THE ROLES OF MITH1 AND SAGA1 IN BIOGENESIS OF THE PYRENOID, THE EUKARYOTIC CO2-CONCENTRATING ORGANELLE

Abstract

Photosynthetic organisms are crucial to the global carbon cycle, as they capture CO2 andconvert it into organic molecules that sustain life. While land plants and algae share much of the machinery that performs photosynthesis, algal cells additionally contain a specialized compartment called the pyrenoid, which allows them to capture CO2 more efficiently than land plants. Understanding the genes needed to build a pyrenoid could allow us to engineer a functional pyrenoid into land plants to improve their growth. Many of the advances made towards understanding the biogenesis and function of the pyrenoid have utilized the model alga Chlamydomonas reinhardtii (Chlamydomonas hereafter). The Chlamydomonas pyrenoid is composed of three distinct regions that provide the organelle with its functionality: 1) A liquid-like matrix consisting primarily of Rubisco and its linker protein EPYC1, 2) Traversing membrane tubules that supply Rubisco in the pyrenoid with concentrated CO2 and are continuous with the photosynthetic thylakoid membranes, and 3) A barrier made out of starch which surrounds the matrix and is thought to prevent rapid escape of CO2 from the pyrenoid. While recent advances have characterized the matrix to the point where it has been successfully reconstituted in plants, how the tubules or starch sheath are formed remains unknown. Here, we confirm that the starch sheath and tubules are needed for an optimally functioning pyrenoid, as mutants lacking these two components grow poorly in conditions where a pyrenoid is needed. We also show that homologous proteins MITH1 and SAGA1 mediate the biogenesis of the pyrenoid membrane tubules. In both the mith1 and saga1 mutants, tubules are disrupted and Rubisco mis-localizes to multiple puncta instead of forming a single pyrenoid, revealing that MITH1 and SAGA1 are required for a canonical tubule network and formation of a single pyrenoid. We provide evidence which suggests that SAGA1 and MITH1 act sequentially to form distinct tubule regions. Our results advance our understanding of tubule formation and represent a key milestone toward engineering a pyrenoid into crops to improve yields.