Fungal cell wall alpha-1,3-glucan synthesis and remodeling

Alpha-1,3-glucan is a major cell wall polysaccharide in many ascomycetes and basidiomycetes. It is absent from Saccharomyces cerevisiae, but in Schizosaccharomyces pombe it is about a third of the wall glucan and is essential. It is made at the plasma membrane by Ags1-family synthases, very large multi-pass proteins with two probable catalytic regions. An intracellular GT5 (glycogen synthase-like) domain makes a short alpha-1,4-glucan primer. An extracellular GH13 (alpha-amylase-like) domain is proposed to join or remodel chains. The mature S. pombe polymer is two alpha-1,3-glucan chains of about 120 residues each, coupled through short alpha-1,4-linked segments. Separate GPI-anchored GH13 enzymes on the cell surface (Aah1 and Aah3 in S. pombe) are needed for normal chain assembly. By analogy to characterised Aspergillus homologs they are thought to be 4-alpha-glucanotransferases, not hydrolases. In S. pombe, Ags1 makes alpha-glucan at growing tips and in the primary septum, alongside the beta-glucan synthase Bgs1, while sporulation-specific paralogs (Mok12, Mok13, Mok14) make the spore-envelope alpha-glucans. UDP-glucose supply, the Rho2-Pck2 signalling that controls Ags1, synthase trafficking, and alpha-glucan degradation (e.g. Agn1 during cell separation) are outside the module.

MODULE:fungal_alpha_1_3_glucan_synthesisDRAFTCONCRETEMetabolic Pathwaymodules/fungal_alpha_1_3_glucan_synthesis.yaml
fungal-type cell wall (1->3)-alpha-glucan biosynthetic processGO:0070600
GO:0070600
fungal-type cell wall (1->3)-alpha-glucan biosynthetic process
Module-level process concept.
PMID:9689051
Identification of a putative alpha-glucan synthase essential for cell wall construction and morphogenesis in fission yeast.
Discovery of ags1 and of its two-catalytic-domain architecture.
the predicted Ags1 protein consists of two probable catalytic domains for alpha-glucan assembly, namely an intracellular domain for alpha-glucan synthesis and an extracellular domain speculated to cross-link or remodel alpha-glucan.
PMID:40668835
α-glucan remodeling by GH13-domain enzymes shapes fungal cell wall architecture.
Aah1 and Aah3 act redundantly with each other but not with Ags1. Both kinds of GH13 activity are needed for alpha-glucan chain assembly.
Aah1 and Aah3 enzymes are not functionally redundant with Ags1, and cells require both types of GH13 domain enzymes to support α-glucan chain assembly
PMID:22891259
Fission yeast Ags1 confers the essential septum strength needed for safe gradual cell abscission.
Ags1 alpha-glucan is a structural component of the primary septum, made alongside Bgs1.
Ags1 synthesizes an α-glucan of the primary septum that is essential to confer it the mechanical strength
7Nodes
2Parts
2Variant Sets
4Variants
4Annotons
1Connections

Derived QC

Recommended-field compliance

80.0% recommended fields populated
  • knowledge_gaps[0] · provenance (0/1)

Module deep research

✗ none found

No MODULE:fungal_alpha_1_3_glucan_synthesis deep-research report alongside the module YAML.

Leaf nodes lacking representative members

✓ every leaf node grounds to a representative protein.

Template conformance

✓ every declared conforms_to bundle matches its template motif.

Function consistency and core coverage

3 gene-function check(s) reflected in core functions · 0 conflict(s) · 5 evidence or coverage gap(s) (8 comparison(s))

Retained noncore functions are reported separately; they do not require promotion to core functions. Support from representative genes does not establish conservation across the whole family. Missing reviews or identifiers leave a claim unverified.

AnnotonParticipantFunctionAssessment
ags1_activity ags1/mok1 (Schizosaccharomyces pombe)
UniProtKB:Q9USK8
Family Representative 		alpha-1,3-glucan synthase activity
GO:0047657 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
mok_spore_activity mok12 (Schizosaccharomyces pombe)
UniProtKB:Q9UUL4
Family Representative 		alpha-1,3-glucan synthase activity
GO:0047657 		Review Missing
No ai-review exists for this protein; its function and core-function coverage cannot yet be checked.
aah1_transglycosylase aah1 (Schizosaccharomyces pombe)
UniProtKB:O74922
Family Representative 		4-alpha-glucanotransferase activity
GO:0004134 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
aah3_transglycosylase aah3 (Schizosaccharomyces pombe)
UniProtKB:Q9Y7S9
Family Representative 		4-alpha-glucanotransferase activity
GO:0004134 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
ags1_activity Ags1 subfamily
PANTHER:PTHR47182:SF2
Family 		alpha-1,3-glucan synthase activity
GO:0047657 		Family Review Missing
No supported structured family review is available for PANTHER:PTHR47182:SF2.
mok_spore_activity Mok12 subfamily (representative of the spore paralogs)
PANTHER:PTHR47182:SF5
Family 		alpha-1,3-glucan synthase activity
GO:0047657 		Family Review Missing
No supported structured family review is available for PANTHER:PTHR47182:SF5.
aah1_transglycosylase Aah1 subfamily of GH13 alpha-amylase-family proteins
PANTHER:PTHR10357:SF215
Family 		4-alpha-glucanotransferase activity
GO:0004134 		Family Function Unreviewed
The family review has no applicable adjudication supporting or excluding GO:0004134.
aah3_transglycosylase Aah3 subfamily of GH13 alpha-amylase-family proteins
PANTHER:PTHR10357:SF224
Family 		4-alpha-glucanotransferase activity
GO:0004134 		Family Function Unreviewed
The family review has no applicable adjudication supporting or excluding GO:0004134.

Gene-review completeness (3/4 grounded genes reviewed)

3 complete review(s) · 1 with deep research · 1 missing review · 2 reviewed but lacking deep research

Gene Review Complete Deep research
aah1 O74922 ✓ ✓ ✓
aah3 Q9Y7S9 ✓ ✓ ✗
ags1 Q9USK8 ✓ ✓ ✗
mok12 (Schizosaccharomyces pombe) Q9UUL4 ✗ — —

Details

Context
fungiNCBITaxon:4751
Fungal cell wall alpha-1,3-glucan synthesis and remodelingMetabolic Pathwayfungal_alpha_1_3_glucan_synthesis
fungal-type cell wall (1->3)-alpha-glucan biosynthetic processGO:0070600
Context
fungiNCBITaxon:4751

NEW module (2026-11), built after reviewing S. pombe ags1 and aah3 and a consistency pass on aah1. It is the alpha-glucan counterpart of fungal_beta_1_3_glucan_synthesis. The two meet at the primary septum, where Ags1 and Bgs1 co-immunoprecipitate, and the PomBase septum GO-CAM 6918f23700003787 contains both. Boundary decisions: (1) Rho2-Pck2 controls Ags1 (PMID:11102532) but is a signalling input, not a holoenzyme subunit like Rho1 for the beta-glucan synthases, so it is outside. (2) Only Aah1/Aah3 are modelled for remodeling. aah2 and aah4 add nothing in vegetative growth, and the meiotic mde5 was not tested (PMID:40668835). (3) Ags1's own extracellular GH13 domain is described in the synthase annoton, not as a separate remodeling part, because its activity is unassayed. (4) The spore paralogs are a lightly grounded variant with Mok12 as exemplar. Mok13 and Mok14 are named but sit in different PANTHER subfamilies. Mok14 is smaller (1369 aa) and its exact role should be checked in a gene review.

Connections

Aah1/Aah3 act on Ags1-made alpha-glucan. Aah3 overproduction cannot rescue ags1-664, and aah3 deletion interacts negatively with it (PMID:40668835).
Part 1: alpha-glucan synthesis at the plasma membrane by an Ags1-family synthase
UDP-glucose to cell wall alpha-1,3-glucan by Ags1-family synthasesReactionalpha_glucan_synthesis
Variant set: Ags1-family synthases for the vegetative wall and the spore envelope by life-cycle stage and wall type (One Or More)

All are PTHR47182 members. GOA IBA rows (GO_REF:0000033) place alpha-1,3-glucan synthase activity at PAINT node PTN000969361, seeded by S. pombe Ags1, Mok12 and Mok13. That node is not in the repository's local PAINT files, so it is cited here rather than asserted as an ancestral_node. Mok11 has no reported vegetative or spore role here and is left out.

Ags1 (vegetative wall and primary septum)Reactionags1_vegetative_variant

Annotons

Ags1 alpha-1,3-glucan synthase
ags1_activity
Participant: Family: Ags1 subfamily
Family:
Ags1 subfamilyPANTHER:PTHR47182:SF2
Representative Members: ags1/mok1 (Schizosaccharomyces pombe)UniProtKB:Q9USK8
Required Function:
alpha-1,3-glucan synthase activityGO:0047657

Function

alpha-1,3-glucan synthase activityGO:0047657 Not assayed in vitro. Inferred from wall alpha-glucan levels in ags1 mutants and from the domains. The GT5 domain is shown to make the alpha-1,4 primer (E1526A abolishes it). The extracellular GH13 domain is essential, but its transglycosylase activity is only suggested.
Substrates: UDP-alpha-D-glucose
Products: cell wall alpha-1,3-glucan UDP

Processes

fungal-type cell wall (1->3)-alpha-glucan biosynthetic processGO:0070600 primary cell septum biogenesisGO:0031671

Locations

plasma membraneGO:0005886

Essential. Works at growing tips and in the primary septum, where it co-immunoprecipitates with the beta-glucan synthase Bgs1 (PMID:22891259). Controlled by Rho2-Pck2 signalling, which is outside the module.

Mok12/Mok13/Mok14 (spore envelope)Reactionmok_spore_variant

Annotons

Sporulation-specific Ags1 paralogs
mok_spore_activity
Participant: Family: Mok12 subfamily (representative of the spore paralogs)
Family:
Mok12 subfamily (representative of the spore paralogs)PANTHER:PTHR47182:SF5
Representative Members: mok12 (Schizosaccharomyces pombe)UniProtKB:Q9UUL4

Function

alpha-1,3-glucan synthase activityGO:0047657 Family-level inference (PAINT PTN000969361, seeded by Mok12 and Mok13). Not assayed.

Processes

ascospore wall assemblyGO:0030476

Make the spore-envelope alpha-glucans. Not reviewed gene by gene here.

PMID:16420355
Mok12, Mok13 and Mok14 are sporulation-specific and localize to the spore envelope.
the proteins localized to the spore envelope
Part 2: assembly of alpha-glucan chains at the cell surface by GPI-anchored GH13 enzymes
Cell-surface GH13 remodeling of alpha-glucan chainsReactionalpha_glucan_chain_remodeling

Needed for normal alpha-1,3-glucan content and wall architecture. In the aah1 aah3 double mutant, rigid-wall alpha-1,3-glucan falls from 44% to 11% (PMID:40668835). The reaction is inferred from Aspergillus niger AgtA/AgtB.

Variant set: Redundant GPI-anchored GH13 paralogs Aah1 and Aah3 by redundant paralog (One Or More)
PMID:40668835
Aah1 and Aah3 are the redundant vegetative GH13 enzymes.
Thus, we concluded that Aah1 and Aah3 act redundantly and are the two S. pombe α-amylase homologs important for vegetative cell growth

Aah1 and Aah3 sit in different PANTHER subfamilies of PTHR10357 (SF215 and SF224) but act redundantly: the strong wall phenotype appears only in the double mutant. aah2 and aah4 add nothing in vegetative growth (PMID:40668835).

Aah1Reactionaah1_variant

Annotons

Aah1 GPI-anchored GH13 enzyme (activity inferred)
aah1_transglycosylase
Participant: Family: Aah1 subfamily of GH13 alpha-amylase-family proteins
Family:
Aah1 subfamily of GH13 alpha-amylase-family proteinsPANTHER:PTHR10357:SF215
Representative Members: aah1 (Schizosaccharomyces pombe)UniProtKB:O74922

Function

4-alpha-glucanotransferase activityGO:0004134 Inferred from A. niger AgtA/AgtB (about 40% identity), which make new alpha-1,4 bonds without hydrolysis. Aah1/Aah3 need their catalytic Asp/Glu in vivo, and no alpha-amylase activity is detectable in S. pombe.

Processes

fungal-type cell wall (1->3)-alpha-glucan biosynthetic processGO:0070600

Locations

external side of plasma membraneGO:0009897
Aah3Reactionaah3_variant

Annotons

Aah3 GPI-anchored GH13 enzyme (activity inferred)
aah3_transglycosylase
Participant: Family: Aah3 subfamily of GH13 alpha-amylase-family proteins
Family:
Aah3 subfamily of GH13 alpha-amylase-family proteinsPANTHER:PTHR10357:SF224
Representative Members: aah3 (Schizosaccharomyces pombe)UniProtKB:Q9Y7S9

Function

4-alpha-glucanotransferase activityGO:0004134 Inferred from A. niger AgtA/AgtB (about 40% identity), which make new alpha-1,4 bonds without hydrolysis. Aah1/Aah3 need their catalytic Asp/Glu in vivo, and no alpha-amylase activity is detectable in S. pombe.

Processes

fungal-type cell wall (1->3)-alpha-glucan biosynthetic processGO:0070600

Locations

external side of plasma membraneGO:0009897