MITH1 (MIssing THylakoids 1; also called SAGA3; locus Cre06.g259100) is a large (1,304 aa, ~138 kDa) nuclear-encoded pyrenoid protein of Chlamydomonas reinhardtii. It is a distant homologue of the pyrenoid starch-sheath and tubule proteins SAGA1 and SAGA2, but it has no CBM20 starch-binding domain and no detectable Rubisco-binding motif. Its sequence is dominated by long predicted coiled-coil segments flanked by disordered, low-complexity N- and C-terminal regions. MITH1 localizes to the thylakoid membrane tubules that traverse the pyrenoid Rubisco matrix. It is most concentrated on the peripheral, cylindrical tubules, sits on the stromal (matrix-facing) side of the membrane, and co-pellets with membranes in a detergent-sensitive way. Under high CO2 it spreads through the whole tubule network, and at limiting CO2 it retreats to the peripheral tubules. MITH1 binds SAGA1, its most abundant specific interactor, and depends on SAGA1 to reach the pyrenoid. Without SAGA1 it is dispersed through the chloroplast. MITH1 is required to extend the membranes through the matrix after SAGA1 has started them. In mith1 mutants, thylakoids cross the starch sheath and stop at the matrix surface, so pyrenoids lack matrix-traversing tubules. The mutants form some extra small Rubisco condensates and grow poorly at limiting CO2, a milder defect than that of saga1. Expressed together with SAGA1 in Arabidopsis plants carrying an EPYC1-Rubisco condensate, MITH1 makes thylakoid membrane sheets traverse the condensate. Neither protein does this alone. MITH1 is proposed to strengthen the adhesive contact between the membrane and the Rubisco matrix that SAGA1 makes.
Definition: The formation of pyrenoid tubules, the thylakoid membranes that enter and traverse the Rubisco matrix of a pyrenoid.
Justification: SAGA1 and MITH1 are necessary for pyrenoid tubules and together sufficient to make thylakoids traverse a Rubisco condensate in Arabidopsis. GO has a pyrenoid tubule cellular component term but no process term for its formation, so the role can only be captured with the generic thylakoid membrane organization term.
Parent term: thylakoid membrane organization
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0160223 pyrenoid tubule | IDA PMID:39548241 SAGA1 and MITH1 produce matrix-traversing membranes in the C... | NEW | Summary: MITH1-Venus (functional, complementing) localizes in a branching pattern with tubule chlorophyll and RBMP1, co-pellets with membranes, and in expansion microscopy lies on the peripheral cylindrical tubules. Reason: GOA has no rows for this gene. Live imaging of a complementing fusion, membrane fractionation (PMID:39548241) and antibody-based expansion microscopy (PMID:42465278) all place MITH1 on the pyrenoid tubules, at their matrix-facing surface. GO:0160223 is part_of GO:1990732 pyrenoid, so no separate pyrenoid row is added. The earlier matrix assignment (PMID:28938113) is superseded. Supporting Evidence: PMID:39548241 Whereas MITH1 was previously proposed to localize to the Chlamydomonas pyrenoid matrix19,20, here we observed that it localizes to and functions at the matrix-traversing membranes. PMID:39548241 Chlamydomonas cell fractionation also supported membrane localization, as MITH1 co-pelleted with membranes in the absence of detergent and was solubilized when detergent was added PMID:42465278 MITH1 signal was largely restricted to the cylindrical tubules at the pyrenoid periphery |
| GO:0010027 thylakoid membrane organization | IMP PMID:39548241 SAGA1 and MITH1 produce matrix-traversing membranes in the C... | NEW | Summary: MITH1 is required to extend thylakoid-derived membranes through the Rubisco matrix, and with SAGA1 it is sufficient to make thylakoid sheets traverse an EPYC1-Rubisco condensate in Arabidopsis. Reason: GO has no pyrenoid tubule biogenesis term, and thylakoid membrane organization is the closest existing process, as for SAGA1. Participation test. MITH1 is not a substrate or a remote requirement. It sits on the membranes it reorganises, binds SAGA1, and is proposed to add the adhesive membrane-matrix force that lets the membrane advance into the matrix. That is a structural contribution to the step itself. The mith1 phenotype is specific (membranes reach the matrix surface and stop), complementation restores it, and heterologous sufficiency with SAGA1 goes beyond necessity evidence. Comparator check. Structural thylakoid shapers such as Arabidopsis VIPP1 and FZL carry GO:0010027, and the sibling review of SAGA1 uses the same term for the initiation step. The open point is how MITH1 contacts the membrane and whether it binds Rubisco directly. Supporting Evidence: PMID:39548241 These observations suggest that pyrenoid membrane formation initiates at the correct cellular location in the mith1 mutant but then stalls before these membranes are extended through the pyrenoid matrix. PMID:39548241 In the complemented strain mith1;MITH1-Venus, an apparently normal matrix-traversing membrane tubule network is restored PMID:39548241 These results demonstrate that SAGA1 and MITH1 together are sufficient to generate matrix-traversing thylakoid membranes in a heterologous system. PMID:39548241 We propose that MITH1 increases the membrane–matrix adhesive force per unit area by binding to SAGA1 |
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Download this section (compressed HTML)Q: Does MITH1 bind Rubisco directly? Hennacy et al. found no yeast two-hybrid interaction and no recruitment to the condensate without SAGA1, but a 2026 preprint cites structural work reporting MITH1 binding to membranes and Rubisco. The answer decides whether MITH1 is a membrane-matrix adaptor in its own right or an amplifier of SAGA1.
Suggested experts: Jonikas MC
Q: How does MITH1 attach to the membrane? It has no predicted transmembrane segment, yet it co-pellets with membranes and is released by detergent. Is the contact through lipids, a thylakoid membrane protein, or SAGA1?
Q: Should GO add a "pyrenoid tubule assembly" process term so that tubule biogenesis factors (SAGA1, MITH1) can be annotated more specifically than GO:0010027 thylakoid membrane organization?
Experiment: Purify MITH1 and fragments (N-terminal disordered region, coiled-coil core, C-terminal region) and test binding to thylakoid-lipid liposomes (flotation), to SAGA1 and to Rubisco (pull-down, SPR). Test which fragments complement the mith1 tubule-extension defect.
Hypothesis: MITH1 bridges membranes and the Rubisco matrix through separable regions.
Type: biochemical reconstitution and genetic complementation
Experiment: Image MITH1 by expansion microscopy across high-to-low CO2 shifts in strains with altered matrix size (for example EPYC1 dosage mutants) and compare its radial distribution with Rubisco.
Hypothesis: The CO2-dependent relocalization of MITH1 tracks Rubisco availability in the matrix.
Type: quantitative imaging
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The molecular activity of MITH1 is not established. It binds SAGA1 and co-pellets with membranes, but no membrane-binding region, lipid binding, or reproducible direct Rubisco binding has been shown.
OPEN BIOLOGY
What is known: MITH1 is a SAGA1-recruited, membrane-associated pyrenoid tubule protein necessary to extend tubules through the matrix.
Provenance (the field's own admissions):
Gap: GO has no process term specific to pyrenoid tubule biogenesis, so MITH1's role can only be captured as thylakoid membrane organization.
OPEN ONTOLOGY
What is known: SAGA1 and MITH1 are necessary and together sufficient for membranes to traverse the pyrenoid matrix.
What would resolve it: Request a new GO BP term "pyrenoid tubule assembly".
Provenance (the field's own admissions):
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