Dai-Chao Xu's team at the Center for Biological and Chemical Cross Research, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, published a research paper entitled: PARP5A and RNF146 phase separation restrains RIPK1-dependent necroptosis in Molecular Cell.
The study reports a new death checkpoint of programmed necrosis and reveals the molecular mechanism by which this checkpoint regulates programmed necrosis in the form of phase separation.
In this study, the team revealed the molecular mechanism by which PARP5A and RNF146 proteins prevent programmed necrosis from occurring by facilitating their poly ADP-ribosylation (PARylation)-dependent ubiquitination modification (PARdU) of the activated RIPK1-K376 locus through cofractionation.
This study found that if RIPK1 is not inhibited by normal checkpoints in TNF-RSC, such as when the K376 locus is not normally ubiquitinated, this portion of RIPK1 undergoes activation and recruits the junction protein TAX1BP1 by denaturing, which further recruits the functional PARP5A and RNF146 protein complexes to the activated RIPK1 and PARP5A-mediated PARylation and RNF146-mediated PARdU modification, respectively, link the ubiquitin chain at the K376 site of activated RIPK1, causing it to be degraded by the proteasome, which in turn prevents programmed necrosis from occurring.
However, PARP5B, a protein homologous to PARP5A, was shown in previous studies to have exactly the same function as PARP5A. The amino acid sequences as well as the structures of the two are so similar that they were once thought to be functionally redundant. However, the researchers found that PARP5B does not regulate programmed necrosis like PARP5A. Further analysis revealed that PARP5A has an additional HPS structural domain of unknown function at its N-terminus compared to PARP5B, which is a typical disordered structural domain (IDR). Since IDRs may drive proteins to phase-separate for specific functions, the researchers found that PARP5A and RNF146 can undergo co-phase-separation at the onset of programmed necrosis and are dependent on the HPS structural domain of PARP5A. If the HPS domain is artificially attached to the PARP5B protein, PARP5B also undergoes phase separation and regulates programmed necrosis. Finally, the researchers demonstrated that co-segregation of PARP5A and RNF146 increases their local concentration, which is essential for the PARP5A-catalyzed modification of RIPK1 PARylation and subsequent RNF146-catalyzed modification of PARdU.
Model of the molecular mechanism by which PARP5A and RNF146 novel cell death checkpoints regulate programmed necrosis through phase separation
Overall, this study links phase separation to RIPK1-mediated programmed necrosis for the first time and conceptualizes the importance of phase separation-triggered PARdU modification at the RIPK1-K376 locus as an alternative cell death checkpoint. This study will inspire the discovery of more novel mechanisms of phase-separated regulation of cell death pathways, which will provide new ideas for drug development based on cell death interventions.
Dai-Chao Xu, a researcher at the Center for Intersection of Biology and Chemistry, Chinese Academy of Sciences (CIBC), is the corresponding author of the paper, while Shou-Qiao Hou, a PhD student at CIBC, and Jian Zhang, an associate professor at Soochow University, are the co-first authors of the paper. Professor Yuan Junying and Researcher Cong Liu of the Center for Intersectional Research in Biology and Chemistry, Chinese Academy of Sciences, provided invaluable assistance for this work.
