Spliceosomal snRNAs are transcribed from specialized promoters, which recruit RNA polymerase II cofactors that aid in proper 3′ end maturation of these non-polyadenylated transcripts.
Like most non-coding RNAs, small nuclear RNAs (snRNAs) use cognate antisense elements to interact with their nucleic acid targets via base pairing.
Assembly of functional small nuclear ribonucleoproteins (snRNPs) involves a series of non-functional intermediates that are often sequestered in subcellular compartments that are distinct from their sites of action.
snRNP function requires multiple protein partners (such as DExD/H helicases or WD box proteins) the roles of which may include modulating RNA structure or tethering an enzyme.
snRNPs recognize specific sequences in pre-mRNAs and assemble into the spliceosome in a stepwise manner. The splicing reaction itself is catalysed by U6/U2 snRNA complex that resembles a self-splicing ribozyme.
Alternative splicing is typically regulated by multiple cis-elements and trans-factors, which form complex interaction networks that may provide a great deal of regulatory plasticity.
Pre-mRNA splicing can be regulated throughout the entire spliceosomal assembly pathway, although the early steps are the main stages of regulation.
One of the most amazing findings in molecular biology was the discovery that eukaryotic genes are discontinuous, with coding DNA being interrupted by stretches of non-coding sequence. The subsequent realization that the intervening regions are removed from pre-mRNA transcripts via the activity of a common set of small nuclear RNAs (snRNAs), which assemble together with associated proteins into a complex known as the spliceosome, was equally surprising. How do cells coordinate the assembly of this molecular machine? And how does the spliceosome accurately recognize exons and introns to carry out the splicing reaction? Insights into these questions have been gained by studying the life cycle of spliceosomal snRNAs from their transcription, nuclear export and re-import to their dynamic assembly into the spliceosome. This assembly process can also affect the regulation of alternative splicing and has implications for human disease.
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Research in the authors' laboratories is supported by US National Institutes of Health grants R01-GM053034 and R01-NS041617 (to A.G.M.), as well as R01-CA158283 and R21-AR061640 (to Z.W.). The authors apologize to those whose work could not be discussed owing to space limitations.
The authors declare no competing financial interests.
- Splice site
The short sequences at exon–intron junctions of pre-mRNA, which include the 5′ splice (splice donor) site and the 3′ splice (splice acceptor) site located at the beginning and the end of an intron, respectively.
- Heterogeneous nuclear RNP
(hnRNP). A diverse class of ribonucleoproteins (RNPs) located in the cell nucleus, and primarily involved in post-transcriptional regulation of mRNAs. The hnRNP proteins are a class of RNA-binding factors, many of which shuttle between the nucleus and cytoplasm, that are involved in regulating the processing, stability and subcellular transport of mRNPs.
- Cajal bodies
Nuclear substructures that are highly enriched in pre-mRNA splicing factors. They are thought to function as sites of ribonucleoprotein assembly and remodelling.
- Tudor domain
A conserved protein structural motif that is thought to bind to methylated arginine or lysine residues, promoting physical interactions with its target protein.
- Nuclear speckles
Sub-nuclear structures highly enriched in pre-mRNA-splicing factors. At the ultrastructural level, they correspond to domains known as interchromatin granule clusters.
- SR proteins
Proteins that contain a domain with repeats of serine (S) and arginine (R) residues and one or more RNA-recognition motifs. SR proteins are best known for their ability to bind exonic splicing enhancers and activate splicing, although some SR proteins also regulate transcription.
- Branch point
A loosely conserved short sequence usually located ∼15–50 nucleotides upstream of the 3′ splice site, before a region rich in pyrimidines (cytosine and uracil). Most branch points include an adenine nucleotide as the site of lariat formation.
- Exon definition
One of two different modes of initial splice site pairing at the early stage of splicing (the other being intron definition). During exon definition, the U1 and U2 small nuclear ribonucleoproteins (snRNPs) interact to pair the splice sites across an exon. For some small introns, the U1 and U2 snRNPs interact to pair the splice sites across introns.
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Matera, A., Wang, Z. A day in the life of the spliceosome. Nat Rev Mol Cell Biol 15, 108–121 (2014). https://doi.org/10.1038/nrm3742
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