Introduction: Organelles carry out specific processes within eukaryotic cells. These processes include the organelle’s specific intracellular functions and the building (or “biogenesis”) of the organelle. For example, chloroplasts are the organelle of plant and algal cells that carry out photosynthesis; the conversion of light energy to chemical energy. While much is known about photosynthesis, less is known about the biogenesis of chloroplasts. My laboratory is revealing complexity of the internal organization of biogenic processes in the chloroplast of the unicellular alga, Chlamydomonas reinhardtii. This green alga is a long-standing model organism for chloroplast biology.
There are currently three projects in my laboratory:
Project 1. Localized photosystem biogenesis. Eukaryotic cells compartmentalize biochemical pathways and other processes to internal organelles. For example, in plant and algal cells, chloroplasts convert light energy to chemical energy by photosynthesis. Photosynthesis is driven by two multi-protein complexes, photosystems I and II, which act like solar-powered battery chargers. Organelles themselves have internal compartments with specialized functions. For example, membranous vesicles in chloroplasts, called thylakoids, have photosystems I and II and other complexes in the photosynthetic electron transport chain and the chloroplast’s ATP synthase. My research program has revealed that the biogenesis of photosystem I and II occurs in a specific compartment of the chloroplast called the translation zone (T-zone). The T-zone is the primary location of the synthesis of the photosystem proteins that are encoded by a small genome in the chloroplast and the biosynthesis of chlorophyll, the photopigment component of the photosystems. Chloroplasts have their own genomes and protein synthesis machinery because they have evolved from photosynthetic bacteria that took up residence in the eukaryotic common ancestor of the plants and green algae billions of years ago.
See: Sun, Y., Valente-Paterno, M., Bakhtiari, S., Law, C., Zhan, Y., & Zerges, W. (2019). Photosystem Biogenesis is Localized to the Translation Zone in the Chloroplast of Chlamydomonas. The Plant Cell, 31(12), 3057-3072.
Project 2. Chloroplast-localized translation for protein targeting. Most proteins of chloroplasts are encoded by nuclear genes, synthesized in the cytoplasm and imported into the chloroplast. Our recent results reveal that at least two of these proteins are synthesized by cytoplasmic ribosomes bound to the chloroplast surface. Concurrent protein synthesis and import or “cotranslational” protein targeting is known to target proteins to the endoplasmic reticulum and mitochondria. This mechanism facilitated import and gets the most hydrophobic (“greasy”) proteins integrated into a membrane before they stick together to form toxic aggregates. Intriguingly, these chloroplast-bound cytoplasmic ribosomes translate on a discrete domain of the chloroplast envelope which closely aligns the T-zone within the chloroplast (Project 1). Together, these results support a spatial coordination of the dual protein synthesis systems on opposing sides of this domain of the chloroplast envelope for localized assembly of the photosystems in the T-zone.
See: Sun, Y., Bakhtiari, S., Valente-Paterno, M., Wu, Y., Law, C., Dai, D., Dhaliwal, J., Huy Bui, K., & Zerges, W. (2021). Chloroplast-localized translation for protein targeting in Chlamydomonas reinhardtii. Preprint.
Project 3. A moonlighting function of a metabolic protein during oxidative stress. We identified a compartment in the chloroplast of Chlamydomonas which forms during oxidative stress and somewhat resembles stress granules, a membrane-less organelle of the cytoplasm in eukaryotic cells. These “chloroplast stress granules” have a metabolic protein, RbcL, the large subunit of the enzyme Rubisco. We then showed that RbcL has a moonlighting function during stress, which we are currently exploring.
See: Zhan, Y., Dhaliwal, J.S., Adjibade, P., Uniacke, J., Mazroui, R., & Zerges, W. (2015). Localized control of oxidized RNA. Journal of Cell Science, 128, 4210-4219.