CAH3 (Q39588, Cre09.g415700) review notes

2026-10-03 session

scripts/deep_research_wrapper.py CHLRE CAH3 falcon --fallback perplexity-lite failed:
falcon returned HTTP 402 Payment Required; the perplexity-lite fallback failed with
"Provider 'perplexity' not available". Not retried. Literature gathered directly from
PubMed (esearch "Cah3 AND Chlamydomonas", "cia3 AND Chlamydomonas") and cached with
ai-gene-review fetch-pmid.

Alpha-type CA, first intracellular alpha-CA found in a photosynthetic eukaryote; lumenal
PMID:9482718.
Bipartite transit peptide with twin-arginine lumen-targeting domain; the cia3 substitutions
lie next to it PMID:38247801.
Crystal structures 4XIW/4XIX; zinc site, disulfide, N-terminal arm-swapped dimer; most
active at mildly acidic pH PMID:25617045.
Recombinant CAH3 highly active PMID:39795314.

ca-1 nonsense allele complemented by CAH3 PMID:9159949.
Important because cia3 (the other allele) is a transit-peptide lesion and might have
mistargeting side effects; ca-1 is a null-type allele.
In vivo, cia3 is limited by CO2 supply to Rubisco, not PSII PMID:12913181.
Epistasis: cah3 suppresses lcib air-dier; internal Ci >20-fold up PMID:19074623.
BST1-3 supply lumenal bicarbonate; cia3 used as CCM-deficient control PMID:31391312.

Immunogold: mostly in thylakoids through the pyrenoid PMID:22709623.
Blanco-Rivero 2012: phosphorylated after low CO2, activity up 5-6 fold, pyrenoid fraction
19% -> 37% PMID:23139834. So even at low CO2 most CAH3 is outside the pyrenoid in this study.
Garde et al. 2026 bioRxiv (preprint): CAH3 in central reticulated tubules, membrane
associated, and unchanged between high and low CO2 PMID:42465278. Conflicts with the
relocalization claim; not yet peer reviewed.

Villarejo 2002: cia3 PSII particles have impaired water splitting, ~2 Mn per RC, rescued
by bicarbonate PMID:11953312.
Shutova 2008: O2 evolution in cia3 PSII preps raised by HCO3- and further by CAH3; model of
bicarbonate as proton carrier PMID:18239688.
Terentyev 2019, 2026 [PMID:31226314; PMID:42511718] further in vitro and cia3
photoinhibition data.
Assessment: supporting data are mainly from isolated PSII membranes and the single cia3
allele (a targeting mutant). Hanson et al. 2003 found no PSII limitation in vivo at low Ci.
Pyrenoid tubules lack PSII (D1 absent from isolated pyrenoids, PMID:23139834), and the
expansion-microscopy preprint finds CAH3 concentrated in tubules even at high CO2, which
sits awkwardly with a large PSII-associated pool. The PSII effect is plausible as a
secondary, local proton-consuming effect of CA activity but is not established as a
physiological function. GO:0009781 photosynthetic water oxidation is obsolete; no
annotation proposed. Recorded as a suggested question/experiment instead.

GO:0004089 IEA: ACCEPT (core MF).
GO:0008270 IEA: ACCEPT (catalytic zinc, structure-confirmed).
NEW GO:0009543 chloroplast thylakoid lumen (IDA, PMID:9482718).
NEW GO:0160223 pyrenoid tubule (IDA, PMID:23139834; also PMID:22709623, PMID:42465278).
No BP proposed: GO has no CCM process term; CAH3 supplies substrate to Rubisco but does
not carry out a step of the reductive pentose-phosphate cycle, so that term would fail the
participation test.