catalytic step 1 spliceosome

GO:0071012 · Ribonucleoprotein · Ribonucleoprotein complex · Proposed

A spliceosomal complex that is formed by the displacement of the two snRNPs from the precatalytic spliceosome; three snRNPs including U5 remain 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 associated snRNPs. [GO:0071012]

Synonyms: activated spliceosome; mammalian spliceosomal complex B*; mammalian spliceosomal complex B2; yeast spliceosomal complex A1

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

Components

Component	Type	Part of / acts on	Grounding	Genes	Stoichiometry	Essential	Role
retained catalytic step 1 snRNPs	Protein complex	part of	Reviewed label only (reviewed name; no accession asserted)				Subtype-specific set of three snRNP constituents, including U5, that remain associated with mRNA in catalytic step 1 spliceosomes. `GO:0071012` GO:0071012 defines the catalytic step 1 spliceosome as retaining three snRNPs including U5 after displacement of two snRNPs from the precatalytic spliceosome. `GO:0071006` GO:0071006 defines the U2-type catalytic step 1 spliceosome by retention of U2, U5 and U6 snRNPs. `GO:0071017` GO:0071017 defines the U12-type catalytic step 1 spliceosome by retention of U12, U5 and U6atac snRNPs.

Mechanism graphs

Three snRNPs remain in catalytic step 1 spliceosomes (ASSEMBLY)

Catalytic step 1 spliceosomes form when precatalytic spliceosomes are remodeled by displacement of two snRNPs while U5 and two subtype-specific snRNPs remain associated with mRNA.

Subject	Predicate	Object	Evidence
retained catalytic step 1 snRNPs	remain associated in	catalytic step 1 spliceosome	`GO:0071012` GO:0071012 defines catalytic step 1 spliceosomes by retention of three snRNPs including U5.
precatalytic spliceosome	is remodeled to form	catalytic step 1 spliceosome	`GO:0071012` GO:0071012 defines the catalytic step 1 spliceosome as formed by displacement of two snRNPs from the precatalytic spliceosome.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral snRNP membership and additional protein boundaries for each catalytic step 1 spliceosome subtype. The first GO:0071012 parent record captures only the generic retention of three snRNPs including U5 after precatalytic remodeling. Child records for U2-type and U12-type catalytic step 1 spliceosomes should carry the exact U2/U5/U6 and U12/U5/U6atac constituent boundaries that apply to those specific states.

Evidence

GO:0071012 Gene Ontology cellular-component term for catalytic step 1 spliceosome and its GO synonyms.
GO:0005681 Gene Ontology cellular-component parent term for spliceosomal complex.
GO:1902494 Gene Ontology cellular-component parent term for catalytic complex.
GO:0071011 Gene Ontology cellular-component term for the preceding precatalytic spliceosome state.
GO:0071006 Gene Ontology cellular-component term for the U2-type catalytic step 1 spliceosome child state.
GO:0071017 Gene Ontology cellular-component term for the U12-type catalytic step 1 spliceosome child state.
GO:0005682 Gene Ontology cellular-component term for the U5 snRNP.
GO:0005686 Gene Ontology cellular-component term for the U2 snRNP.
GO:0005688 Gene Ontology cellular-component term for the U6 snRNP.
GO:0005693 Gene Ontology cellular-component term for the U12 snRNP.
GO:0005691 Gene Ontology cellular-component term for the U6atac snRNP.

Curation history

2026-09-25T08:37:13Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-25T08:37:55Z codex · EXPANDED_RECORD — Added GO exact and narrow synonyms, spliceosomal and catalytic-complex parentage, a reviewed-label-only retained catalytic step 1 snRNP component, a conservative snRNP-retention graph and a TODO for exact U2-type and U12-type catalytic step 1 spliceosome boundaries.

Source: data/structures/ribonucleoprotein/catalytic_step_1_spliceosome.yaml