U2-type precatalytic spliceosome

GO:0071005 · Ribonucleoprotein · Ribonucleoprotein complex · Proposed

A spliceosomal complex that is formed by the recruitment of the preassembled U4/U6.U5 tri-snRNP to the prespliceosome. Although all 5 snRNPs are present, the precatalytic spliceosome is catalytically inactive. The precatalytic spliceosome includes many proteins in addition to those found in the U1, U2 and U4/U6.U5 snRNPs. [GO:0071005]

Synonyms: major precatalytic spliceosome

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Role
U1 snRNP	Protein complex	part of	`GO:0005685`	Small nuclear ribonucleoprotein present in the U2-type precatalytic spliceosome before U1 release during activation. `GO:0071005` GO:0071005 defines the U2-type precatalytic spliceosome as a complex in which the U1 snRNP is present. `GO:0005685` GO:0005685 is the Gene Ontology cellular-component term for the U1 snRNP.
U2 snRNP	Protein complex	part of	`GO:0005686`	Small nuclear ribonucleoprotein that recognizes the branch-point region and interfaces with U4/U6.U5 tri-snRNP in the precatalytic spliceosome. `GO:0071005` GO:0071005 defines the U2-type precatalytic spliceosome as a complex in which the U2 snRNP is present. `GO:0005686` GO:0005686 is the Gene Ontology cellular-component term for the U2 snRNP. `DOI:10.1038/nature22799` Plaschka, Lin and Nagai 2017 solved a yeast precatalytic B-complex structure and modeled contacts between U2 snRNP and U4/U6.U5 tri-snRNP.
U4/U6 x U5 tri-snRNP complex	Protein complex	part of	`GO:0046540`	Preassembled tri-snRNP subcomplex recruited to the prespliceosome to form the U2-type precatalytic spliceosome. `GO:0071005` GO:0071005 defines the U2-type precatalytic spliceosome as formed by recruitment of preassembled U4/U6.U5 tri-snRNP to the prespliceosome. `GO:0046540` GO:0046540 is the Gene Ontology cellular-component term for the U4/U6 x U5 tri-snRNP complex. `DOI:10.1038/nature22799` Plaschka, Lin and Nagai 2017 reported a near-atomic cryo-EM structure of the Saccharomyces cerevisiae pre-catalytic B-complex spliceosome.

Taxonomic distribution

NCBITaxon:4932 Saccharomyces cerevisiae — COMMON: The U2-type precatalytic B-complex spliceosome is directly evidenced in budding yeast; broader eukaryotic distribution is left for follow-up curation. [DOI:10.1038/nature22799]

Canonical examples

NCBITaxon:4932 Saccharomyces cerevisiae — Budding-yeast model whose pre-catalytic B-complex spliceosome was resolved by cryo-electron microscopy at near-atomic resolution. [DOI:10.1038/nature22799]

Mechanism graphs

U4/U6.U5 tri-snRNP recruitment forms the precatalytic B complex (ASSEMBLY)

The U2-type precatalytic spliceosome forms when preassembled U4/U6.U5 tri-snRNP joins the U1- and U2-containing prespliceosome on pre-mRNA, creating a catalytically inactive complex poised for RNA-RNA remodeling.

Subject	Predicate	Object	Evidence
U1 snRNP	associates with	U2-type prespliceosome	`GO:0071004` GO:0071004 defines the U2-type prespliceosome by 5-prime splice-site association with U1 snRNP and branch-point recognition by U2 snRNP.
U4/U6 x U5 tri-snRNP complex	is recruited to form	U2-type precatalytic spliceosome	`GO:0071005` GO:0071005 defines the U2-type precatalytic spliceosome as the state formed by recruitment of U4/U6.U5 tri-snRNP to the prespliceosome.
U2-type prespliceosome	receives	U4/U6 x U5 tri-snRNP complex	`GO:0071005` GO:0071005 defines the U2-type precatalytic spliceosome as formed by tri-snRNP recruitment to the prespliceosome.
U2 snRNP	interfaces with	U4/U6 x U5 tri-snRNP complex	`DOI:10.1038/nature22799` Plaschka, Lin and Nagai 2017 modeled the yeast B-complex U2/U6 helix II and U2 SF3b to U4/U6 di-snRNP contacts.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral component boundaries for Prp38, Snu23, Spp381, Prp8/Brr2/Snu114 and other proteins observed in U2-type precatalytic B complexes. GO grounds the snRNP subcomplexes that define this assembly state. The first record does not split the many B-complex proteins into orthologous source-neutral families or distinguish Saccharomyces and metazoan accessory factors.

Evidence

GO:0071005 Gene Ontology cellular-component term for the U2-type precatalytic spliceosome and its exact synonym.
GO:0071011 Gene Ontology cellular-component parent term for precatalytic spliceosome.
GO:0005684 Gene Ontology cellular-component parent term for U2-type spliceosomal complex.
GO:0005685 Gene Ontology cellular-component term for the U1 snRNP.
GO:0005686 Gene Ontology cellular-component term for the U2 snRNP.
GO:0046540 Gene Ontology cellular-component term for the U4/U6 x U5 tri-snRNP complex.
GO:0071004 Gene Ontology cellular-component term for the U2-type prespliceosome.
DOI:10.1038/nature22799 Plaschka, Lin and Nagai 2017, Structure of a pre-catalytic spliceosome, Nature.
PMID:28530653 PubMed identifier for Plaschka, Lin and Nagai 2017 as reported by PubMed.
DOI:10.1101/cshperspect.a003707 Will and Luehrmann 2011, Spliceosome structure and function, Cold Spring Harbor Perspectives in Biology.
PMID:21441581 PubMed identifier for Will and Luehrmann 2011 as reported by PubMed.

Curation history

2026-09-24T12:44:49Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-24T12:46:01Z codex · EXPANDED_RECORD — Added GO synonymy, U2/precatalytic parentage, GO-grounded U1, U2 and U4/U6 x U5 snRNP components, budding-yeast distribution, Plaschka/Will evidence, a conservative preactivation function, a U4/U6.U5 recruitment graph and a B-complex protein-component TODO.
2026-09-24T12:46:34Z codex · REFINED_GRAPH — Removed the redundant U2/U6 helix II standalone node and left the helix-II evidence on the U2 snRNP to U4/U6 x U5 tri-snRNP edge.
2026-09-24T12:58:09Z codex · ADDRESS_REVIEW — #1489: removed the passive preactivation state from functions; #1490: connected the assembly graph to the GO:0071005 structure node.

Source: data/structures/ribonucleoprotein/u2_type_precatalytic_spliceosome.yaml