Respiration is the process used by all forms of life to turn organic matter from food into energy that cells can use to live and grow. This process’s final stage relies on an intricate chain of protein complexes that produce the molecule that cells use for energy.
Complexes in the chain are made up of specific proteins that are carefully assembled, often into discrete modules or intermediate complexes, before coming together to form the full protein complex.
A study by the University of California, Davis, has offered the first-ever, atomic-level, 3-D structure of the most abundant protein complex (complex I) involved in the plant mitochondrial electron transport chain. Scientists now have higher resolution structures of the entire electron transport chain and even supercomplexes, complexes of complexes, but for plants.
This detailed information on the structure and functionality of these plant protein complexes could help scientists improve agriculture and even design better pesticides.
James Letts, assistant professor in the Department of Molecular and Cellular Biology, College of Biological Sciences, said, “We can also design better-targeted pesticides or fungicides that will kill the fungus but not the plant and not the human who eats the plant.”
Chloroplasts are organelles found in plant cells and eukaryotic algae that conduct photosynthesis. Chloroplasts absorb sunlight and use it in conjunction with water and carbon dioxide gas to produce food for the plant. But chloroplasts can pose a problem to scientists studying the molecular minutiae of the mitochondrial electron transport chain.
Maria Maldonado, a postdoctoral researcher in the lab of James Letts, said, “Plants have mitochondria, and they also have chloroplasts, which make the plant green, but the organelles are very similar in size and have very similar physical properties.”
“These similarities make it difficult to isolate mitochondria from chloroplasts in a lab setting.”
To get around this, scientists used “etiolated” mung beans (Vigna radiata). They grew the plants in the dark, preventing chloroplasts from developing and causing the plants to appear bleached. Without chloroplasts, the plants are unable to photosynthesize, limiting their energy streams.
By separating mitochondria from chloroplasts, the researchers gained a clearer structural image of complex I and its subcomplexes.
They did this by using single-particle cryoelectron microscopy, which reveals the structure of the complexes. With these structures, scientists can see, at the atomic level, how the building block proteins of complex I are assembled and how those structures and their assembly differs compared to the complexes present in the cells of mammals, yeast, and bacteria.
Scientists noted, “Our structure shows us for the first time the details of a complex I module that is unique to plants. Our experiments also gave us hints that this assembly intermediate may not just be a step towards the fully assembled complex I, but may have a separate function of its own.”
With the structure of complex I now in hand, Scientists are now planning to conduct functional experiments. Further understanding complex I’s functionality could open the doorway to making crop plants more energy efficient.
Maria Maldonado et al., Atomic structure of a mitochondrial complex I intermediate from vascular plants, eLife (2020). DOI: 10.7554/eLife.56664