Isp3 layer of spore wall

GO:1990916 · Spore · Envelope layer · Proposed

The outermost layers of the spore wall, as described in Schizosaccharomyces pombe. [GO:1990916]

Part of: ascospore wall (GO:0005619)

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
Isp3 spore wall protein	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	isp3		ESSENTIAL	Major proteinaceous constituent of the outermost fission-yeast ascospore wall layer. `DOI:10.1091/mbc.E13-12-0731` Fukunishi et al. 2014 identified p23 as the Isp3/Meu4 protein, found that Isp3 remains firmly associated with the spore-wall fraction, and localized Isp3 to the outermost electron-dense layer by immunoelectron microscopy.

Taxonomic distribution

NCBITaxon:4896 Schizosaccharomyces pombe — COMMON: The Isp3 layer is the outermost spore-wall layer directly characterized in fission yeast; no broader fungal distribution is asserted in this first standalone layer record. [DOI:10.1091/mbc.E13-12-0731]

Canonical examples

NCBITaxon:4896 Schizosaccharomyces pombe — Fission-yeast model in which electron microscopy and immunoelectron microscopy show Isp3 in the outermost spore-wall layer. [DOI:10.1091/mbc.E13-12-0731]

Functions

outer spore-wall stress barrier — The Isp3-rich outer layer of the fission-yeast spore wall contributes to resistance against heat, digestive-enzyme, and ethanol stress.
- DOI:10.1091/mbc.E13-12-0731 Fukunishi et al. 2014 found decreased heat, digestive-enzyme, and ethanol tolerance in isp3-deletion spores.

Mechanism graphs

Isp3 builds the outer fission-yeast spore-wall layer (FUNCTION)

In Schizosaccharomyces pombe, the Isp3 spore-wall protein localizes to and forms the outermost electron-dense layer of the ascospore wall, where the layer contributes to spore stress resistance.

Subject	Predicate	Object	Evidence
Isp3 spore wall protein	forms	Isp3 layer of spore wall	`DOI:10.1091/mbc.E13-12-0731` Fukunishi et al. 2014 identified Isp3/Meu4 as the p23 spore-wall protein and showed that the outermost electron-dense layer is absent from isp3-deletion spores.
Isp3 layer of spore wall	is outermost layer of	ascospore wall	`GO:1990916` GO:1990916 defines the Isp3 layer as the outermost layers of the Schizosaccharomyces pombe spore wall. `GO:0005619` GO places GO:1990916 as part of the ascospore wall.
Isp3 layer of spore wall	contributes to	spore environmental stress resistance	`DOI:10.1091/mbc.E13-12-0731` Fukunishi et al. 2014 measured decreased heat, digestive-enzyme, and ethanol tolerance when isp3 was deleted.

Discussions

CURATION_TODO (OPEN): Resolve whether Isp3 has an exact source-neutral protein-family grounding suitable for the component. The component is curated as reviewed-label-only because this pass verified the GO layer and Isp3 spore-wall role but did not find an exact InterPro, Pfam, NCBIfam, or GO protein-family identifier for fission-yeast Isp3/Meu4.
INTERPRETATION (RESOLVED): Keep the Schizosaccharomyces Isp3 layer separate from the Saccharomyces chitosan and dityrosine-rich ascospore-wall layers. GO:1990916 denotes the Schizosaccharomyces pombe Isp3 layer, whereas GO:0005631 and GO:0005630 denote Saccharomyces chitosan and dityrosine spore-wall layers.

Evidence

GO:1990916 Gene Ontology cellular-component term for the Schizosaccharomyces pombe Isp3 layer of the spore wall.
GO:0005619 Gene Ontology cellular-component term for the broader ascospore wall that contains GO:1990916.
DOI:10.1091/mbc.E13-12-0731 Fukunishi et al. 2014, The fission yeast spore is coated by a proteinaceous surface layer comprising mainly Isp3, Molecular Biology of the Cell.

Curation history

2026-09-23T01:08:07Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-23T01:09:44Z codex · EXPANDED_RECORD — Added ascospore-wall parthood, a reviewed-label-only Isp3 protein component, Schizosaccharomyces pombe scope, a stress barrier function, GO/DOI evidence, a nonmechanistic Isp3 layer topology graph, and boundary discussions for Isp3 component grounding and Saccharomyces layer separation.

Source: data/structures/spore/isp3_layer_of_spore_wall.yaml