrepresentation of synaptic molecules as Lys 63 -ub interactors. Inhibition of the 19 S 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.