Biology subjects

Garde, A.

Publications and source records attributed to Garde, A..

6 recordsLinked to original sources

bioRxiv2026

RBMP2 shapes specialized membranes for CO₂ delivery in the pyrenoid condensate

Approximately one-third of global CO2 fixation occurs in the algal pyrenoid, a condensate of the CO2-fixing enzyme Rubisco traversed by membranes that supply it with CO2. In the model alga Chlamydomonas reinhardtii, CO2 is delivered by a specialized central reticulated region of these membranes. Here we define how this region is built and identify RBMP2 as the first factor required for its biogenesis. Loss of RBMP2 prevents reticulated-region formation and impairs pyrenoid-dependent growth. RBMP2 domains perform separable roles in extending membrane tubules toward the pyrenoid center and remodeling them into narrow tubules of the reticulated region. The CO2-releasing carbonic anhydrase CAH3 localizes not only to the reticulated region but also to helical interfaces between tubules and minitubules, which expand in some RBMP2 truncation mutants. Our findings advance the basic understanding of pyrenoid function and establish the pyrenoid as a powerful system for studying membrane remodeling within a phase-separated condensate.

Plant Biology↗

bioRxiv2026

Super-resolution expansion microscopy reveals nanoscale protein domains and CO 2 -dependent remodeling of Chlamydomonas pyrenoid-traversing membranes

Within the algal carbon-assimilating organelle, the pyrenoid, specialized traversing membranes perform the essential function of delivering concentrated CO2 to Rubisco. In Chlamydomonas reinhardtii, these membranes consist of peripheral cylindrical tubules that connect to a central reticulated region. However, due to resolution limitations, the spatial distribution of their structural and functional proteins has remained unclear. Here, we achieve an [~]11-fold improvement in resolution by combining ultrastructure expansion microscopy with super-resolution instantaneous structured illumination microscopy, revealing protein localizations and condition-dependent remodeling of these membranes. At air levels of CO2, the tubule-initiating protein SAGA1 forms narrow rings at the pyrenoid edge, the tubule-extending protein MITH1 surrounds the peripheral tubules, and the putative transporter BST4 surrounds tubules more centrally, suggesting that the cylindrical tubules contain multiple distinct protein domains. The CO2-delivering carbonic anhydrase CAH3 localizes to the inner face of the central reticulated region, suggesting that this region is specialized for CO2 delivery. CAH3 remains in the reticulated region at high CO2, suggesting that the cell maintains a minimal CO2-delivery apparatus even when dispensable. Finally, at high CO2, cylindrical tubule diameter narrows, and MITH1 relocalizes throughout the pyrenoid-traversing membrane network. Together, our study elucidates sub-pyrenoid protein organization and CO2-dependent reorganization.

Plant Biology↗

bioRxiv2026

SAGA1 and SAGA2 localize the starch sheath to the pyrenoid in Chlamydomonas reinhardtii

Most algae enhance their CO2 assimilation by concentrating CO2 within the pyrenoid, a biomolecular condensate that contains the CO2-fixing enzyme Rubisco. Many pyrenoids are surrounded by a starch sheath that is thought to slow the escape of CO2 from the pyrenoid, but how the starch sheath is localized to the pyrenoid remains poorly understood. Here, in the leading model alga Chlamydomonas reinhardtii, we find that the protein SAGA2 is necessary for early pyrenoid starch sheath biogenesis and works redundantly with its homolog, SAGA1, to localize the starch sheath to the pyrenoid. SAGA2 and SAGA1 were enriched in different regions of the pyrenoid-starch sheath interface, suggesting that they play complementary roles. Both saga2 and saga1 mutants showed defects in starch sheath coverage early during pyrenoid formation that were improved at a later timepoint. Strikingly, a saga1;saga2 double mutant did not have a starch sheath around the pyrenoid and showed decreased overall starch content. SAGA1 and SAGA2 starch-binding domains bound to starch, the starch mimic molecule {beta}-cyclodextrin, and the starch precursor molecule maltoheptaose, suggesting a role for SAGA1 and SAGA2 in starch granule initiation. We propose a model where SAGA1 and SAGA2 each locally prime starch sheath initiation in a distinct region of the pyrenoid surface by enriching starch precursor molecules around the pyrenoid. These findings advance the understanding of algal starch sheath biogenesis and provide insights into the associations between biomolecular condensates and other cellular structures. Significance StatementEukaryotic algae enhance their carbon assimilation using an organelle called the pyrenoid, where concentrated CO2 is supplied to the CO2-fixing enzyme Rubisco. In many algae, a starch sheath surrounding the pyrenoid is thought to enhance CO2 fixation, but how starch is localized to pyrenoids is unknown. Here, we show that two proteins, SAGA1 and SAGA2, each bind to starch precursor molecules and redundantly localize starch to the pyrenoid in the alga Chlamydomonas reinhardtii. Our results suggest that SAGA1 and SAGA2 promote starch sheath initiation at the pyrenoid, rather than merely tethering starch, as previously thought. This work advances the understanding of the proteins and molecular mechanisms involved in pyrenoid starch sheath biogenesis and lays the foundations for their further study.

molecular biology↗

bioRxiv2024

Proteomic analysis of the pyrenoid-traversing membranes of Chlamydomonas reinhardtii reveals novel components

O_LIPyrenoids are algal CO2-fixing organelles that mediate approximately one-third of global carbon fixation. Most pyrenoids are traversed by membranes that are thought to supply them with concentrated CO2. Despite the critical nature of these membranes for pyrenoid function, they are poorly understood, with few protein components known in any species. C_LIO_LIHere, we identify protein components of the pyrenoid-traversing membranes from the leading model alga, Chlamydomonas reinhardtii, by affinity purification and mass spectrometry of membrane fragments. Our proteome includes previously-known proteins as well as novel candidates. C_LIO_LIWe further characterize two of the novel pyrenoid-traversing membrane-resident proteins: Cre10.g452250, which we name Pyrenoid Membrane Enriched 1 (PME1), and Cre02.g143550, also known as Low-CO2-Induced 16 (LCI16). We confirm the pyrenoid-traversing membrane localization of LCI16 and observe that PME1 and LCI16 physically interact. We find that neither protein is required for normal membrane morphology or growth under CO2-limiting conditions, but that both mutants show a similar proteomic profile to those of established pyrenoid mutants. C_LIO_LITaken together, our study identifies the proteome of the pyrenoid-traversing membranes and initiates the characterization of a novel pyrenoid-traversing membrane complex, building toward a mechanistic understanding of the pyrenoid. C_LI

molecular biology↗

bioRxiv2024

De novo lipid synthesis and polarized prenylation drives cell invasion through basement membrane

SummaryInvasive cells form large, specialized protrusions to break through basement membrane (BM) matrix barriers. Park et al., reveal a crucial requirement for de novo lipid synthesis and a dynamic polarizing prenylation system to rapidly construct invasive protrusions that breach BMs. To breach basement membrane (BM), cells in development and cancer use large, transient, specialized lipid-rich membrane protrusions. Using live imaging, endogenous protein tagging, and cell-specific RNAi during C. elegans anchor cell (AC) invasion, we demonstrate that the lipogenic SREBP transcription factor SBP-1 drives expression of the fatty acid synthesis enzymes POD-2 and FASN-1 prior to invasion. We show that phospholipid producing LPIN-1 and sphingomyelin synthase SMS-1, which use fatty acids as substrates, produce lysosome stores that build the ACs invasive protrusion, and that SMS-1 also promotes protrusion localization of the lipid raft partitioning ZMP-1 matrix metalloproteinase. Finally, we discover that the endoplasmic reticulum (ER)-associated HMG-CoA reductase HMGR-1, which generates isoprenoids for prenylation, enriches at the AC invasive front, and that the final ER prenylation enzyme, ICMT-1, localizes to ER exit sites that dynamically polarize to deliver prenylated GTPases for protrusion formation. Together, these results reveal a collaboration between lipogenesis and a polarized lipid prenylation system that drives invasive protrusion formation.

cell biology↗

bioRxiv2022

The C. elegans Anchor Cell Transcriptome: Ribosome Biogenesis Drives Cell Invasion through Basement Membrane

Cell invasion through basement membrane (BM) barriers is important in development, immune function, and cancer progression. As invasion through BM is often stochastic, capturing gene expression profiles of cells actively transmigrating BM in vivo remains elusive. Using the stereotyped timing of C. elegans anchor cell (AC) invasion, we generated an AC transcriptome during BM breaching. Through a focused RNAi screen of transcriptionally enriched genes, we identified new invasion regulators, including TCTP (Translationally Controlled Tumor Protein). We also discovered gene enrichment of ribosomal proteins. AC-specific RNAi, endogenous ribosome labeling, and ribosome biogenesis analysis revealed a burst of ribosome production occurs shortly after AC specification, which drives the translation of proteins mediating BM removal. Ribosomes also strongly localize to the ACs endoplasmic reticulum (ER) and the endomembrane system expands prior to invasion. We show that AC invasion is sensitive to ER stress, indicating a heightened requirement for translation of ER trafficked proteins. These studies reveal key roles for ribosome biogenesis and endomembrane expansion in cell invasion through BM and establish the AC transcriptome as a resource to identify mechanisms underlying BM transmigration.

cell biology↗