# [Spliceosome Composition](/knowledge/molecular-biology/spliceosome-made): Core snRNPs and Protein Factors

## Introduction to the Spliceosome and Its Composition

The spliceosome is a large, dynamic ribonucleoprotein (RNP) complex that catalyzes the removal of introns from pre-messenger RNA (pre-mRNA) in the nucleus of eukaryotic cells. This process, termed pre-mRNA splicing, is an essential step in gene expression: it produces mature messenger RNA (mRNA) that can be exported to the cytoplasm and translated into protein. The spliceosome is composed of five small nuclear ribonucleoproteins (snRNPs)—U1, U2, U4, U5, and U6—along with a large number of non-snRNP protein factors. In total, the assembled spliceosome can contain over 150 distinct polypeptides and five RNA molecules, making it one of the most complex molecular machines in the cell.

A common point of confusion is the distinction between the spliceosome and its components. The spliceosome is not a single static entity; rather, it is assembled *de novo* on each intron from its constituent parts. The term "spliceosome composed" therefore refers to the complete, functional complex that forms during splicing, which differs in composition at each step of the reaction. The snRNPs provide the catalytic core and the primary RNA–RNA recognition elements, while accessory proteins—including helicases, [RNA-binding proteins](/knowledge/molecular-biology/rna-binding-protein), and scaffolding factors—regulate assembly, conformational rearrangements, and fidelity. Understanding the composition of the spliceosome is fundamental to understanding how splicing is achieved with remarkable accuracy despite the sequence diversity of introns.

This article details the molecular components of the spliceosome, the stepwise assembly pathway, the key RNA–RNA and RNA–protein interactions, and the experimental methods that have revealed its composition. For a broader overview of the machine's architecture, see the [Spliceosome Structure](/knowledge/molecular-biology/spliceosome-structure) entry.

## The Core snRNPs: U1, U2, U4/U6, and U5

Each snRNP consists of a small nuclear RNA (snRNA) associated with a set of proteins. The snRNAs—U1, U2, U4, U5, and U6—range from approximately 100 to 200 nucleotides in length and are highly conserved across eukaryotes. With the exception of U6, all snRNAs are transcribed by RNA polymerase II and acquire a 5′ trimethylguanosine cap. U6 is transcribed by RNA polymerase III and retains a γ-monomethyl phosphate cap.

All snRNPs except U6 share a common structural core: seven Sm proteins (B/B′, D1, D2, D3, E, F, and G) assemble into a ring around a conserved Sm site—a single-stranded region with the consensus sequence AU4-6G—on the snRNA. The U6 snRNP instead contains seven LSm (Sm-like) proteins (LSm2–8) that bind a similar but distinct sequence. These Sm/LSm rings stabilize the snRNA and serve as platforms for additional protein interactions.

The table below summarizes the core components of each snRNP.

| snRNP | snRNA length (human) | Sm/LSm core | Key specific proteins | Primary function |
|-------|----------------------|-------------|------------------------|------------------|
| U1 | 164 nt | Sm B/B′, D1, D2, D3, E, F, G | U1-70K, U1-A, U1-C | 5′ splice site recognition |
| U2 | 187 nt | Sm core | U2-A′, U2-B″ (SF3a, SF3b complexes) | Branch point recognition |
| U4 | 144 nt | Sm core | hPrp4, hPrp3, hPrp31, cyclophilin H | Chaperone for U6; base-pairs with U6 |
| U6 | 106 nt | LSm2–8 | Prp8, Brr2, Snu114, Prp3, Prp4, Prp31 | Catalytic center; base-pairs with U2 |
| U5 | 116 nt | Sm core | Prp8, Brr2, Snu114, hPrp28, hSnu66 | Exon alignment; 5′ and 3′ splice site juxtaposition |

### U1 snRNP

The U1 snRNP is the first component to recognize the pre-mRNA during [spliceosome assembly](/knowledge/molecular-biology/spliceosome-assembly). Its snRNA contains a 5′ single-stranded region that base-pairs with the 5′ splice site consensus sequence (AG/GURAGU, where R is a purine and the slash indicates the exon–intron boundary). This base-pairing is short—only 6 to 8 nucleotides—and is stabilized by the U1-specific proteins.

The U1-specific proteins include U1-70K, U1-A, and U1-C. U1-70K is an RNA-binding protein that contacts the stem-loop I of U1 snRNA and also interacts with the SR protein ASF/SF2 (see below), linking U1 snRNP recruitment to exon definition. U1-A binds to stem-loop II of U1 snRNA and has a role in regulating U1 snRNP assembly. U1-C, a small zinc-finger protein, stabilizes the U1 snRNA–5′ splice site duplex and is required for the subsequent recruitment of U2 snRNP. Although U1 snRNP is essential for early recognition, it does not remain stably associated with the catalytic core; it is released during later stages of assembly.

### U2 snRNP

The U2 snRNP recognizes the branch point sequence (BPS) in the intron, typically the consensus YNYURAC (where Y is a pyrimidine, N is any nucleotide, and the underlined A is the branch adenosine). The U2 snRNA base-pairs with the BPS, but this interaction is longer and more stable than the U1–5′ splice site interaction, involving approximately 10 nucleotides.

The U2 snRNP is the most protein-rich of the snRNPs. In addition to the Sm core, it contains the U2-A′ and U2-B″ proteins, which bind stem-loop IV of U2 snRNA. More importantly, the U2 snRNP carries two large multiprotein complexes: SF3a and SF3b. SF3a consists of three subunits (SF3a120, SF3a66, and SF3a60 in humans), while SF3b contains at least six subunits (SF3b155, SF3b145, SF3b130, SF3b49, SF3b14a, and SF3b10). The SF3b155 subunit directly contacts the branch point region and helps position the branch adenosine for the first catalytic step. These proteins are essential for stable U2 binding and are among the first targets of splicing inhibitors used in cancer therapy, such as pladienolide B, which binds SF3b155.

### U4/U6 snRNP

U4 and U6 snRNAs exist as a single di-snRNP particle. The two RNAs are held together by extensive base-pairing between their 5′ ends, forming a Y-shaped structure with two intermolecular helices (stem I and stem II). In this complex, U6 is sequestered in an inactive conformation. The U4/U6 di-snRNP also associates with the U5 snRNP to form the U4/U6·U5 tri-snRNP, which is recruited as a pre-assembled unit during [spliceosome assembly](/knowledge/molecular-biology/spliceosome-assembly).

The U4/U6 di-snRNP contains several proteins that are shared with the U5 snRNP in the tri-snRNP, including Prp31, Prp3, Prp4, and cyclophilin H. Prp31 is a key scaffolding protein that bridges the U4/U6 di-snRNP to the U5 snRNP. The protein hPrp4 is a kinase that phosphorylates the U5 protein Prp8, and cyclophilin H is a peptidyl-prolyl isomerase that may regulate protein conformation. The LSm proteins (LSm2–8) bind the 3′ end of U6 snRNA and are required for U6 stability and for the assembly of the U4/U6 complex.

### U5 snRNP

The U5 snRNP is unique in that it does not base-pair with the pre-mRNA directly. Instead, its snRNA contains a conserved loop that interacts with the exonic sequences at both the 5′ and 3′ splice sites, aligning the two exons for ligation. The U5 snRNP also carries the largest and most conserved protein in the spliceosome: Prp8. Prp8 is a 220-kDa protein that sits at the heart of the catalytic center, making contacts with U5 and U6 snRNAs, the pre-mRNA, and the branch point. It is thought to act as a scaffold that coordinates the RNA rearrangements required for catalysis.

Other U5-specific proteins include Brr2, a DExD/H-box RNA helicase that unwinds the U4/U6 interaction during activation, and Snu114, a GTPase that regulates Brr2 activity. The U5 snRNP also contains hPrp28, another DExD/H-box helicase that destabilizes the U1–5′ splice site interaction to allow U6 to take over the 5′ splice site. Together, these proteins make the U5 snRNP a central hub for the conformational changes that drive splicing forward.

## Protein-Only Components and Accessory Factors

Beyond the snRNPs, the spliceosome contains numerous non-snRNP proteins that are not stably associated with any single snRNP but are recruited during assembly. These proteins perform a wide range of functions: they recognize regulatory sequences, promote or antagonize snRNP binding, remodel RNA–RNA interactions, and proofread the splicing reaction. Many of these factors are members of the SR protein family or the DExD/H-box helicase family.

### SR Proteins

SR proteins are a family of serine/arginine-rich RNA-binding proteins that are essential for constitutive splicing and are major regulators of [alternative splicing](/blog/guides/alternative-splicing). They contain one or two N-terminal RNA recognition motifs (RRMs) and a C-terminal arginine/serine (RS) domain rich in RS dipeptide repeats. The RS domain mediates protein–protein interactions with other splicing factors, particularly the U1-70K protein and the U2 snRNP component SF3a120.

SR proteins bind to exonic splicing enhancers (ESEs)—short, degenerate sequences in exons—and promote spliceosome assembly by recruiting U1 snRNP to the 5′ splice site and U2 snRNP to the branch point. The prototypical SR protein is ASF/SF2 (encoded by the *SRSF1* gene), which was the first splicing factor shown to be required for spliceosome assembly *in vitro*. Other well-studied SR proteins include SC35 (SRSF2), SRp20 (SRSF3), and SRp40 (SRSF5). The activity of SR proteins is regulated by phosphorylation: kinases such as SRPK1 and Clk/Sty phosphorylate the RS domain, while phosphatases such as PP1 dephosphorylate it. This phosphorylation cycle controls the subcellular localization and activity of SR proteins during splicing.

SR proteins are not permanent components of the spliceosome; they are required for early assembly but are largely released before the catalytic steps. However, their presence is essential for the fidelity of splice site selection, and their dysregulation is linked to many human diseases, including cancer and spinal muscular atrophy.

### DExD/H-box Helicases

The spliceosome contains at least eight DExD/H-box RNA helicases, which use ATP hydrolysis to remodel RNA–RNA and RNA–protein interactions. These enzymes are named for the conserved DExD/H amino acid motif in their ATPase domain. They do not processively unwind long duplexes like canonical helicases; instead, they act as ATP-dependent RNA-binding proteins that destabilize specific RNA duplexes or protein–RNA contacts.

Key helicases in the spliceosome include:

- **UAP56** (also called DDX39B): Recruited early, it promotes U2 snRNP binding to the branch point by remodeling the SF3b complex. It is also involved in mRNA export.
- **Prp5**: An ATPase required for stable U2 snRNP addition. It proofreads the U2–branch point interaction.
- **Prp28**: A U5-associated helicase that disrupts the U1–5′ splice site duplex, allowing U6 snRNA to base-pair with the 5′ splice site.
- **Brr2**: A U5-associated helicase that unwinds the U4/U6 interaction, releasing U4 snRNA and activating U6 for catalysis.
- **Prp2**: A helicase that acts after the first catalytic step to remodel the spliceosome, removing the SF3a/SF3b proteins and allowing the second catalytic step to proceed.
- **Prp16**: A helicase that proofreads the first catalytic step and promotes the conformational change required for exon ligation.
- **Prp22**: A helicase that releases the mature mRNA from the spliceosome after the second catalytic step.
- **Prp43**: A helicase that disassembles the post-catalytic spliceosome and recycles the snRNPs.

These helicases are not merely passive motors; they are key fidelity factors. For example, Prp16 and Prp22 use ATP hydrolysis to reject suboptimal substrates, ensuring that only correctly paired splice sites are used. Mutations in these helicases lead to increased splicing errors and are associated with developmental disorders.

## The Spliceosome Assembly Pathway

The spliceosome assembles in a strictly ordered, stepwise manner on each intron. This process is often described as a series of discrete complexes—E, A, B, Bact, B*, C, and post-catalytic—each with a distinct protein composition. The assembly pathway is conserved from yeast to humans, although the number of accessory factors is greater in higher eukaryotes.

### Early Complexes (E and A)

The first step is the formation of the **E (early) complex**, also called the commitment complex. This begins with the binding of U1 snRNP to the 5′ splice site via RNA base-pairing. Simultaneously, the branch point is recognized by the splicing factor SF1 (also called BBP in yeast) and the U2 auxiliary factor U2AF. U2AF is a heterodimer consisting of a 65-kDa subunit (U2AF65) that binds the polypyrimidine tract downstream of the branch point, and a 35-kDa subunit (U2AF35) that binds the 3′ splice site AG dinucleotide. SR proteins bound to exonic enhancers stabilize these interactions by bridging U1 snRNP and U2AF.

The E complex is ATP-independent and is not yet catalytically committed—the intron can still be released and the pre-mRNA can be used for other purposes. However, the E complex is the point at which splice site pairing is first established.

The **A complex** (also called the pre-spliceosome) forms when U2 snRNP stably binds the branch point. This requires ATP hydrolysis by the helicase UAP56 and the proofreading activity of Prp5. U2 snRNA base-pairs with the branch point sequence, and the branch adenosine is bulged out of the duplex. The SF3a and SF3b complexes stabilize this interaction. At this stage, the 5′ splice site and branch point are brought into proximity, but the catalytic center has not yet formed.

### Catalytic Complexes (B and C)

The **B complex** forms when the pre-assembled U4/U6·U5 tri-snRNP is recruited. This step is promoted by the protein Prp31 and requires ATP hydrolysis by the helicase Prp28, which disrupts the U1–5′ splice site interaction. At this point, the spliceosome contains all five snRNPs and is primed for activation.

The **Bact complex** (activated complex) is generated by a major conformational rearrangement. The helicase Brr2 unwinds the U4/U6 interaction, releasing U4 snRNP from the complex. This allows U6 snRNA to refold and base-pair with U2 snRNA, forming the U2/U6 helix I, which is the core of the catalytic center. U6 also replaces U1 in base-pairing with the 5′ splice site. The Bact complex is catalytically competent but is still inactive until the helicase Prp2 remodels the complex, removing SF3a/SF3b and other proteins. This yields the **B*** complex, which is the first catalytically active form.

The **C complex** forms after the first catalytic step: the 2′-hydroxyl of the branch adenosine attacks the 5′ splice site phosphate, producing a free 5′ exon and a lariat intron–3′ exon intermediate. This step is catalyzed by the U2/U6 RNA network, with the divalent metal ions coordinated by U6 snRNA. After the first step, the helicase Prp16 proofreads the reaction and promotes a conformational change that positions the 5′ exon for the second step.

The second catalytic step—exon ligation—occurs in the **C*** complex. The 3′-hydroxyl of the 5′ exon attacks the 3′ splice site phosphate, joining the two exons and releasing the lariat intron. The helicase Prp22 then releases the mature mRNA, and Prp43 disassembles the remaining complex, recycling the snRNPs for another round of splicing.

The ordered steps of assembly are summarized below:

1. U1 snRNP binds the 5′ splice site; SF1 and U2AF bind the branch point and polypyrimidine tract (E complex).
2. U2 snRNP binds the branch point with ATP hydrolysis (A complex).
3. U4/U6·U5 tri-snRNP joins; Prp28 displaces U1 (B complex).
4. Brr2 unwinds U4/U6; U6 refolds to form the catalytic center (Bact complex).
5. Prp2 removes SF3a/SF3b; first catalytic step occurs (B* → C complex).
6. Prp16 promotes rearrangement; second catalytic step occurs (C* complex).
7. Prp22 releases mRNA; Prp43 disassembles the spliceosome.

For a more detailed treatment of the assembly intermediates, see the [Spliceosome Assembly](/knowledge/molecular-biology/spliceosome-assembly) and [Spliceosome Complex](/knowledge/molecular-biology/spliceosome-complex) entries.

## RNA-RNA and RNA-Protein Interactions in the Spliceosome

The catalytic core of the spliceosome is formed by RNA, not protein. This has led to the classification of the [spliceosome as a ribozyme](/knowledge/molecular-biology/spliceosome-a-ribozyme)—a view supported by the fact that the U2/U6 RNA network can catalyze a two-metal-ion mechanism similar to that of self-splicing group II introns. For a discussion of this evolutionary and mechanistic connection, see [Spliceosome a Ribozyme](/knowledge/molecular-biology/spliceosome-a-ribozyme).

The key RNA–RNA interactions are:

- **U1 snRNA–5′ splice site**: A 6–8 base-pair duplex that is the first recognition event. This interaction is short and is later replaced by U6 snRNA.
- **U2 snRNA–branch point**: A ~10 base-pair duplex that bulges the branch adenosine. The bulged adenosine is the nucleophile for the first catalytic step.
- **U6 snRNA–5′ splice site**: After U1 is displaced, U6 snRNA base-pairs with the 5′ splice site, positioning it for the first catalytic step.
- **U2/U6 helix I**: The central catalytic structure. This helix brings together conserved sequences in U2 and U6 that coordinate two magnesium ions required for catalysis.
- **U2/U6 helix II**: A second, less conserved interaction that stabilizes the U2/U6 structure.
- **U5 snRNA–exon sequences**: The conserved loop of U5 snRNA interacts with the last nucleotide of the 5′ exon and the first nucleotide of the 3′ exon, aligning them for ligation.
- **U4/U6 interaction**: The base-paired U4/U6 structure sequesters U6 in an inactive form. This interaction is unwound by Brr2 during activation.

RNA–protein interactions are equally critical. The Sm/LSm rings stabilize the snRNAs and serve as binding platforms. Prp8, the largest spliceosomal protein, contacts the U5 and U6 snRNAs and the pre-mRNA at the catalytic center, effectively clamping the RNA network in place. The SF3b complex wraps around the U2–branch point duplex, protecting it from premature unwinding. Many accessory proteins, such as the SR proteins and U2AF, make sequence-specific contacts with the pre-mRNA that determine splice site choice.

The overall architecture is one of a dynamic RNA–protein machine in which the RNA provides the catalytic chemistry and the proteins provide the structural framework, regulatory control, and proofreading functions. This division of labor is reflected in the fact that the protein composition of the spliceosome changes dramatically during assembly, while the RNA network remains relatively constant once the catalytic core is formed.

## Methods to Study Spliceosome Composition

Understanding the composition of the spliceosome has required a combination of biochemistry, genetics, and structural biology. The major techniques are described below.

### Affinity Purification

The most direct way to determine the protein composition of the spliceosome is to purify it from cell extracts. This is typically done using a tagged protein component. For example, a tandem affinity purification (TAP) tag can be fused to a core spliceosomal protein such as Prp8 or Brr2. The tagged protein is expressed in cells, the spliceosome is assembled *in vitro* on a pre-mRNA substrate, and the complex is purified by sequential affinity chromatography steps. The purified complex is then analyzed by mass spectrometry to identify its protein components.

This approach has been used to define the composition of each spliceosomal complex (E, A, B, C) in both yeast and human cells. The results show that the spliceosome gains and loses proteins at each step: for example, the B complex contains over 100 proteins, while the C complex has a distinct but overlapping set. Affinity purification can also be combined with quantitative mass spectrometry to measure the stoichiometry of each component.

### Cryo-Electron Microscopy

Cryo-electron microscopy (cryo-EM) has revolutionized the study of [spliceosome structure](/knowledge/molecular-biology/spliceosome-structure). Because the spliceosome is large (1.5–2 MDa) and conformationally heterogeneous, it was difficult to crystallize. Cryo-EM, which does not require crystals, has allowed researchers to determine the structures of the B, Bact, B*, C, and post-catalytic complexes at near-atomic resolution (3–5 Å). These structures reveal the precise positions of the snRNAs, the protein components, and the pre-mRNA substrate.

For example, cryo-EM structures of the *Saccharomyces cerevisiae* and human spliceosomes have shown how Prp8 coordinates the U2/U6 RNA network, how the SF3b complex protects the branch point, and how the U5 snRNA loop aligns the exons. These structures have also revealed the conformational changes that occur during activation and catalysis. The structures are consistent with the biochemical data and have confirmed that the spliceosome is a protein-directed ribozyme.

Other methods, such as crosslinking and immunoprecipitation (CLIP), have been used to map RNA–protein contacts *in vivo*, and genetic screens in yeast have identified the genes encoding spliceosomal proteins. Together, these approaches have provided a comprehensive picture of the spliceosome's composition and dynamics.

## Common Misconceptions and Pitfalls

Students often make several errors when learning about spliceosome composition. These are worth addressing directly.

**Misconception 1: The spliceosome is a single, pre-formed complex.** The spliceosome assembles *de novo* on each intron. It is not a stable complex that travels along the pre-mRNA. The composition changes at each step, and the snRNPs are recycled after each splicing event.

**Misconception 2: snRNPs are the same as the spliceosome.** snRNPs are the RNA–protein subunits that assemble into the spliceosome. The spliceosome contains snRNPs plus many non-snRNP proteins. A snRNP alone cannot catalyze splicing.

**Misconception 3: The spliceosome is composed only of protein.** The spliceosome is composed of both RNA and protein. The snRNAs are essential for catalysis; they are not merely structural scaffolds. The catalytic center is formed by U2 and U6 snRNAs.

**Misconception 4: The spliceosome is composed of enzymes in the traditional sense.** The spliceosome is a ribonucleoprotein complex, not a single enzyme. It contains multiple enzymatic activities, including ATP-dependent helicases and the RNA-based catalytic center. The term "enzyme" is sometimes used loosely, but the spliceosome is better described as a molecular machine.

**Misconception 5: U1 snRNP remains in the spliceosome throughout splicing.** U1 snRNP is released during the B complex formation, after U6 has taken over the 5′ splice site. It is not present in the catalytic complexes.

**Misconception 6: The number of proteins in the spliceosome is fixed.** The number varies depending on the stage of assembly and the organism. The human B complex contains over 100 proteins, while the C complex contains a different set. No single number applies to all spliceosomes.

**Misconception 7: All introns are spliced by the same set of proteins.** While the core snRNPs are universal, many accessory proteins are specific to particular introns or cell types. Alternative splicing is regulated by the differential expression and activity of SR proteins and other factors.

## Summary: The Spliceosome as a Dynamic Ribonucleoprotein Machine

The spliceosome is composed of five snRNPs (U1, U2, U4, U5, and U6) and a large number of accessory proteins that assemble in a stepwise manner on each intron. The snRNPs provide the RNA components that recognize the splice sites and catalyze the two transesterification reactions, while the protein components provide structural stability, regulatory control, and proofreading. The assembly pathway proceeds through defined intermediates—E, A, B, Bact, B*, and C—each with a distinct composition. The spliceosome is therefore not a static entity but a dynamic machine that is assembled, activated, and disassembled for every splicing event.

The study of spliceosome composition has been central to understanding the mechanism of splicing. Affinity purification and mass spectrometry have cataloged the protein components, while cryo-EM has revealed their spatial organization. These studies have confirmed that the spliceosome is a ribozyme guided by proteins, and they have provided a framework for understanding how mutations in splicing factors cause human disease. For further reading on the molecular details of the catalytic mechanism, see [Spliceosome Splicing](/knowledge/molecular-biology/spliceosome-splicing) and [Spliceosome Made](/knowledge/molecular-biology/spliceosome-made).

## Frequently Asked Questions

### What is the spliceosome composed of?

The spliceosome is composed of five small nuclear ribonucleoproteins (snRNPs)—U1, U2, U4, U5, and U6—and numerous non-snRNP protein factors. Each snRNP contains a small nuclear RNA (snRNA) and a set of proteins, including the seven Sm or LSm proteins that form a common core. The non-snRNP proteins include SR proteins, DExD/H-box helicases, and other accessory factors that regulate assembly and catalysis.

### Are spliceosomes composed of RNA and protein?

Yes. The spliceosome is a ribonucleoprotein complex containing both RNA and protein. The RNA components (the snRNAs) are essential for catalysis: U2 and U6 snRNAs form the catalytic center, and U1, U5, and U6 snRNAs recognize the splice sites. The protein components provide structural support, regulatory control, and ATP-dependent remodeling activities.

### What are the main components of the spliceosome?

The main components are the five snRNPs (U1, U2, U4/U6, and U5) and the non-snRNP proteins. The snRNPs contain the snRNAs and the Sm/LSm core proteins. The non-snRNP proteins include SR proteins (e.g., ASF/SF2), helicases (e.g., Prp2, Prp16, Prp22, Brr2), and scaffolding proteins such as Prp8.

### Is the spliceosome composed of enzymes?

The spliceosome is not a single enzyme but a complex containing multiple enzymatic activities. The catalytic reaction is performed by the RNA components (U2 and U6 snRNAs), making the [spliceosome a ribozyme](/knowledge/molecular-biology/spliceosome-a-ribozyme). In addition, the spliceosome contains several protein enzymes, including ATP-dependent RNA helicases and kinases, that drive conformational changes and regulate the reaction.

### How many proteins are in the spliceosome?

The number varies by stage and organism. The human B complex contains more than 100 distinct proteins, while the C complex contains a different but overlapping set of approximately 80–100 proteins. The core snRNPs contribute about 30–40 proteins, and the rest are accessory factors.

### What is the role of snRNPs in the spliceosome?

The snRNPs perform the primary recognition and catalytic functions. U1 snRNP recognizes the 5′ splice site, U2 snRNP recognizes the branch point, U5 snRNP aligns the exons, and U4/U6 snRNPs provide the U6 RNA that forms the catalytic center after U4 is released. Without the snRNPs, splicing cannot occur.

## Key Takeaways

- The spliceosome is composed of five snRNPs (U1, U2, U4, U5, U6) and many non-snRNP proteins, with a total of over 100 proteins in the assembled complex.
- Each snRNP contains a snRNA and a common Sm/LSm protein core, plus specific proteins such as U1-70K, SF3b, Prp8, and Brr2.
- The spliceosome assembles stepwise on each intron through E, A, B, Bact, B*, and C complexes, with distinct protein compositions at each stage.
- Catalysis is performed by RNA: the U2/U6 snRNA network forms the active site, making the spliceosome a ribozyme.
- Non-snRNP proteins, including SR proteins and DExD/H-box helicases, regulate splice site selection, promote conformational changes, and proofread the reaction.
- The spliceosome is a dynamic machine, not a static complex; it is assembled, activated, and disassembled for every splicing event.
- Cryo-EM and mass spectrometry have revealed the near-atomic structure and complete protein composition of the spliceosome, confirming its role as a protein-directed ribozyme.

## Related Clinical & Scientific Guides

* [MAPK Pathway: Mechanism, Function, and Clinical Relevance](/knowledge/molecular-biology/mapk-pathway)
* [Mammalian Cell Culture Bioreactors: A Practical Guide](/knowledge/molecular-biology/mammalian-cell-culture-bioreactor)
* [Nucleotide Formation: Biosynthesis and Assembly of DNA/RNA Building Blocks](/knowledge/molecular-biology/nucleotide-formation)