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Crystallized protein sheds light on how crops could survive high temperatures.

Plants use an enzyme, ribulose-1,5-bisphosphate carboxylase oxygenase (rubisco), to capture carbon dioxide from the atmosphere and, with energy from the sun and nutrients from the soil, build up the shoots, leaves and stems that make up the plant itself.

When the temperature is too hot, a rubisco helper protein, rubisco activase, shuts down, photosynthesis stops, and the plant stops growing. Heat unravels the activase protein, and when it does, the result is a less bountiful harvest. Different plants shut down photosynthesis at different temperatures, and the process of unraveling the activase protein is known as denaturation.

Now, USDA-ARS scientists are collaborating to crystallize rubisco activase. Crystallization will allow researchers to study the activase protein more closely, to visualize its structure and possibly manipulate its sequence so that it doesn't unravel at higher temperatures. The findings could help in the search for genes that cue plants to synthesize more heat-stable versions of the protein. Crops with these enhanced proteins could thrive at higher temperatures.

Investigators discovered the existence of rubisco activase in 1985 and proved that it activates rubisco. Globally, scientists have been trying to crystallize rubisco activase ever since. But, we're told, you need to know what the protein looks like to understand how it works. For proteins, the tougher and more rigid a structure, the easier it is to crystallize them. But the problem has been that most plant activase proteins do not have rigid or even regular structures.

The scientists have managed to crystallize the activase protein from the creosote bush, which is a shrub that's abundant in the Arizona desert. They wanted to find the most temperature-tolerant activase possible, and they chose creosote because it can survive and photosynthesize at relatively high temperatures.

The researchers suspected that creosote's activase proteins would remain relatively stable under most conditions. They cloned the plant's activase proteins and reproduced parts of them that were stable enough so that crystals could be produced from them.

The next step will be to use the crystal structure to guide efforts to improve the thermal stability of the rubisco activase protein utilizing rationale design and DNA shuffling. The scientists are also using transgenic approaches to supplement cooler season plants (wheat) with the more heat-stable rubisco activase from warmer season plants (cotton).

Further information. Dr. Michael E. Salvucci, USDA-ARS Arid Land Agricultural Research Center, 21881 N. Cardon Lane, Maricopa, AZ 85138; phone: 520-316-6355; email:
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Publication:Emerging Food R&D Report
Date:Apr 1, 2013
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