Biologists Identify Plant-specific Protein Essential for Survival

(Brookhaven National Laboratory) Despite their fundamental differences, plants, animals, and fungi share certain metabolic processes. Biologists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have discovered that a unique protein plays a key role in one such process — yet the protein is found only in plants.

As described in a study just published in Science Advances, a deeper understanding of this plant-specific protein could lead to more robust bioenergy crop plants — and potentially a new way to eliminate weeds.

Chang-Jun Liu, a senior scientist in Brookhaven’s Biology Department, studies the role of cytochrome b5 proteins in Arabidopsis plants, which are commonly used for understanding basic plant biology. These proteins, classified as electron carriers, help drive metabolic processes in plants, animals, and fungi by transferring electrons between molecules.

In earlier work, Liu and his team discovered a cytochrome b5-like protein (CB5LP) in Arabidopsis plants. Its genetic sequence is similar to that of conventional cytochrome b5 proteins, but its major functional pieces, or domains, are arranged differently.

Brookhaven Lab biologists engineered Arabidopsis plants to produce less cytochrome b5-like protein (CB5LP). At two weeks old, the CB5LP mutant plants (circled in magenta) were noticeably smaller than their wild-type counterparts, indicating that CB5LP is essential for typical plant growth and development. (Brookhaven National Laboratory)

“CB5LP is clearly a special protein,” said Liu. “But we did not know what it did. We were curious to know if it functions like conventional cytochrome b5 proteins.”

Xianhai Zhao, a postdoctoral researcher in Liu’s group, began the investigation by engineering Arabidopsis plants that did not produce CB5LP. The outcome was striking: The plants could not survive without CB5LP.

“We routinely edit plants by deleting specific genes to study the roles of various proteins or compounds,” said Zhao. “There are usually several effects, but it’s rare that removing just one protein kills them.”

This drastic consequence suggested that CB5LP is involved in synthesizing an important compound — and that other cytochrome b5 proteins could not act as a substitute.

Protein partners

Cytochrome b5 proteins often partner up with cytochrome P450 enzymes to carry out metabolic processes. So, the scientists thought that CB5LP may also work with a molecular partner whose function could help reveal its role.

With collaborators at the Carnegie Institution for Science, they used proximity labeling analysis to identify proteins in the vicinity of CB5LP that could be potential partners.

“We uncovered a set of candidate proteins,” said Zhao. “Then we examined, one by one, the metabolic processes each one likely participates in and the compounds they help produce.”

One of the potential partners was a cytochrome P450 enzyme involved in the synthesis of sterols — compounds that are essential for the integrity and function of cell membranes in plants, animals, and fungi. Sterols are so critical for survival that they are typically the target of antifungal drugs, which disrupt sterol synthesis to prevent fungal growth or reproduction. Sterols in plants and animals — known as phytosterols and cholesterols, respectively — are also precursors to hormones and vitamins vital for growth and overall health.

Using genetic and biochemical analyses, the researchers revealed that CB5LP, like cytochrome b5 proteins, functions as an electron carrier, specifically in the sterol synthesis pathway.

They also discovered that CB5LP is found only in plants and not in animals or fungi. This was especially surprising because many aspects of sterol synthesis, including key cytochrome P450 proteins, are shared across all three groups.

This unique distinction could help inform the development of novel herbicides, or “weed killers” that interfere with essential plant processes, like sterol synthesis. In particular, the findings suggest that targeting plant-specific CB5LP — and disrupting sterol synthesis — could offer a new strategy for eliminating weeds while avoiding potential adverse effects for animals or fungi.

Liu said, “This work also contributes to our knowledge of plant metabolism, which will ultimately help us engineer stronger and more productive bioenergy crops.”

This work was supported by the DOE Office of Science. The researchers used a confocal microscope at the Center for Functional Nanomaterials (CFN) to study where CB5LP was located within Arabidopsis plant cells. CFN is a DOE Office of Science user facility at Brookhaven Lab.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.