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

Perception of the environment is essential for vegetal development and adaptation to environmental constraints. This information is integrated and transmitted to lead in fine to appropriate cellular and molecular responses, involving mechanisms for regulating gene expression mediated by small interfering RNA/siRNA (RNA silencing) and Argonaute proteins. In this context, post-translational modifications can be used as fast and cost-effective switches to directly modulate the action of a protein, making them a preferred route of communication. However, there is very few evidence that this type of modification regulates the activity of RNA silencing effectors in plants, which is all the more surprising given the central role played by the Argonaute 1/AGO1 protein in the development and responses of plants exposed to biotic or abiotic stresses.

The "Gene Regulation and RNA silencing in Plants" team at LGDP has just highlighted the presence of two types of modification in the Arabidopsis thaliana AGO1 protein : an asymmetric methylation/aDMA and symmetric methylation/sDMA detected on arginine residues located in boxes clustered in an unstructured region of the protein's N-terminal extension. These boxes are also found in other members of the Argonaute/PIWI family, but the presence of these two marks is unique among RNA silencing effectors. Moreover, this signature has so far been found exclusively in three other proteins in all kingdoms.

The team also showed that the absence of the PRMT5 (Protein arginine/R Methyl Transferases) enzyme, widely conserved in eukaryotes, alters the methylation profile observed in AGO1 and impacts its loading of phased siRNAs, a species known to regulate plant responses to stress.

This discovery highlights the incredible complexity of the regulations in place to control the action of the AGO1 protein. It also raises important questions about the role(s) of arginine methylation in plant responses to its environment, and in particular in plant-pathogen interactions.