Stuxnet Regulates PRC1-mediated Epigenetic Silencing by Promoting Ubiquitinated Polycomb Protein for Degradation. Juan Du¹, Junzheng Zhang¹, Feng Tie², Ying Su¹, Peter Harte², Alan Jian Zhu¹. 1) Department of Cellular & Molecular Medicine, Lerner Research Institute, Cleveland Clinic, Cleveland, OH; 2) Department of Genetics and Genome Sciences, Case Western Reserve University School of Medicine, Cleveland, OH.

   By utilizing an in vivo RNAi screen strategy, we identify a new gene, stuxnet, that functions as a key component of Notch signaling, a process at the core of cell fate decisions in development, adult tissue homeostasis and cancer. To further study the biological function of stuxnet, we generate a stuxnet null allele and confirm that the transcript of the Notch receptor gene is reduced. Surprisingly, this stuxnet lethal mutation can be rescued by reducing the activity of Polycomb (Pc), an essential component of the Polycomb Repressive Group complex 1 (PRC1) that is known to epigenetically silence target genes critical for animal development. In accordance with a genetic interaction between stuxnet and Pc, Stuxnet protein physically interacts with and subsequently destabilizes ubiquitinated Pc protein through a proteasome-mediated degradation pathway. Our detailed structure/function analyses suggest that Stuxnet utilizes its ubiquitin-like domain (Ubl) to interact with the proteasome to facilitate Pc degradation. Consistently, overexpressed stuxnet leads to stereotypical homeotic transformation phenotypes associated with loss of PRC activity. Further chromatin immunoprecipitation experiments indicate that Stuxnet protein functions through Pc to epigenetically regulate transcription of a panel of PRC target genes, including Notch, Ubx and Antp. Thus, our work uncovers a novel mechanism for the control of the activity and stability of the PRC1 transcriptional silencing machinery in development.