Activity-dependent coordination of protein synthesis and protein degradation through a neuronal-specific plasma membrane 20S proteasome complex

[摘要] In the nervous system, rapidly occurring processes such as neuronal transmission and calcium signaling are affected by short-term inhibition of proteasome function. It is unclear how proteasomes are able to acutely regulate such processes, as this action is inconsistent with their canonical role in proteostasis. We discovered a mammalian nervous-system-specific membrane 20S proteasome complex that directly and rapidly modulates neuronal function by degrading intracellular proteins into extracellular peptides that can stimulate neuronal signaling. This proteasome complex is closely associated with neuronal plasma membranes, exposed to the extracellular space, and catalytically active. Selective inhibition of the membrane proteasome complex by a cell-impermeable proteasome inhibitor blocked the production of extracellular peptides and attenuated neuronal-activity-induced calcium signaling. Moreover, we observed that membrane-proteasome-derived peptides were sufficient to induce neuronal calcium signaling. Analyzing the composition of the neuronal membrane proteasome (NMP), we did not find canonical ubiquitin-proteasome components required for recognizing a ubiquitiylated protein. This raised the fundamental question of how substrates were being targeted to the NMP for degradation into extracellular peptides. Remarkably, we observed newly synthesized polypeptides were rapidly turned over by the NMP in a stimulation-dependent manner. This turnover correlated with enhanced production of NMP-derived peptides in the extracellular space. Using parameters determined in these experiments, we constructed Markov process chain models in silico which predicted that the kinetics of this process necessitate coordination of translation and degradation. In a series of biochemical analyses, this predicted coordination was instantiated by NMP-mediated and ubiquitin-independent degradation of ribosome-associated nascent polypeptides. Using in-depth, global, and unbiased mass spectrometry, we identified the nascent protein substrates of the NMP. Among these substrates, we found that immediate-early gene products c-Fos and Npas4 are targeted by the NMP during ongoing activity-dependent protein synthesis, prior to activity-induced transcriptional responses. Our findings challenge the prevailing notion that proteasomes function primarily to maintain proteostasis, and highlight a form of neuronal communication that takes place through the NMP. Together, these findings generally define an activity-dependent protein quality control program unique to the nervous system through the neuronal membrane proteasome.

[发布机构] Johns Hopkins University

[效力级别] Biochemistry

[关键词] ProteasomeProteostasisProtein HomeostasisMolecular NeuroscienceMembrane BiologyCo-translational DegradationCo-translation protein quality controlImmediate early genesProtein synthesis;Biochemistry