Fungal chitin synthesis and deacetylation to chitosan

Chitin, a linear beta-1,4-linked polymer of N-acetylglucosamine, is a minor but essential structural polysaccharide of the fungal cell wall, septa and spore walls. It is made at the plasma membrane by chitin synthases, processive multi-pass membrane GT-2 glycosyltransferases that transfer N-acetylglucosamine from cytosolic UDP-N-acetylglucosamine to the growing chain and extrude the chain through a transmembrane channel to the cell surface. Fungi have several chitin synthase classes with different cellular jobs. In budding yeast, chitin synthase II (Chs2) makes the primary septum at the bud neck, chitin synthase III (catalytic Chs3 activated by the Chs4 or, in sporulation, Shc1 subunit) makes most cell-wall chitin, the chitin ring at bud emergence and the chitin of the spore wall, and chitin synthase I (Chs1) repairs the birth scar after cell separation. Loss of all three is lethal. Some nascent chitin is then deacetylated by secreted or membrane-anchored CE4-family chitin deacetylases to give chitosan, a beta-1,4-glucosamine polymer. In budding yeast this happens only in the ascospore wall, where the sporulation-specific deacetylases Cda1 and Cda2 convert Chs3-made chitin into the chitosan layer; in other fungi, such as Cryptococcus neoformans, deacetylases also make chitosan in the vegetative wall. The supply of UDP-N-acetylglucosamine (hexosamine pathway), chitin synthase trafficking and localisation factors, and chitin cross-linking or degradation are outside the module.

MODULE:fungal_chitin_chitosan_synthesisDRAFTCONCRETEMetabolic Pathwaymodules/fungal_chitin_chitosan_synthesis.yaml
chitin biosynthetic processGO:0006031
GO:0006031
chitin biosynthetic process
Module-level process concept.
GO:0006030
chitin metabolic process
GO has no chitosan biosynthesis term; the deacetylation step is grounded as chitin metabolism within ascospore wall assembly.
GO:0030476
ascospore wall assembly
Process context of the S. cerevisiae Chs3 plus Cda1/Cda2 chitosan arm.
YeastPathways:PWY3O-15
chitin biosynthesis (S. cerevisiae, SGD YeastPathways)
YeastCyc lists CHITIN-SYNTHASE-RXN with CHS1, CHS2 and CHS3; all three were retained, separated by synthase class. The comment names Chs4 as essential for Chs3 activity but does not attach it; Chs4 (SKT5) and its sporulation paralog Shc1 were added as activating subunits of chitin synthase III.
YeastPathways:PWY3O-70
chitosan biosynthesis (S. cerevisiae, SGD YeastPathways)
YeastCyc lists CHITIN-DEACETYLASE-RXN with CDA1 and CDA2. Both retained.
PMID:8257107
Genetics and molecular biology of chitin synthesis in fungi.
Review of the three S. cerevisiae chitin synthases and their distinct roles; chitin synthase III makes about 90% of cellular chitin.
Chitin synthase III is required for the synthesis of 90% of the cellular chitin, including the chitin in the bud scars and lateral wall.
PMID:12228806
The genetic complexity of chitin synthesis in fungi.
Fungi have one (S. pombe) to seven (Aspergillus fumigatus) chitin synthase isoenzymes, which supports modelling synthase classes as variants.
Chitin synthesis is based on the regulation of distinct chitin synthase isoenzymes whose number ranges from one in Schizosaccharomyces pombe to seven in some filamentous fungi, such as Aspergillus fumigatus.
PMID:16760306
Cell wall assembly in Saccharomyces cerevisiae.
Review of yeast cell wall composition and assembly, including chitin (cited by YeastPathways).
PMID:2050738
The function of chitin synthases 2 and 3 in the Saccharomyces cerevisiae cell cycle.
Chs2 is specific for the primary septum; Chs3 makes the chitin ring, lateral-wall chitin and the chitosan of spore walls; loss of all three chitin synthases is lethal.
Therefore, Chs2 is specific for primary septum formation, whereas Chs3 is responsible for chitin in the ring at bud emergence and in the cell wall.
PMID:2523889
Chitin synthase 1, an auxiliary enzyme for chitin synthesis in Saccharomyces cerevisiae.
Chs1 is an auxiliary repair enzyme that prevents lysis at the birth scar under acidic conditions.
The latter emerges as an auxiliary or emergency enzyme.
PMID:1293886
DIT101 (CSD2, CAL1), a cell cycle-regulated yeast gene required for synthesis of chitin in cell walls and chitosan in spore walls.
CHS3 (DIT101/CSD2) is required for spore-wall chitosan as well as for vegetative chitin.
Chemical measurements showed that this mutant does not synthesize chitosan during sporulation.
PMID:9234668
Characterization of CHS4 (CAL2), a gene of Saccharomyces cerevisiae involved in chitin biosynthesis and allelic to SKT5 and CSD4.
Chs4 is a post-translational activator required for full chitin synthase III activity.
we therefore propose that Chs4p would be an essential component of the CSIII complex, acting as a post-translational regulator of this activity.
PMID:17142567
Prenylation of Saccharomyces cerevisiae Chs4p Affects Chitin Synthase III activity and chitin chain length.
Chs4 physically interacts with Chs3 and is indispensable for its activity in vivo.
Chs4p (Cal2/Csd4/Skt5) was identified as a protein factor physically interacting with Chs3p, the catalytic subunit of chitin synthase III (CSIII), and is indispensable for its enzymatic activity in vivo.
PMID:11918806
Control of chitin synthesis through Shc1p, a functional homologue of Chs4p specifically induced during sporulation.
Shc1 is a sporulation-specific Chs4 homolog needed for normal spore-wall chitosan.
Shc1p is required for proper maturation of the ascospore through its participation in the synthesis of the chitosan layer.
PMID:10366589
Chs7p, a new protein involved in the control of protein export from the endoplasmic reticulum that is specifically engaged in the regulation of chitin synthesis in Saccharomyces cerevisiae.
Chs7 is an ER protein needed for Chs3 export; it is a trafficking factor kept outside the module.
In the absence of CHS7 product, Chs3p, but not other secreted proteins, is retained in the ER
PMID:8940152
Two sporulation-specific chitin deacetylase-encoding genes are required for the ascospore wall rigidity of Saccharomyces cerevisiae.
CDA1 and CDA2 account for all chitin deacetylase activity and are sporulation-specific.
We have cloned the CDA1 and CDA2 genes which together account for the total chitin deacetylase activity of the organism.
PMID:17400891
Chitosan, the deacetylated form of chitin, is necessary for cell wall integrity in Cryptococcus neoformans.
In Cryptococcus neoformans, three chitin deacetylases convert Chs3-made chitin into vegetative cell-wall chitosan, showing that the synthase-to-deacetylase coupling is not restricted to spores.
The data suggest a model for chitosan production in vegetatively growing C. neoformans where the three chitin deacetylases convert chitin generated by the chitin synthase Chs3 into chitosan.
PMID:36131020
Structural basis for directional chitin biosynthesis.
Cryo-EM structures of an oomycete chitin synthase show the reaction chamber and polymer-translocating channel of the membrane enzyme.
The key reactions of chitin biosynthesis are catalysed by chitin synthase2-4, a membrane-integrated glycosyltransferase that transfers GlcNAc from UDP-GlcNAc to a growing chitin chain.
6Nodes
2Parts
1Variant Sets
3Variants
5Annotons
2Connections

Derived QC

Recommended-field compliance

100.0% recommended fields populated

All recommended fields populated.

Module deep research

✗ none found

No MODULE:fungal_chitin_chitosan_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

7 gene-function check(s) reflected in core functions · 0 conflict(s) · 0 evidence or coverage gap(s) (12 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
chs3_activity CHS3 (Saccharomyces cerevisiae)
UniProtKB:P29465
Family Representative 		chitin synthase activity
GO:0004100 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
chs4_activator_activity SKT5/CHS4 (Saccharomyces cerevisiae)
UniProtKB:P34226
Family Representative 		enzyme activator activity
GO:0008047 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
chs4_activator_activity SHC1 (Saccharomyces cerevisiae)
UniProtKB:P39000
Family Representative 		enzyme activator activity
GO:0008047 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
chs2_activity CHS2 (Saccharomyces cerevisiae)
UniProtKB:P14180
Family Representative 		chitin synthase activity
GO:0004100 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
chs1_activity CHS1 (Saccharomyces cerevisiae)
UniProtKB:P08004
Family Representative 		chitin synthase activity
GO:0004100 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
chitin_deacetylase_activity CDA1 (Saccharomyces cerevisiae)
UniProtKB:Q06702
Family Representative 		chitin deacetylase activity
GO:0004099 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
chitin_deacetylase_activity CDA2 (Saccharomyces cerevisiae)
UniProtKB:Q06703
Family Representative 		chitin deacetylase activity
GO:0004099 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
chs3_activity class IV chitin synthase (Chs3 subfamily)
PANTHER:PTHR22914:SF16
Family 		chitin synthase activity
GO:0004100 		Family Supported
Family review supports GO:0004100 FAMILY_WIDE. Asserted by fungal_chitin_chitosan_synthesis for SF9, SF38 and SF16; accepted by CHS1, CHS2 and CHS3.
chs4_activator_activity Chs4/Skt5-type chitin synthase III activator
PANTHER:PTHR46430:SF1
Family 		enzyme activator activity
GO:0008047 		Family Supported
Family review supports GO:0008047 for PANTHER:PTHR46430:SF1. Chitin synthase III activator, as fungal_chitin_chitosan_synthesis selects; accepted by SKT5 and SHC1.
chs2_activity class II chitin synthase (Chs2 subfamily)
PANTHER:PTHR22914:SF38
Family 		chitin synthase activity
GO:0004100 		Family Supported
Family review supports GO:0004100 FAMILY_WIDE. Asserted by fungal_chitin_chitosan_synthesis for SF9, SF38 and SF16; accepted by CHS1, CHS2 and CHS3.
chs1_activity class I chitin synthase (Chs1 subfamily)
PANTHER:PTHR22914:SF9
Family 		chitin synthase activity
GO:0004100 		Family Supported
Family review supports GO:0004100 FAMILY_WIDE. Asserted by fungal_chitin_chitosan_synthesis for SF9, SF38 and SF16; accepted by CHS1, CHS2 and CHS3.
chitin_deacetylase_activity CE4 chitin deacetylase
PANTHER:PTHR10587:SF133
Family 		chitin deacetylase activity
GO:0004099 		Family Supported
Family review supports GO:0004099 for PANTHER:PTHR10587:SF133. The fungal chitin deacetylases (SF133, SF98, SF135), as fungal_chitin_chitosan_synthesis selects SF133. SF133 also contains bacterial PgdA peptidoglycan deacetylases, so even this subfamily is not uniformly chitin-specific.

Gene-review completeness (7/7 grounded genes reviewed)

7 complete review(s) · 7 with deep research · 0 missing review · 0 reviewed but lacking deep research

Gene Review Complete Deep research
CDA1 Q06702 ✓ ✓ ✓
CDA2 Q06703 ✓ ✓ ✓
CHS1 P08004 ✓ ✓ ✓
CHS2 P14180 ✓ ✓ ✓
CHS3 P29465 ✓ ✓ ✓
SHC1 P39000 ✓ ✓ ✓
SKT5 P34226 ✓ ✓ ✓

Details

Context
fungiNCBITaxon:4751
Fungal chitin synthesis and deacetylation to chitosanMetabolic Pathwayfungal_chitin_chitosan_synthesis
chitin biosynthetic processGO:0006031
Context
fungiNCBITaxon:4751

NEW module (YeastPathways crosswalk, 2026-10) combining PWY3O-15 (chitin biosynthesis) and PWY3O-70 (chitosan biosynthesis). The polymerisation step is modelled with chitin-synthase class variants (CSIII, CSII, CSI); the deacetylation step is an optional second part, linked specifically to CSIII. Corrections and additions to YeastCyc: (1) the three synthases share one reaction in YeastCyc; they are separated here because they build different structures (PMID:2050738, PMID:2523889, PMID:8257107). (2) The CSIII activator Chs4/Skt5, named in the YeastCyc comment, and its sporulation paralog Shc1 were added as non-catalytic activator subunits (GO:0008047). (3) The chitin synthases are placed in the plasma membrane (CSII at the bud neck), not the cytosol. GO-CAM disagreements: the YeastPathways PWY3O-15 model puts all three chitin synthase activities in the cytosol and omits Chs4; there is no GO-CAM for PWY3O-70. Consistency with gene reviews: the CHS7 review gives Chs7 a protein folding chaperone function in ER-to-Golgi transport of Chs3, so Chs7, like the exomer (Chs5/Chs6) and the chitin-ring scaffold Bni4, is treated as trafficking and localisation machinery outside the module. UDP-N-acetylglucosamine supply is the hexosamine_biosynthesis module (GFA1, GNA1, PCM1, QRI1 in yeast). GO has no chitosan biosynthetic process term, so the deacetylation part uses chitin metabolic process (GO:0006030) and its S. cerevisiae annoton uses ascospore wall assembly; a chitosan biosynthesis term could be requested. The Cryptococcus deacetylases (PMID:17400891) generalise the step to vegetative walls but are not used as exemplars because their UniProt accessions were not verified. No PomBase GO-CAM is linked; S. pombe is not used as an exemplar.

Connections

Spore-wall chitosan requires Chs3-made chitin (chs3/dit101 spores have no chitosan, PMID:1293886); Chs1 and Chs2 do not feed this step.
Part 1: chitin polymerisation at the plasma membrane
UDP-N-acetylglucosamine + chitin(n) to chitin(n+1) + UDPReactionchitin_polymerisation
Variant set: Chitin synthase classes with distinct cellular roles by enzyme class and cellular role (One Or More)

All classes catalyse the same reaction (EC 2.4.1.16) and belong to one PANTHER family (PTHR22914), but differ in subfamily, regulation, location and the wall structure they build. Budding yeast uses all three variants; S. pombe has a single chitin synthase used in the spore wall (PMID:12228806).

Chitin synthase III (Chs3 with Chs4/Shc1 activator)Protein Complexchs3_lateral_wall_variant

Makes most cell-wall chitin, the chitin ring at the bud base and the spore-wall chitin that is deacetylated to chitosan.

Annotons

Chitin synthase 3 catalytic subunit
chs3_activity
Participant: Family: class IV chitin synthase (Chs3 subfamily)
Family:
class IV chitin synthase (Chs3 subfamily)PANTHER:PTHR22914:SF16
Representative Members: CHS3 (Saccharomyces cerevisiae)UniProtKB:P29465
Required Function:
chitin synthase activityGO:0004100

Function

chitin synthase activityGO:0004100
Substrates: UDP-N-acetyl-alpha-D-glucosamine chitin chain (n residues)
Products: chitin chain (n+1 residues) UDP

Processes

chitin biosynthetic processGO:0006031

Locations

plasma membraneGO:0005886

Catalytic subunit; reaches the plasma membrane only with the help of the ER export factor Chs7 and the exomer, which are outside the module.

Chs4/Shc1 activating subunit of chitin synthase III
chs4_activator_activity
Participant: Family: Chs4/Skt5-type chitin synthase III activator
Family:
Chs4/Skt5-type chitin synthase III activatorPANTHER:PTHR46430:SF1
Representative Members: SKT5/CHS4 (Saccharomyces cerevisiae)UniProtKB:P34226 SHC1 (Saccharomyces cerevisiae)UniProtKB:P39000

Function

enzyme activator activityGO:0008047 Raises the Vmax of chitin synthase III; Chs4 farnesylation further modulates activity and chain length.
Targets: chitin synthase 3 (Chs3)

Processes

chitin biosynthetic processGO:0006031

Locations

plasma membraneGO:0005886 prospore membraneGO:0005628

Non-catalytic activating subunit. Chs4 acts in vegetative growth and is degraded in sporulation, where the sporulation-induced Shc1 takes over (PMID:11918806).

Connections

chs4_activator_activity -> chs3_activity Positively Regulates
Chs4 (or Shc1) binds and activates Chs3.
Chitin synthase II (primary septum)Reactionchs2_septum_variant

Annotons

Chitin synthase 2
chs2_activity
Participant: Family: class II chitin synthase (Chs2 subfamily)
Family:
class II chitin synthase (Chs2 subfamily)PANTHER:PTHR22914:SF38
Representative Members: CHS2 (Saccharomyces cerevisiae)UniProtKB:P14180
Required Function:
chitin synthase activityGO:0004100

Function

chitin synthase activityGO:0004100
Substrates: UDP-N-acetyl-alpha-D-glucosamine
Products: chitin UDP

Processes

division septum assemblyGO:0000917

Locations

cellular bud neckGO:0005935

Builds the chitinous primary septum at cytokinesis; without it the primary septum is absent and cells clump.

Chitin synthase I (birth-scar repair)Reactionchs1_repair_variant

Annotons

Chitin synthase 1
chs1_activity
Participant: Family: class I chitin synthase (Chs1 subfamily)
Family:
class I chitin synthase (Chs1 subfamily)PANTHER:PTHR22914:SF9
Representative Members: CHS1 (Saccharomyces cerevisiae)UniProtKB:P08004
Required Function:
chitin synthase activityGO:0004100

Function

chitin synthase activityGO:0004100
Substrates: UDP-N-acetyl-alpha-D-glucosamine
Products: chitin UDP

Processes

chitin biosynthetic processGO:0006031

Locations

plasma membraneGO:0005886

Auxiliary enzyme that repairs the birth scar after chitinase-mediated cell separation, preventing lysis at acidic pH.

Part 2: deacetylation of nascent chitin to chitosan (optional)
chitin + n H2O to chitosan + n acetateReactionchitin_deacetylation

Optional at the level of the module because it is restricted to the ascospore wall in budding yeast and absent from many fungi; in Cryptococcus neoformans it also occurs in vegetative cells (PMID:17400891).

chitin metabolic processGO:0006030

Annotons

CE4-family chitin deacetylase
chitin_deacetylase_activity
Participant: Family: CE4 chitin deacetylase
Family:
CE4 chitin deacetylasePANTHER:PTHR10587:SF133
Representative Members: CDA1 (Saccharomyces cerevisiae)UniProtKB:Q06702 CDA2 (Saccharomyces cerevisiae)UniProtKB:Q06703
Required Function:
chitin deacetylase activityGO:0004099

Function

chitin deacetylase activityGO:0004099
Substrates: chitin water
Products: chitosan acetate

Processes

ascospore wall assemblyGO:0030476

Locations

chitosan layer of spore wallGO:0005631

Deacetylates nascent Chs3-made chitin; cda1 cda2 spores lack wall rigidity and dityrosine fluorescence but remain viable.