RNA Splicing

The spliceosome is a large ribonucleoprotein complex composed of five snRNPs (U1, U2, U4, U5, and U6) and several proteins that catalyzes RNA splicing. It recognizes specific nucleotide sequences at exon-intron boundaries and orchestrates the removal of introns while joining exons together through transesterification reactions.

The spliceosome catalyzes two sequential transesterification reactions. First, the branch point adenine attacks the 5' splice site, forming a lariat loop containing the intron. Then, the 3' end of the upstream exon attacks the 5' end of the downstream exon, releasing the lariat and ligating the exons together.

Alternative splicing combines different exon combinations from a single pre-mRNA to produce multiple mature mRNA variants and protein isoforms. This process generates proteins with different functions, cellular localizations, and interactions, enabling tissue- and environment-specific gene expression from a single gene.

Alternative splicing patterns include exon skipping, where shorter exons are omitted; alternative 5' or 3' splice sites, which change exon boundaries; and intron retention, where short introns escape removal. These patterns are determined by exon and intron length and the strength of splicing signals at splice sites.

Aberrant splicing produces abnormal protein isoforms that can cause diseases including cancer. For example, the BCL2L1 gene produces BCL-XL, which promotes cell survival and is overexpressed in cancers, while the pro-death BCL-XS isoform is suppressed, disrupting normal cell death pathways.

The spliceosome recognizes a GU-containing sequence at the 5' end of the intron and an AG-containing sequence near the 3' end, along with a branch point sequence containing adenine toward the intron's 3' end. These conserved sequences signal where splicing should occur.

snRNPs are small nuclear ribonucleoproteins consisting of small nuclear RNAs and proteins. The five snRNPs (U1, U2, U4, U5, U6) sequentially bind to splice sites and branch points, bringing the 5' splice site close to the branch point adenine to facilitate the transesterification reactions that remove introns.