Gld2 activity is regulated by phosphorylation in the N-terminal domain

The de-regulation of microRNAs (miRNAs) is associated with multiple human diseases, yet cellular mechanisms governing miRNA abundance remain largely elusive. Human miR-122 is required for Hepatitis C proliferation, and low miR-122 abundance is associated with hepatic cancer. The adenylyltransferase Gld2 catalyses the post-transcriptional addition of a single adenine residue (A + 1) to the 3'-end of miR-122, enhancing its stability. Gld2 activity is inhibited by binding to the Hepatitis C virus core protein during HepC infection, but no other mechanisms of Gld2 regulation are known. We found that Gld2 activity is regulated by site-specific phosphorylation in its disordered N-terminal domain. We identified two phosphorylation sites (S62, S110) where phosphomimetic substitutions increased Gld2 activity and one site (S116) that markedly reduced activity. Using mass spectrometry, we confirmed that HEK 293 cells readily phosphorylate the N-terminus of Gld2. We identified protein kinase A (PKA) and protein kinase B (Akt1) as the kinases that site-specifically phosphorylate Gld2 at S116, abolishing Gld2-mediated nucleotide addition. The data demonstrate a novel phosphorylation-dependent mechanism to regulate Gld2 activity, revealing tumour suppressor miRNAs as a previously unknown target of Akt1-dependent signalling.