Orientation of Human Microprocessor on Primary MicroRNAs

Abstract

Single-stranded microRNAs (miRNAs) regulate gene expression by triggering mRNA degradation and/or inhibiting mRNA translation. miRNAs play important roles in various critical cellular processes and are associated with numerous human diseases, including cancer and neurodegenerative diseases. miRNA sequences are embedded in the primary miRNA transcripts (pri-miRNAs) that are initially processed by the Microprocessor complex in the nucleus. Microprocessor can orient itself on pri-miRNAs in two ways: one orientation results in subsequent miRNA production, and the other leads to cleavage of the miRNA sequence. Therefore, orienting Microprocessor on pri-miRNAs is a fundamental mechanism for determining the accuracy and efficiency of pri-miRNA processing and, in turn, miRNA production. Multiple mechanisms controlling Microprocessor orientation on pri-miRNAs, involving both cis-acting RNA elements and trans-acting factors, have recently been revealed. In this review, we discuss these exciting mechanisms and consider potential unknown mechanisms that might regulate Microprocessor orientation on pri-miRNAs.

Mismatched and wobble base pairs govern primary microRNA processing by human Microprocessor

“…High-throughput pri-miRNA processing assays. Since DROSHA can cleave either the basal or apical junction of a typical two-junction pri-miRNA 13,14,20,27 , we decided to use the short pri-miRNA substrate, arti1, which contains just one junction, to investigate the substrate recognition ability of DROSHA ( Fig. 1c).…”

“…Another part of the pri-miRNA structure is called the basal junction, this is formed between the stem and the 5pbasal and 3p-basal RNA segments [17][18][19] . The stem is sub-divided by DROSHA cleavage sites into the upper and lower stems, and both DROSHA and DGCR8 are coordinated to interact with multiple RNA elements to process pri-miRNA accurately and efficiently 4,5,20 .…”

“…DROSHA is known to cleave at the basal junction (referred to as 'productive' cleavage); it measures the distance and determines the site of cleavage at~13 nt from the basal junction 13,14,17 . It has been suggested that DROSHA might also be able to cleave at the apical junction via a process called 'unproductive' cleavage 13,14,17,20 (Fig. 1b).…”

Bulges control pri-miRNA processing in a position and strand-dependent manner

“…Microprocessor is a trimetric complex consisting of an RNase III enzyme called DROSHA and a dimer of the RNA-binding protein, DGCR8 [3,[6][7][8][9][10][11] (Fig. S1A).…”

Select amino acids in DGCR8 are essential for the UGU-pri-miRNA interaction and processing

“…The apical loop connects to one end of the stem via the apical junction, whereas two ssRNA segments, namely the 5p-and 3p-basal segments, flank the other end, making a basal junction 2,15 . Pri-miRNAs contain several RNA elements, such as UG, UGU, mGHG, CNNC, seedMW, and midMW, which are critical for the efficiency and accuracy of pri-miRNA processing by Microprocessor 8,10,17,[22][23][24][25][26][27][28] . The recent report demonstrated that the internal loop in the lower stem of pri-miRNAs facilitates the single cleavage of pri-miRNAs on the 5p-strand and reduces the double cleavages of Microprocessor, thereby controlling miRNA biogenesis 29 .…”