Biogenesis, engineering and function of membranes in the CO ₂ -fixing pyrenoid

Area of Science:

• Cell Biology
• Plant Science
• Biochemistry

Background:

• The pyrenoid is a crucial organelle for carbon dioxide (CO2) assimilation in algae and some plants, contributing significantly to global CO2 uptake.
• Specialized internal membranes within the pyrenoid are hypothesized to enhance CO2 delivery and concentration, but their formation and roles remain largely uncharacterized.

Purpose of the Study:

• To experimentally investigate the biogenesis and function of pyrenoid membrane tubules.
• To identify the molecular factors responsible for the formation and maintenance of these essential structures.
• To assess the role of pyrenoid membranes in CO2 delivery and pyrenoid organization.

Main Methods:

• Utilized the model alga *Chlamydomonas reinhardtii* and the model plant *Arabidopsis thaliana* for genetic and reconstitution experiments.
• Investigated the localization and function of proteins SAGA1 and MITH1 using microscopy and mutant analysis.
• Assessed the physiological impact of tubule deficiency on algal growth under varying CO2 conditions.

Main Results:

• Identified SAGA1 and MITH1 as homologous proteins essential for pyrenoid membrane tubule biogenesis in *Chlamydomonas reinhardtii*.
• Demonstrated that SAGA1 and MITH1 can reconstitute pyrenoid-traversing membranes in *Arabidopsis thaliana*, a heterologous system.
• Showed that tubule-deficient mutants exhibit impaired growth under CO2-limiting conditions and form aberrant pyrenoid matrix condensates.
• SAGA1 initiates tubule formation, while MITH1 is crucial for tubule extension.

Conclusions:

• SAGA1 and MITH1 are key mediators of pyrenoid membrane tubule biogenesis, essential for CO2 delivery and pyrenoid integrity.
• The successful reconstitution of pyrenoid membranes in a plant offers a pathway for engineering enhanced CO2 assimilation in crops.
• This study provides fundamental insights into pyrenoid membrane function and opens avenues for further molecular characterization across diverse organisms.