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| RIKEN Press Release | November 27, 2007 |
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Dissecting the mechanism of plant immunityAn international team of scientists has uncovered how select proteins come together to help plants maintain an efficient immune system. | |
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A recent study by an international team of scientists, including researchers at RIKEN's Plant Science Center, has revealed the molecular underpinnings of a plant immune response system. This work could eventually pave the way toward development of crops able to outmaneuver pathogens that can now evade a plant's immune responses by mutating into modified versions unrecognizable to its immune system. The project, led by Raphael Guerois of the Commissariat a` l'Energie Atomique, Institut de Biologie et Technologies de Saclay in France and by Ken Shirasu of RIKEN's Plant Science Center in Yokohama, was published in the latest online issue of The Plant Cell. Plants possess a panoply of 'resistance,' or R proteins, the receptor proteins targeting specific pathogens. When these protective R proteins encounter their corresponding pathogens, infected cells are induced to 'commit suicide,' thereby denying the invaders their strategic foothold. Numerous genes coding R proteins have been identified, but our understanding of the molecular mechanisms underlying this immunity has lagged far behind advances in genetics. To address this imbalance, Shirasu and colleagues used a potent approach combining genetic, biochemical and physiological assays, in close concert with a structure-modeling system, to unveil the interactions between three proteins that are keys to plant immunity. HSP90, the best characterized of these proteins, is a ubiquitous, multifunctional 'molecular chaperone' that binds to and maintains the native folding structure of many signaling proteins or 'clients' during a client maturation process. How this maturation progresses remains mysterious, but answers may lie in the link between client release and HSP90-directed chemical splitting of a nucleotide called adenosine triphosphate (ATP) - the ephemeral, high-energy storage molecule that drives cellular functions - and with co-chaperones important for client recruitment. SGT1 and RAR1, which are important for the stabilization of various R proteins, also interact with HSP90 as suspected co-chaperones. To parse how these three proteins interface to confer disease resistance, the researchers began by dissecting SGT1, using a complex assay system. An SGT1 protein version from Arabidopsis, a common weed studied extensively in labs, was used. The host testing system was either a wild-type tobacco (WT) plant or a transgenic version, NB - genetically manipulated to silence its own endogenous SGT1 gene and to express the potato protein, Rx, that confers resistance to a potato virus (PVX). A variation of PCR-based molecular engineering generated a library of mutant SGT1 proteins, which were then screened for activity either in host NB plants infected with PVX or in host WT plants inoculated with both PVX and the Rx gene. PVX accumulation, which correlated strongly with the occurrence of necrotic spots on the leaves of infected NB plants, was easily measured and indicated how efficiently the SGT1 mutants mediated Rx-protein activity. These experiments singled out relevant mutant forms of SGT1 while subsequent biochemical and genetic analyses of these mutants defined the specific protein region - the CS domain - that is critical for resistance. Nuclear magnetic resonance (NMR) studies - which hinge on the magnetic properties, at a quantum mechanical level, of an atom's nucleus-allowed the scientists to model the structure of the CS domain and map the interacting regions between SGT1/RAR1 and between SGT1/HSP90. Biochemical and genetic analyses of SGT1 mutants, in tandem with NMR, were employed to further pinpoint the specific amino acids - the building blocks of proteins - crucial for the binding interactions. The researchers suggest a multi-protein assembly model featuring SGT1 and RAR1 functioning as co-chaperones but, unusually, without affecting how HSP90 splits ATP. SGT1 can interact directly with HSP90 but RAR1 enhances the HSP90-SGT1 association, stabilizing the resulting ternary complex. However, more study on RAR1's role is necessary as it can also out-compete SGT1 for binding to HSP90, suggesting that specific R protein systems may require RAR1 to either work antagonistically against or synergistically with SGT1.
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