Casein kinase 1 and 2 phosphorylate Argonaute proteins to regulate miRNA‐mediated gene silencing

MicroRNAs (miRNAs) together with Argonaute (AGO) proteins form the core of the RNA‐induced silencing complex (RISC) to regulate gene expression of their target RNAs post‐transcriptionally. Argonaute proteins are subjected to intensive regulation via various post‐translational modifications that can affect their stability, silencing efficacy and specificity for targeted gene regulation. We report here that in Caenorhabditis elegans, two conserved serine/threonine kinases – casein kinase 1 alpha 1 (CK1A1) and casein kinase 2 (CK2) – regulate a highly conserved phosphorylation cluster of 4 Serine residues (S988:S998) on the miRNA‐specific AGO protein ALG‐1. We show that CK1A1 phosphorylates ALG‐1 at sites S992 and S995, while CK2 phosphorylates ALG‐1 at sites S988 and S998. Furthermore, we demonstrate that phospho‐mimicking mutants of the entire S988:S998 cluster rescue the various developmental defects observed upon depleting CK1A1 and CK2. In humans, we show that CK1A1 also acts as a priming kinase of this cluster on AGO2. Altogether, our data suggest that phosphorylation of AGO within the cluster by CK1A1 and CK2 is required for efficient miRISC‐target RNA binding and silencing.