Ubiquitin pathway blockade reveals endogenous ADP-ribosylation marking PARP7 and AHR for degradation.

ADP-ribosylation is an important protein post-translational modification catalysed by a family of PARP enzymes in humans and is involved in DNA damage and immunity among other processes. While poly-ADP-ribosylation has been established as a protein degradation signal in several cases, the role of mono-ADP-ribosylation in protein turnover has remained elusive and mostly relies on overexpression systems. Here, we describe a way to visualise high levels of endogenous ADP-ribosylation by inhibiting the ubiquitin pathway. By blocking ubiquitylation/proteasome, we found that ADP-ribosylation by at least three different PARPs (PARP7, PARP1 and TNKS) can be greatly induced. We discovered that specific activation of the aryl hydrocarbon receptor (AHR) pathway in combination with the ubiquitin pathway inhibition promotes quantitative ADP-ribosylation of PARP7 targets, including the mono-ADP-ribosyltransferase PARP7 itself and AHR. We found that DTX2 is the E3 ligase responsible for degrading ADP-ribosylated PARP7, AHR and other PARP7 substrates. This PARP7-DTX2 crosstalk establishes a mechanism to rapidly shut down AHR-mediated transcription by decreasing its protein levels. Taken together, our findings uncover a paradigm where mono-ADP-ribosylation acts as a degradation mark.