Bioenergetics and gene silencing approaches for unraveling nucleotide recognition by the human EIF2C2/Ago2 PAZ domain

Gene silencing and RNA interference are major cellular processes that control gene expression via the cleavage of target mRNA. Eukaryotic translation initiation factor 2C2 (EIF2C2, Argonaute protein 2, Ago2) is considered to be the major player of RNAi as it is the core component of RISC complexes. While a considerable amount of research has focused on RNA interference and its associated mechanisms, the nature and mechanisms of nucleotide recognition by the PAZ domain of EIF2C2/Ago2 have not yet been characterized. Here, we demonstrate that the EIF2C2/Ago2 PAZ domain has an inherent lack of binding to adenine nucleotides, a feature that highlights the poor binding of 3'-adenylated RNAs with the PAZ domain as well as the selective high trimming of the 3'-ends of miRNA containing adenine nucleotides. We further show that the PAZ domain selectively binds all ribonucleotides (except adenosine), whereas it poorly recognizes deoxyribonucleotides. In this context, the modification of dTMP to its ribonucleotide analogue gave a drastic improvement of binding enthalpy and, hence, binding affinity. Additionally, higher in vivo gene silencing efficacy was correlated with the stronger PAZ domain binders. These findings provide new insights into the nature of the interactions of the EIF2C2/Ago2 PAZ domain.