GO:0071006 U2-type catalytic step 1 spliceosome

The U2-type catalytic step 1 spliceosome (GO:0071006) is a cellular-component term describing the spliceosomal complex that performs the first catalytic step of pre-mRNA splicing for U2-type introns. It is formed by the displacement of the U1 and U4 snRNPs from the precatalytic spliceosome, while the U2, U5 and U6 snRNPs remain associated with the mRNA. This complex is sometimes called the activated spliceosome and is the catalytically active form of the spliceosome, containing many proteins in addition to those found in the U2, U5 and U6 snRNPs. Because it represents the point at which the spliceosome becomes catalytically competent, GO:0071006 is central to understanding how introns are removed and exons are joined. Researchers studying RNA processing, gene regulation and splicing-related disease need to know how this complex is assembled, what its protein composition is, and how its activity can be experimentally manipulated. The term is also known by several synonyms, including GT-AG catalytic step 1 spliceosome, major catalytic step 1 spliceosome, mammalian U2-type spliceosomal complex B*, mammalian U2-type spliceosomal complex B2, and yeast U2-type spliceosomal complex A1. These synonyms reflect the conservation of the activated spliceosome across species and its role in the major, U2-type splicing pathway.

In simple terms, GO:0071006 defines the activated spliceosome that carries out the first chemical step of splicing for U2-type introns. According to the QuickGO definition, it is a spliceosomal complex formed by the displacement of the U1 and U4 snRNPs from the precatalytic spliceosome, with the U2, U5 and U6 snRNPs remaining associated with the mRNA. This complex, sometimes called the activated spliceosome, is the catalytically active form of the spliceosome and includes many proteins in addition to those found in the U2, U5 and U6 snRNPs. It is a cellular_component term, meaning it describes a physical assembly rather than a process or a molecular function. The complex is specific to U2-type (major, GT-AG) introns and is distinct from the U12-type spliceosome. Its formation is a key checkpoint in spliceosome activation, and its catalytic step 1 activity is required for subsequent exon ligation.

The U2-type catalytic step 1 spliceosome is important because it is the catalytically active form of the spliceosome that executes the first step of pre-mRNA splicing for the majority of human introns. Understanding this complex helps explain how splice-site signals are recognized and how RS-domain proteins and splicing signals interact to promote catalysis. Because splicing is essential for gene expression, perturbations in the assembly or activity of this complex can alter mRNA and protein output and have been associated with human disease. Studying GO:0071006 therefore provides a mechanistic window into RNA processing, gene regulation and the molecular basis of splicing-related disorders.

The following genes and proteins are associated with the U2-type catalytic step 1 spliceosome and its regulation, based on the verified literature.

The formation and activity of the U2-type catalytic step 1 spliceosome are regulated by the displacement of U1 and U4 snRNPs and the retention of U2, U5 and U6 snRNPs on the mRNA. RS domain-splicing signal interactions contribute to the splicing of U12-type and U2-type introns, indicating that signal recognition and protein-RNA contacts influence the activation state of the spliceosome. The complex also includes many proteins in addition to those found in the U2, U5 and U6 snRNPs, which provides additional layers of regulation beyond the core snRNP machinery.

Because the U2-type catalytic step 1 spliceosome is the catalytically active form of the spliceosome, alterations in its components or in the signals it recognizes can perturb pre-mRNA splicing. RS domain-splicing signal interactions are important for splicing of U12-type and U2-type introns, linking signal recognition to disease-relevant splicing outcomes. Research into GO:0071006 therefore helps clarify how splicing defects arise at the level of the activated complex.

Splicing is essential for gene expression, and the activated spliceosome is a central node in this process. Perturbations that affect the formation or activity of the U2-type catalytic step 1 spliceosome can change mRNA and protein output, which is relevant to cancer biology. Studying the complex provides a mechanistic basis for understanding how splicing changes contribute to altered gene expression programs.

Accurate splicing is required for normal cellular function, and defects in spliceosomal complexes have been linked to human disease. The U2-type catalytic step 1 spliceosome is part of the core splicing machinery, so its dysfunction can affect transcripts important for neural and other tissues. Model systems that perturb this complex can help test hypotheses about splicing-related disease mechanisms.

RNA-seq can be used to measure changes in splicing after perturbation of genes associated with the U2-type catalytic step 1 spliceosome. Because the complex acts on U2-type introns, RNA-seq readouts can reveal how its disruption affects intron removal and exon joining. This approach links the molecular function of GO:0071006 to transcript-level outcomes.

Proteomics can identify the proteins that associate with the activated spliceosome, including the many proteins beyond the U2, U5 and U6 snRNP components. Tagged knock-in models enable affinity purification of the complex for mass spectrometry. This helps define the composition of GO:0071006 in different cell types.

Imaging approaches can visualize the localization and assembly of spliceosomal components in cells. Such methods help determine when and where the U2-type catalytic step 1 spliceosome forms relative to other splicing complexes. They complement biochemical and sequencing-based readouts.

CRISPR knockout, point mutation, knock-in and overexpression models can be combined with RNA-seq and proteomics to test the function of genes associated with GO:0071006. This integrated approach connects genotype to splicing phenotype. It is well suited to studying the activated spliceosome in disease-relevant contexts.

Researchers studying U2-type catalytic step 1 spliceosome-related genes often need to determine whether a candidate gene is causally involved in splicing, how specific domains contribute, and which transcripts depend on the complex. EDITGENE provides CRISPR-based cell models and screening services designed to answer these questions with publication-ready rigor.

The U2-type catalytic step 1 spliceosome (GO:0071006) is the activated, catalytically active spliceosomal complex that performs the first step of splicing for U2-type introns. Its formation involves displacement of U1 and U4 snRNPs and retention of U2, U5 and U6 snRNPs on the mRNA, together with many additional proteins. Understanding its composition, assembly and regulation is essential for mechanistic studies of RNA processing and for investigating splicing-related disease. CRISPR-based knockout, point-mutation, knock-in and overexpression models, combined with RNA-seq and proteomics, provide a powerful toolkit for dissecting this complex.