A Long-Sought Goal: Crystallizing an Elusive Protein

A U.S. Department of Agriculture (USDA) scientist and his colleagues have opened the door to development of more heat-tolerant crops by crystallizing a plant protein that plays a key role in photosynthesis.

Plants use an enzyme known as Rubisco in photosynthesis, the life-giving process that involves capturing carbon dioxide from the atmosphere and combining it with sunlight and nutrients in the soil to build up the shoots, leaves and stems that make up the plant, according to Michael E. Salvucci, a plant physiologist with the Agricultural Research Service (ARS) in Maricopa, Ariz. ARS is USDA's principal intramural scientific research agency.

Temperature plays an important role in that process. When it gets too hot, a Rubisco helper protein called Rubisco activase shuts down, photosynthesis stops, and the plant no longer grows. Heat literally unravels the activase protein, and when it does, the result is a less-bountiful harvest. Different plants shut down photosynthesis at different temperatures.

Scientists have known about Rubisco for decades. But Salvucci was part of an ARS team that discovered Rubisco activase in 1985 and proved that it activates Rubisco. Scientists around the world have been trying to crystallize Rubisco activase ever since. Crystallizing it would allow researchers to visualize it with X-ray diffraction, and visualizing it would help scientists better understand how it works.

For proteins, the tougher and more rigid the structure, the easier it is to crystallize. But most plant activase proteins do not have rigid or even regular structures.

Salvucci, who works at the ARS U.S. Arid-Land Agricultural Research Center in Maricopa, is now part of another team that has found a way to crystallize Rubisco activase. That discovery will allow researchers to possibly manipulate its sequence so that it doesn't unravel at higher temperatures.

Salvucci and Arizona State University cooperators Rebekka Wachter and Nathan Henderson wanted to find the most heat-tolerant activase protein possible. They knew that activase from the creosote bush would remain relatively stable at high temperatures because the plant is so heat tolerant. The bush is found in abundance in the Arizona desert and has no connection to the tar-like preservative in many wood products.

They cloned the activase genes from the bush and generated parts of the protein that were stable enough to produce crystals. The findings, published in the Journal of Biological Chemistry, could help in the search for genes that cue plants to synthesize more heat-stable versions of the protein and thrive at higher temperatures. With climate change expected to alter landscapes and growing cycles, the work is considered extremely relevant.

Read more about this research in the March 2013 issue of Agricultural Research magazine.