Regulation of transcription by the ubiquitin–proteasome system

It’s hard to imagine two processes that are more different than transcription and ubiquitin-mediated proteolysis. Transcription is the first step in the life of any protein; proteolysis is the last. Despite the disparate nature of these processes, it is clear that ubiquitin, the ubiquitylation machinery, and the proteasome play direct roles in the regulation of gene activity. The direct involvement of the UPS in transcription reveals a previously unanticipated level of gene control that we wish to explore. Our research has three specific areas of focus:

Transcriptional activators. As a group, transcriptional activators tend to be very unstable proteins. We have found that their destruction by the UPS is intimately linked to their ability to activate transcription. We suggest that activity-coupled destruction of transcriptional activators serves as important role in micromanaging their activity, as well providing directionality to the process of transcription. We seek to understand the mechanisms involved and the significance of this process to gene control.

RNA polymerase II. We identified a family of ubiquitin-ligases, the “RPC family”, that bind directly to the initiated form of RNA polymerase II (pol II). The budding yeast RPC protein, Asr1, ubiquitylates the largest subunit of pol II, and these ubiquitylation events—which are not associated with destruction of RNA polymerase—lead to the ejection of two subunits of the polymerase complex and inactivation of pol II enzymatic activity. This non-proteolytic repression mechanism is important for silencing the expression of genes that are close to telomeres. We want to understand how Asr1 is targeted to subtelomeric chromatin, to reveal its other functions, and to understand activities of other RPC family members across evolution.

The Proteasome. Proteasome components associate with chromatin during the transcription process, where they facilitate multiple steps in gene regulation including co-activator recruitment, transcriptional elongation, and histone modifications. Some of these functions require the proteolytic functions of the proteasome, whereas others appear to be non-proteolytic, and mediated by ATPases in the 19S base. We have established that the 26S proteasome is the form that associates with active genes. We now seek to understand how it is recruited, the extent of its binding across the genome, and how the proteolytic versus non-proteolytic functions of this complex are put to work to drive transcriptional processes.

© Bill Tansey 2017