MicroRNAs (miRNAs) are short non-codingRNAs that function in RNA silencing and post-transcriptional regulation of geneexpression. In animals, pri-miRNAs are cleaved by Drosha to generatepre-miRNAs, which are subsequently cleaved by Dicer to generate mature miRNAs.Instead of being cleaved by two different enzymes, both cleavages in plants areperformed by Dicer-like 1 (DCL1). With a similar domain architecture as humanDicer, it is mysterious how DCL1 recognizes pri-miRNAs and executes twocleavages sequentially.
In recent years, the research team led byProf. FENG Yu from the Sir Run Run Shaw Hospital, Zhejiang University School ofMedicine has conducted a systematic research into microRNA processing in plantsat a molecular level. The findings were published in the journal Nature Plantson September 30.
Researchers employed single-particlecryo-electron microscopy (cryo-EM) to determine the structures of ArabidopsisDCL1 complexed with a pri-miRNA and a pre-miRNA at a nominal resolution of 4.6and 4.9 Å, respectively. These structures capture DCL1 in dicing states andreveal that both pri-miRNA and pre-miRNA threads through the helicase module asdouble-stranded RNA.
Cryo-EM structures of DCL1 in complex with pri-miRNA and pre-miRNA
Based on this work and previous genetic and biochemical studies,researchers proposed a model for miRNA processing in plants. First, the PAZdomain recognizes and binds the internal loop of pri-miRNA. In the meantime,the helicase module clamps the duplex of pri-miRNA. In this configuration, thecleavage sites are aligned with the RNase III active centers with theassistance of dsRBD1. After the first cleavage, pre-miRNA translocates towardsthe PAZ domain in an ATP-dependent manner. As soon as the 3′ overhang reachesthe PAZ domain, it is captured by the 3′ overhang binding pocket, which alignsthe cleavage sites with the RNase III active centers again.
Proposed model of miRNA processing in plants
“These structures uncover the plasticity of the PAZ domain, which iscritical for the recognition of both pri-miRNA and pre-miRNA, and suggest thatthe helicase module serves as an engine that transfers the substrate between twosequential cleavage events.” said Prof. FENG, “Ourresearch explains the sequential cleavage activity of DCL1 and sheds light onthe dicing state of human Dicer, as well.”
