Spliceosome

The spliceosome is a large RNA-protein complex in the nucleus of eukaryotic cells that removes introns from pre-mRNA and ligates exons. It is responsible for precise excision and the generation of mature mRNA. It is composed of five small nuclear ribonucleoprotein particles (snRNPs): U1, U2, U4, U5, and U6, along with a multitude of non-snRNP proteins. Each snRNP consists of a small nuclear RNA (snRNA) and associated proteins; snRNAs participate in recognizing splice sites and catalysis. The snRNAs U1, U2 recognize 5' splice site and branch point; U4/U6.U5 tri-snRNP is involved in assembly.

Mechanism: Spliceosome assembly proceeds through ordered steps. U1 base-pairs with the 5' splice site to define the boundary, U2 associates with the branch point, aided by SF1 and U2 auxiliary factors to form the A complex. The U4/U6.U5 tri-snRNP joins to form the B complex, which then rearranges as U1 and U4 depart to form the activated B* and catalytic core. The first transesterification cuts at the 5' splice site and forms a lariat intermediate with the branch point. The second transesterification joins the exons and releases the intron as a lariat, completing splicing. The catalytic site is largely RNA-based, supported by proteins.

Biological significance: Splicing is essential for gene expression and contributes to transcript diversity via alternative splicing. Spliceosome assembly and function are regulated by additional proteins and RNA elements, and dysfunction can lead to disease. The spliceosome is a dynamic, highly conserved RNP machine found in the nucleus; its discovery led to the 1993 Nobel Prize in Physiology or Medicine for demonstrating RNA splicing.