Overview
The spliceosome is a large, dynamic ribonucleoprotein assembly that removes noncoding sequences (introns) from newly transcribed precursor messenger RNA (pre-mRNA) and ligates the surrounding exons. This process, called splicing, occurs in the nucleus of eukaryotic cells and is essential for producing mature mRNAs capable of directing protein synthesis. Splicing is intimately connected with other steps of gene expression, including transcription and RNA processing, and influences which protein isoforms are produced.
Composition and structure
Each major spliceosome is formed from five small nuclear ribonucleoprotein particles (snRNPs), named U1, U2, U4, U5 and U6. Each snRNP contains a small nuclear RNA (snRNA) and several associated proteins; overall the machine includes more than fifty protein components as well as numerous transient cofactors. The RNA components act as guides for sequence recognition and participate directly in catalysis; see RNA. Protein subunits provide structural support, regulation and enzymatic activities; see protein.
Assembly and mechanism
Spliceosome assembly proceeds stepwise on each intron: early splice-site recognition is followed by ordered recruitment of snRNPs and many protein factors to generate complexes commonly termed E, A, B and C. Recognition relies on conserved sequence elements such as the 5' splice site, a branch-point adenine, a polypyrimidine tract and the 3' splice site. Pairing between snRNA and pre-mRNA and ATP-dependent rearrangements mediated by helicases drive conformational changes that position reactive groups for chemistry.
Catalysis and chemistry
The catalytic core carries out two sequential transesterification reactions. In the first step the branch-point adenosine attacks the 5' splice site, producing a lariat-shaped intermediate. In the second step the freed 3' end of the upstream exon attacks the downstream splice site, releasing the intron lariat and joining the exons. The RNA-rich active site shows functional parallels with self-splicing group II introns, and structural studies have revealed RNA–RNA and RNA–protein contacts that stabilize the catalytic center.
Alternative and minor spliceosomes
Alternative splicing allows a single gene to produce multiple mRNA isoforms by varying splice-site choice, exon inclusion or intron retention; this expands proteomic diversity and enables tissue-specific regulation. A distinct, less common machinery called the minor spliceosome recognizes a rare class of introns and uses related but different snRNAs (for example U11 and U12) to catalyze splicing of U12-type introns.
Regulation, disease and research
Splicing is regulated by sequence elements in the RNA and by numerous regulatory proteins that enhance or repress splice-site use. Mutations that disrupt splicing signals or spliceosome components can cause inherited diseases and contribute to cancer by altering gene expression or producing harmful protein variants. Therapeutic approaches that modulate splicing include antisense oligonucleotides and small molecules. Structural and biochemical methods, including cryo-electron microscopy and high-throughput sequencing, continue to refine our understanding of spliceosome dynamics and function.
Context and resources
The discovery of introns and the spliceosome fundamentally changed molecular biology. For introductions to related concepts see materials on transcribed RNA, general descriptions of introns, and summaries of small nuclear particles and ribonucleoprotein assemblies. Further technical reviews and textbooks discuss the mechanistic details and the expanding biomedical implications of splicing.