RNA Interference by the Numbers: Explaining Biology Through Enzymology: A Dissertation

Abstract

Small silencing RNAs function in almost every aspect of cellular biology. Argonaute proteins bind small RNA and execute gene silencing. The number of Argonaute paralogs range from 5 in Drosophila melanogaster , 8 in Homo sapiens to an astounding 27 in Caenorhabditis elegans. This begs several questions: Do Argonaute proteins have different small RNA repertoires? Do Argonaute proteins behave differently? And if so, how are they functionally and mechanistically distinct? To address these questions, we examined the thermodynamic, kinetic and functional properties of fly Argonaute1 (dAgo1), fly Argonaute2 (dAgo2) and mouse Argonaute2 (mAGO2). Our studies reveal that in fly, small RNA duplexes sort into Argonaute proteins based on their intrinsic structures: extensively paired siRNA duplex is preferentially sorted into dAgo2 while imperfectly paired miRNA duplex is channeled into dAgo1. The sorting of small RNA is uncoupled from its biogenesis. This is exemplified by mir-277, which is born a miRNA but its extensive duplex structure licenses its entry into dAgo2. In the Argonaute protein, the small RNA guide partitions into functional domains: anchor, seed, central, 3' supplementary and tail. Of these domains, the seed initiates binding to target. Both dAgo2 and mAGO2 (more closely related to and a surrogate for dAgo1 in our studies) bind targets at astonishing diffusion-limited rates (~107–108 M−1s−1). The dissociation kinetics between dAgo2 and mAGO2 from their targets, however, are different. For a fully paired target, dAgo2 dissociates slowly (t½ ~2 hr) but for a seed-matched target, dAgo2 dissociates rapidly (t½ ~20 s). In comparison, mAGO2 does not discriminate between either targets and demonstrates an equivalent dissociation rate (t½ ~20 min). Regardless, both dAgo2 and mAGO2 demonstrate high binding affinity to perfect targets with equilibrium dissociation constants, KD ~4–20 pM. Functionally, we also showed that dAgo1 but not dAgo2 silence a centrally bulged target. By contrast, dAgo2 cleaved and destroyed perfectly paired targets 43-fold faster than dAgo1. In target cleavage, dAgo2 can tolerate mismatches, bulged and internal loop in the target but at the expense of reduced target binding affinities and cleavage rates. Taken together, our studies indicate that small RNAs are actively sorted into different Argonaute proteins with distinct thermodynamic, kinetic and functional behaviors. Our quantitative biochemical analysis also allows us to model how Argonaute proteins find, bind and regulate their targets.