3D Structure of Artificial Photosynthetic Antenna Protein Complex Revealed

Humans can do plenty, but plants have an ability we don't: they make energy straight from sunlight, a superpower called photosynthesis. Yet new research shows that scientists are closing that gap.

Osaka Metropolitan University researchers have revealed the 3D structure of an artificial photosynthetic antenna protein complex, known as light-harvesting complex II (LHCII), and demonstrated that the artificial LHCII closely mirrors its natural counterpart. This discovery marks a significant step forward in understanding how plants harvest and manage solar energy, paving the way for future innovations in artificial photosynthesis.

The researchers, led by Associate Professor Ritsuko Fujii and then graduate student Soichiro Seki of the Graduate School of Science and Research Center for Artificial Photosynthesis, had their study published in PNAS Nexus.

Photosynthesis turns sunlight into usable energy, an extremely complicated process that involves hundreds of different molecules and proteins. Among them is LHCII, the most common pigment-protein complex found in chloroplasts of plants and green algae; it is responsible for capturing sunlight and efficiently channeling energy to drive photosynthesis. LHCII itself consists of many proteins and pigment molecules; understanding how this photosynthetic antenna works its magic and then mimicking it is far from simple.

Various attempts have been made to recreate LHCII. The question is: are these imitations anywhere close to nature's own creation?

Traditional methods fall short of determining the exact structure of in vitro reconstituted LHCII."

Dr. Soichiro Seki, Graduate School of Science and Research Center for Artificial Photosynthesis

In vitro reconstitution is a lab technique that allows scientists to rebuild LHCII outside of plants by synthesizing the protein portion of LHCII in E. coli and combining it with natural pigments and lipids.

The research team therefore turned over a new leaf, using cryo-electron microscopy to analyze the 3D structure of the LHCII they reconstituted. This technique, which won the 2017 Nobel Prize in Chemistry, captures images of samples frozen at extremely low temperatures, making it possible for the researchers to gain a highly detailed look at how pigments and proteins are arranged within the complex.

"Our results showed that the lab-created LHCII was nearly identical to the natural version, with only a few minor differences," Dr. Seki said.

These findings validate the effectiveness of the in vitro reconstitution technique and open up new opportunities for exploring the inner workings of LHCII and its role in photosynthesis, laying the groundwork for future advances in artificial photosynthesis and plant production technologies.

"Our result provides not only a structural foundation for reconstituted LHCII, but also evaluates the functions based on the structure of the reconstituted LHCII," Professor Fujii said. "We hope this will facilitate further studies on the molecular mechanisms by which plants utilize sunlight for chemical reactions."