RNA interference (RNAi): microRNA (miRNA), siRNA, Argonaute (Ago) proteins, & a comparison to CRISPR

RNA interference (RNAi) is a post-transcriptional regulatory mechanism that uses small RNAs (including microRNAs (miRNAs) and small interfering RNAs (siRNAs)) as guides to direct a protein called Argonaute (Ago) to bind to and repress target messenger RNAs (mRNAs) containing sequence complementarity to that small RNA. This triggers mRNA degradation and/or translational repression - bottom line: less of the corresponding protein gets made. ⠀Full text & figures on blog: http://bit.ly/microRNARNAi

microRNA (miRNA) - they may be “micro” in size but they’re “macro” in importance! - they work to regulate over half of all our genes. miRNAs have their own recipes in DNA and get written (transcribed) just like the like protein-coding genes do, but they get processed differently. They’re written (transcribed) as long hairpins but they get chopped a couple times and, after processing, they’re ~20 nucleotide (nt, RNA-letter) long & single-stranded. (more on their “birth” (biogenesis) in a minute).⠀
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On their own they can’t do much but when bound to a protein called Argonaute (Ago) it’s like an address typed into the GPS of a self-driving car.⠀
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Different miRNAs contain different “addresses” for a specific mRNA or set of mRNA “targets” to be silenced (recipes to find & destroy). This address gets entered into a self-driving car (binds a protein called Argonaute (Ago)), and the car takes it to that address and does its thing. (We call this address-loaded car the RNAi-induced-silencing complex (RISC))⠀
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The car knows where to “park” because the miRNA’s sequence is partially complementary to a sequence in the mRNA target. Complementary RNA letters can base pair with each other - like matching puzzles pieces - “A” binds “U” and “C” binds “G.” So you can get 2 complementary sequences to bind to each other, and thus a sequence can act as a guide to direct silencing machinery to a complementary sequence.⠀
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Once parked, the car can either directly silence it or recruit other helper proteins to repress translation of the mRNA (temporarily keep it from being made into protein) and/or degrade the mRNA completely so they can’t be used to make more protein (don’t worry – these are just copies of the “original” DNA instructions that are still held safely in the nucleus).⠀
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Which way it goes depends in part on how well the sequences match. There’s a critical “seed sequence” of ~6-8 letters in the beginning that serves as the “code word” - this “has” to match, but the whole thing doesn’t have to match in order for Ago to bind and recruit mRNA degradation machinery. What happens next, in the “effector phase” depends largely on the amount of matching. ⠀
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If the whole sequence does match, and it’s in Ago2 (the only one of the 4 human versions of Ago that can slice under normal conditions) Ago can cut the target sequence. This type of fully-complementary guide usually comes from a different type of small RNA (sRNA) called small interfering RNA (siRNA). siRNA typically comes from double-stranded RNA (dsRNA) that usually comes from exogenous (outside sources). This long dsRNA gets chopped into ~22 nt sRNAs by a protein called Dicer.⠀
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One source of dsRNA is viruses, so plants, insects, and some other critters use siRNA for antiviral defense and stuff (e.g. using pieces of an invading virus’ RNA to target that virus). But we have a more complex immune system to take care of those, so our RNAi system has shifted to focusing on miRNA-mediated repression. But our cells still have the capacity to deal with siRNA, so we can hijack the system to direct Ago to specific genes. Often if cell biologists want to see what a specific protein does in cells, they’ll use RNAi - they put double-stranded RNA that gets chopped into siRNA containing the protein’s mRNA address into those cells to selectively take that item off the menu. This is often called “genetic knockdown.”⠀
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But most of the time, our cells are dealing with miRNA & its imperfect matches, so Ago calls for backup, recruiting (with the help of a scaffolding protein called GW182) deadenylation (poly(A) tail removing) complexes & decapping complexes which remove the mRNA’s protective ends so they can be chewed up.⠀
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Your body uses miRNA-mediated mRNA regulation as a sort of thermostat to regulate levels of proteins being made inside each of your billions of cellular homes. It’s always working (or it better be!) and it works on tons of targets using miRNA it makes.

Since RNAi is my favorite topic, I’ve done several more in-depth posts on various aspects of all this stuff, and you can find links to them (and tons of other topics) here 👉 http://bit.ly/2OllAB0⠀
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And here’s a link to an RNAi overview figure I shared on WikiMedia Commons for download & use: https://commons.wikimedia.org/wiki/Fi...