Question

How microRNA inhibit translation ?

Answer 1

MicroRNAs are a class of short (~22 bp) RNAs that can silence target mRNAs by inhibiting translation and subsequently initiating mRNA decay.

In general, a protein called exportin-5 transports a hairpin primary micro-RNA our of the nucleus. An enzyme called dicer trims the primary micro-RNA and removes the hairpin loop, leaving a double-stranded micro-RNA duplex molecule. Meanwhile, one of the strands joins a group of proteins, forming a micro-RNA-protein complex. The other strand is discarded. The micro-RNA-protein complex binds to the mRNA based on the complementarity, which further blocks the binding of translation initiation units, and initiates quick deadenylation (degradation of poly-A tail from 3'-end if mRNA), decapping (removal of 7-methyl-guanidine from 5'-end of mRNA) and decaying. miRNAs inhibit protein synthesis by first repressing mRNA translation and subsequently initiating mRNA deadenylation, decapping, and decaying.

More specifically, micro-RNAs recruit the micro-RNA-induced silencing complex (miRISC), a ribonucleoprotein complex composed of a micro-RNA-loaded Argonaute (AGO) protein, and the AGO-interacting protein GW182. GW182 also interacts with the poly-A binding protein (PABP) and facilitates mRNA deadenylation by recruiting the PAN2–PAN3 and CCR4–NOT deadenylase complexes.