spliceosomal complex

Definition

Target type: cellularcomponent

Any of a series of ribonucleoprotein complexes that contain snRNA(s) and small nuclear ribonucleoproteins (snRNPs), and are formed sequentially during the spliceosomal splicing of one or more substrate RNAs, and which also contain the RNA substrate(s) from the initial target RNAs of splicing, the splicing intermediate RNA(s), to the final RNA products. During cis-splicing, the initial target RNA is a single, contiguous RNA transcript, whether mRNA, snoRNA, etc., and the released products are a spliced RNA and an excised intron, generally as a lariat structure. During trans-splicing, there are two initial substrate RNAs, the spliced leader RNA and a pre-mRNA. [GOC:editors, GOC:mah, ISBN:0198547684, PMID:19239890]

The spliceosomal complex is a large and dynamic ribonucleoprotein (RNP) machine responsible for removing introns from pre-mRNA molecules during RNA splicing. This complex is composed of five small nuclear ribonucleoproteins (snRNPs), each containing a small nuclear RNA (snRNA) molecule and several associated proteins. The snRNPs are designated U1, U2, U4, U5, and U6, and they assemble in a specific order onto the pre-mRNA to form the functional spliceosome.

The cellular component of the spliceosomal complex is highly dynamic and undergoes significant structural rearrangements during the splicing process. It exists in both the nucleus and cytoplasm, but its primary function occurs in the nucleus.

Here is a more detailed description of its key components:

**snRNAs:**
- U1 snRNA: Recognizes the 5' splice site of an intron.
- U2 snRNA: Base pairs with the branch point sequence within the intron.
- U4 snRNA: Binds to U6 snRNA and helps in the assembly of the spliceosome.
- U5 snRNA: Brings the 5' and 3' splice sites together for the splicing reaction.
- U6 snRNA: Catalyses the transesterification reactions that remove the intron and join the exons.

**Proteins:**
- **Sm proteins:** These proteins form a ring-like structure that binds to the conserved Sm site on all snRNAs except U6.
- **Lsm proteins:** Bind to the 3' end of U6 snRNA.
- **Other proteins:** A variety of other proteins are involved in the assembly, function, and disassembly of the spliceosome, including:
- **SR proteins:** These proteins bind to exonic splicing enhancers and promote the recognition of splice sites.
- **Prp proteins:** These proteins are involved in the catalytic steps of splicing.
- **SF3A1 and SF3B1:** These proteins bind to the U2 snRNP and help in the recognition of the branch point sequence.

**Assembly and Function:**
The spliceosomal complex assembles on pre-mRNA in a stepwise manner. The process begins with the recruitment of U1 snRNP to the 5' splice site and U2 snRNP to the branch point sequence. U4/U6.U5 tri-snRNP then joins the complex, followed by a series of conformational changes that lead to the catalytic activation of the spliceosome.

The spliceosome then removes the intron through two transesterification reactions:
1. The 5' splice site is cleaved, and the 5' end of the intron is joined to the branch point sequence, forming a lariat structure.
2. The 3' splice site is cleaved, and the two exons are ligated together.

The spliced mRNA is then released from the complex, and the spliceosome is disassembled for use in subsequent splicing events.

**Regulation:**
Splicing is a highly regulated process, and the spliceosomal complex plays a key role in ensuring accurate and efficient splicing. Factors that influence splicing include:
- **Alternative splicing:** Different spliceosomal complexes can recognize different splice sites, leading to the production of multiple protein isoforms from a single gene.
- **Splicing factors:** Various proteins, such as SR proteins and hnRNPs, can bind to specific sequences in pre-mRNA and influence splicing decisions.
- **Chromatin structure:** The accessibility of pre-mRNA to the spliceosome is also influenced by chromatin structure.

In conclusion, the spliceosomal complex is a vital component of the cellular machinery responsible for gene expression. Its intricate composition and dynamic nature enable the precise removal of introns from pre-mRNA, ultimately leading to the production of functional proteins.'
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Proteins (7)

Compounds (238)