For our recent paper in Annual Review of Biochemistry, we made some movies to help explain splicing. The first movie gives a quick overview of the amazing conformational changes in the spliceosome. The next six movies then break down each conformational change and explain what is going on. For movies 2-7, turn the volume up to hear the dulcet narration of our very own Chris Norman! We really hope these movies make splicing understandable, and that they could be useful for teaching etc.
There are two main points to take away from these movies:
1) The spliceosome doesn’t start with an active site! It has to build the active site on the intron out of the U6 snRNA – always shown in red.
2) Every conformational change gets closer to achieving two goals: making the active site, and bringing the reacting groups of the pre-mRNA into this active site
If you ever get lost, just remember: the intron is in black, and the two exons are in gold.
Movie 1: An overview of pre-mRNA splicing
This movie gives a quick view of all the conformational changes that occur when going from an mRNA precursor to a fully spliced mRNA.
Movie 2: Assembly of the A complex
This shows the first step in spliceosome assembly: how U1 and U2 snRNPs recognise the 5′ splice site and branch point
Movie 3: Assembly of pre-B complex and its transition to B complex
This shows how the tri-snRNP joins, and the transfer of the 5′ splice site from U1 snRNA to U6 snRNA
Movie 4: Spliceosome activation, showing the transition from B complex to Bact complex and formation of the active site
This is a dramatic change! This shows how U6 snRNA is freed from its U4 snRNA chaperone by Brr2 helicase, allowing U6 to form the active site. A huge number of proteins dissociate and associate at this stage.
Movie 5: The branching reaction, showing the transition from Bact complex to C complex via B* complex
Here the spliceosome is finally fully activated for chemistry, and can perform the branching reaction (step 1)
Movie 6: The exon ligation reaction, showing the transition from C complex to P complex via C* complex
After step 1, the spliceosome is slightly rejigged to let it do step 2, exon ligation. That’s shown here.
Movie 7: Release of mRNA and spliceosome disassembly, showing the transition from P complex to ILS
Now that chemistry is done, we need to release the products and recycle the spliceosome subunits. This is surprisingly complicated! (In part because this pathway can also be used for proofreading at many of the above stages)