proteorhodopsin phototrophy

Deep research

Generated by just research-trait; source: research/traits/metabolism/proteorhodopsin_phototrophy-deep-research-falcon.md

# Curation report: proteorhodopsin phototrophy

## Trait record and scope

- **Trait label:** proteorhodopsin phototrophy
- **Trait identifier:** **`traitmech:000036`**
- **Category / kind / status:** METABOLISM / CLASS / REVIEWED
- **Parent:** `traitmech:000037`
- **Synonym:** rhodopsin-based phototrophy

### Recommended operational definition

Proteorhodopsin phototrophy is the physiological capacity to use a **retinal-bound, outward proton-pumping proteorhodopsin in the cytoplasmic membrane** to convert absorbed light into a transmembrane electrochemical proton gradient. That proton-motive force can support ATP synthesis and, depending on organism and conditions, transport, motility, maintenance, growth, or survival. It is generally an **energy-supplementing photoheterotrophic mechanism**, not carbon-fixing photosynthesis: proteorhodopsin supplies energy but does not itself provide a reaction center, electron-transfer chain, reducing power, or carbon-fixation pathway. PR-bearing SAR86 genomes, for example, lack autotrophic carbon-fixation pathways despite having PR-based energetic potential. (feng2023isproteorhodopsina pages 17-23, dupont2012genomicinsightsto pages 8-9)

The minimum mechanistic phenotype should require evidence for functional light-driven proton translocation, or a sufficiently complete genotype plus functional validation. A proteorhodopsin-like sequence alone is not enough: retinal availability, membrane expression, pump direction, and ion specificity can determine whether the phenotype exists. SAR86 illustrates this distinction because some genomes carry PR but apparently lack retinal biosynthesis and may have to acquire the chromophore externally. (dupont2012genomicinsightsto pages 8-9)

### Boundary cases

**Include:** outward H⁺-pumping bacterial proteorhodopsins that bind retinal and generate proton motive force under illumination, including systems that principally improve maintenance or stress survival rather than measurable exponential growth.

**Exclude or model separately:**

1. **Sensory rhodopsins**, light-gated channels, inward proton pumps, and chloride/sodium pumps; sequence homology to microbial rhodopsins does not establish PR phototrophy.
2. **Chlorophyll-based photosynthesis and aerobic anoxygenic phototrophy**, which use reaction centers and electron-transfer machinery absent from the minimal PR system.
3. **Retinal biosynthesis alone** or an unexpressed PR gene.
4. **Viral or eukaryotic proton-pumping rhodopsins** unless TraitMech explicitly intends the trait to extend beyond cellular bacterial PR systems. Viral rhodopsins can bind retinal and pump protons, but are evolutionarily and host-contextually distinct. (needham2019adistinctlineage pages 10-10)
5. **Autotrophy as an intrinsic consequence.** A 2024 culture study found no significant light–dark difference in inorganic-carbon assimilation, showing that PR-dependent ATP generation does not automatically imply carbon fixation.

| Subject | Predicate | Object | Confidence | Key evidence |
|---|---|---|---|---|
| light | activates | retinal-bound proteorhodopsin | High | PR is a "light-driven proton pump" whose mechanism depends on retinal/photoisomerization; foundational identification in marine bacteria and later mechanistic summaries support light activation (DOI:10.1126/science.289.5486.1902; DOI:10.1038/35081051; DOI:10.25959/23241740) (feng2023isproteorhodopsina pages 17-23, feng2023isproteorhodopsina pages 28-34) |
| retinal-bound proteorhodopsin | exports | protons (H+) across the membrane | High | Direct functional evidence from illuminated cells and heterologous expression shows outward proton pumping / external pH change; recent work summarizes PR as bacterial proton pumps (DOI:10.1128/AEM.02425-09; DOI:10.25959/23241740; DOI:10.1126/sciadv.adu5303) (johnson2010enhancementofsurvival pages 1-2, feng2023isproteorhodopsina pages 89-95, bukhdruker2025proteorhodopsininsightsinto pages 18-19) |
| proton export by proteorhodopsin | generates | proton motive force | High | Multiple sources explicitly state PR translocates protons to create membrane potential/PMF; direct in engineered Shewanella, established across PR literature (DOI:10.1128/AEM.02425-09; DOI:10.25959/23241740) (johnson2010enhancementofsurvival pages 1-2, feng2023isproteorhodopsina pages 28-34, feng2023isproteorhodopsina pages 17-23) |
| proton motive force | drives | F-type ATP synthase | Medium-High | Established bioenergetic coupling rather than directly reconstituted in every PR study: sources state PR-generated PMF drives ATP synthesis via F0F1-ATPase / proton-translocating ATPase (DOI:10.25959/23241740; DOI:10.1128/AEM.02425-09) (feng2023isproteorhodopsina pages 28-34, feng2023isproteorhodopsina pages 17-23, feng2023isproteorhodopsina pages 34-38, johnson2010enhancementofsurvival pages 1-2) |
| F-type ATP synthase | produces | ATP | High | Established ATP synthase function; PR studies cite ATP increases or ATP synthesis supported by PR-generated PMF in heterologous/native systems (DOI:10.1128/AEM.02425-09; DOI:10.25959/23241740) (johnson2010enhancementofsurvival pages 1-2, feng2023isproteorhodopsina pages 28-34) |
| beta-carotene cleavage / retinal supply | enables | functional proteorhodopsin | High | Genome and biochemical evidence link PR function to retinal availability: PR-containing marine genomes carry retinal biosynthesis genes including blh and carotenoid genes; PR requires retinal/retinol for functionality (DOI:10.1073/pnas.0712027105; DOI:10.1038/ismej.2011.189) (dupont2012genomicinsightsto pages 8-9) |


*Table: Compact curation table of the strongest mechanistic edges for proteorhodopsin phototrophy, from light activation and proton export to PMF-coupled ATP production, plus retinal supply. It distinguishes directly shown PR steps from the broader established ATP synthase coupling used in TraitMech curation.*

## Candidate graph nodes

Identifiers below are conservative suggestions; exact ontology labels and releases should be checked during repository validation. No identifier is proposed for a strain-specific PR protein without a verified accession.

### Genes, proteins, and complexes

| Candidate node | Role | Suggested grounding |
|---|---|---|
| proteorhodopsin gene (`pr`) | Encodes the retinal-binding outward H⁺ pump | Label-only unless a taxon-specific gene/protein accession is used |
| proteorhodopsin holoprotein | Light-activated integral-membrane proton pump | GO:0015078, proton transmembrane transporter activity; optionally label-only for protein identity |
| proteorhodopsin apoprotein | PR before retinal incorporation | Label-only |
| retinal–proteorhodopsin Schiff-base complex | Photoactive holoprotein state | Label-only |
| conserved retinal-binding lysine | Forms the protonated Schiff base with retinal | Label-only; residue numbering is protein-specific (reported as Lys230 in one PR context) (feng2023isproteorhodopsina pages 17-23) |
| `blh` β-carotene 15,15′-dioxygenase | Cleaves β-carotene to supply retinal | EC:1.13.11.63; use a verified UniProt accession only for a specified organism |
| `crtE`, `crtB`, `crtI`, `crtY` | Carotenoid/β-carotene precursor synthesis | Gene-label nodes unless exact enzyme reaction and taxon are specified |
| F-type H⁺-transporting ATP synthase | Uses PMF to phosphorylate ADP | GO:0015986, ATP synthesis coupled proton transport |
| carotenoid–PR antenna complex | Expanded light harvesting in certain Bacteroidota PRs | Label-only; recent and taxon-restricted |

Polaribacter sp. MED152 contains PR together with carotenoid/retinal synthesis genes `crtEBIY` and `blh`, supporting inclusion of a retinal-supply module, although not every PR-bearing genome contains that complete module. (dupont2012genomicinsightsto pages 8-9)

Showing the first 60 of 227 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.