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The unique dual targeting of AGO1 by two types of PRMT enzymes promotes phasiRNA loading in Arabidopsis thaliana

ABSTRACT: Arginine/R methylation (R-met) of proteins is a widespread post-translational modification (PTM), deposited by a family of protein arginine/R methyl transferase enzymes (PRMT). Regulations by R-met are involved in key biological processes deeply studied in metazoan. Among those, post-transcriptional gene silencing (PTGS) can be regulated by R-met in animals and in plants. It mainly contributes to safeguard processes as protection of genome integrity in germlines through the regulation of piRNA pathway in metazoan, or response to bacterial infection through the control of AGO2 in plants. So far, only PRMT5 has been identified as the AGO/PIWI R-met writer in higher eukaryotes. We uncovered that AGO1, the main PTGS effector regulating plant development, contains unique R-met features among the AGO/PIWI superfamily. We pinpointed that AGO1 contains both symmetric and asymmetric R-dimethylations (sDMA and aDMA respectively) and is dually targeted by PRMT5 and by another type I PRMT in Arabidopsis thaliana. We showed that loss of sDMA didn’t compromise AtAGO1 subcellular trafficking in planta. Interestingly, we underscored that AtPRMT5 specifically promotes phasiRNA loading in AtAGO1. All our observations bring to consider this dual regulation of AtAGO1 in plant development and response to environment, and pinpoint the complexity of its post-translational regulation.

INSTRUMENT(S): Q Exactive

ORGANISM(S): Arabidopsis Thaliana (mouse-ear Cress)

SUBMITTER: Lauriane Kuhn

LAB HEAD: Jacinthe Azevedo-Favory

PROVIDER: PXD043460 | Pride | 2024-01-18

REPOSITORIES: Pride

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Project description:PRMT5 is the major enzyme that catalyzes the symmetric dimethylation of arginine (sDMA) in mammalian cells. Its activity is crucial for many biological processes including transcriptional regulation, mRNA processing, as well as DNA damage and repair. However, the protein substrates identified for PRMT5 have yet been limited in numbers, hindering the understanding of PRMT5 functions in normal as well as diseased cells. Herein we developed an optimized strategy for proteomic profiling of cellular sDMA and apply it to the discovery of novel PRMT5 substrates. Our results show that a combination of proteolytic digestion by the metalloprotease ulilysin and using electron-transfer dissociation with supplemental activation as the mass spectrometry method significantly increased sDMA site identification and localization after immuno-affinity enrichment. By employing SILAC-based differential quantitative proteomic approach and a PRMT5 specific inhibitor, we identified 325 sDMA sites being regulated by PRMT5, 220 being novel sites, which greatly expanded the number of PRMT5 substrates. Biochemical studies confirmed the newly discovered PRMT5 substrate, Plasminogen activator inhibitor 1 RNA-binding protein (SERBP1) and revealed that the RGG/RG motif in the central region of SERBP1 contains both PRMT5-catalyzed sDMA and Type I PRMT-catalyzed asymmetric dimethylation of arginine (aDMA). Unexpectedly, using the optimized methylarginine profiling strategy, we also discovered arginine trimethylation, a previously unknown type of modification, on the central RGG/RG region of SERBP1. By mutational studies we demonstrated that the trimethyl modification on R172 of SERBP1 regulates its recruitment to stress granule (SG) under oxidative stress, indicating a functional role of arginine trimethylation in the cell. Overall, the optimized profiling strategy not only enabled the identification of extensive, non-overlapping sDMA sites and novel PRMT5 substrates, but also revealed a potentially important new PTM, arginine trimethylation. The work lays the foundation for further investigations on the functional interplay of different methylation modifications on arginines, especially in the heavily methylated RGG/RG motif of many RNA-binding proteins.

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The unique dual targeting of AGO1 by two types of PRMT enzymes promotes phasiRNA loading in Arabidopsis thaliana

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