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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# Spliceosome

The spliceosome is a large RNA–protein machine that cuts introns out of pre-mRNA and joins the exons; its catalytic core is RNA.

Updated 2026-11-01

Loading the 3D model…

## Key facts

- Five snRNPs (U1, U2, U4, U5, U6), each with an snRNA and Sm proteins, plus about 150 accessory proteins.
- U1 recognises the 5' site (GU), U2 the branch point (A), U2AF the pyrimidine tract and the 3' AG.
- U6 snRNA together with U2 positions the Mg²⁺ ions that catalyse both transesterifications: it is a ribozyme.
- It is assembled anew for each intron and recycled; it uses ATP in the helicases that rearrange it.
- A 'minor' spliceosome (U11, U12, U4atac, U6atac) handles a small group of introns.

**Also called**
Splicing machinery, snRNPs

**EC number**
[—](https://www.enzyme-database.org/query.php?ec=—)

**Genes**
SF3B1, PRPF8, SNRNP200, U2AF1, SRSF2 (snRNA U1, U2, U4, U5, U6)

**Structure (PDB)**
[6EN4](https://www.rcsb.org/structure/6EN4)

## Regulation

- SR proteins (activate) and hnRNPs (repress) bind splicing enhancers and silencers and decide which exons are kept.
- SR protein phosphorylation and Pol II speed modulate alternative splicing.

## Inhibitors and toxins

- Pladienolide B, E7107, H3B-8800 (SF3B1 modulators; investigational)
- Splice-switching antisense oligonucleotides (nusinersen in spinal muscular atrophy; exon-skipping ones in Duchenne)

## Clinical notes

- Somatic mutations in SF3B1 (myelodysplastic syndromes with ring sideroblasts), SRSF2 and U2AF1 are common in myeloid neoplasms.
- Anti-Sm (lupus) and anti-U1 RNP (mixed connective tissue disease) antibodies target spliceosome proteins.
- Mutations in PRPF8 and other factors cause retinitis pigmentosa.

## Related conditions

- [Thalassemias (α and β)](https://durgical.com/en/conditions/thalassemias/)
- [Systemic lupus erythematosus](https://durgical.com/en/conditions/systemic-lupus-erythematosus/)

## Related

- [RNA processing (cap, poly-A tail and splicing)](https://durgical.com/en/biochemistry/topics/rna-processing/)
- [Transcription](https://durgical.com/en/biochemistry/topics/transcription/)
- [Types of mutations](https://durgical.com/en/biochemistry/topics/types-of-mutations/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which spliceosome component carries out catalysis?

RNA: U6 and U2 snRNAs, which position the Mg²⁺ ions; the spliceosome is a ribozyme.

## Which snRNP recognises the 5' splice site?

U1 snRNP, which pairs with the GU at the start of the intron.

## Which somatic spliceosome mutation is linked to ring sideroblasts?

SF3B1 mutations in myelodysplastic syndromes.

## What does the 3D model 6EN4 show?

The human SF3b core (with SF3B1 and PHF5A) bound to pladienolide B (BGZ), a splicing modulator.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [SF3B1: Splicing factor 3B subunit 1 (human)](https://www.uniprot.org/uniprotkb/O75533/entry) (UniProt, 2026); [6EN4: SF3b core in complex with a splicing modulator](https://www.rcsb.org/structure/6EN4) (RCSB Protein Data Bank, 2026); [mRNA Splicing](https://reactome.org/PathwayBrowser/#/R-HSA-72172) (Reactome, 2026); [Intron](https://www.genome.gov/genetics-glossary/Intron) (National Human Genome Research Institute (NHGRI), 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.

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