Salicylic Acid Binding Protein 2

Centuries ago, both the ancient Greeks and native Americans discovered that willow bark and other plants can dull pain. By analyzing these traditional cures, scientists in the nineteenth century extracted the active molecule, salicylic acid, and developed a modified molecule with better drug properties, acetylsalicylic acid or aspirin. In our bodies, aspirin blocks an enzyme that builds one of the molecules of pain signaling. Plants, on the other hand, use salicylic acid for an entirely different type of signaling.

Defensive Action

Plants have developed a complex and multi-layered system to protect themselves from attack by bacteria and viruses. When a leaf gets infected, for instance by tobacco mosaic virus, the local cells make the ultimate sacrifice, inducing a form of programmed cell death. This helps control the spread of the virus by proactively removing all infectable cells in the neighborhood. At the same time, the plant launches a more systemic defense. It sends a signal to all of its distant parts, telling them to build defensive proteins and ready themselves for attack. These defenses are costly, and may result in stunted growth, but this is better than completely losing the battle against the attacker.

Aromatic Signals

Methyl salicylate, the methyl ester of salicylic acid, is one of the signals that spreads through plants, readying them for attack. Methyl salicylate is a familiar molecule, since it provides the distinctive taste and smell of wintergreen flavorings. It is used as a neutral messenger, which is created by cells under attack and delivered to cells throughout the plant. Then, the enzyme SABP2 (salicylic acid binding protein 2) takes methyl salicylate and cleaves off the methyl group, releasing active salicylic acid, which then stimulates the production of defensive proteins in the target cells.

SABP2 in Action

SABP2 was originally discovered based on its ability to bind to salicylic acid (hence its name), but the recent structure of the protein solved by researchers at the NESG revealed its role in cleavage of methyl salicylate and inhibition of the reaction by the product, salicylic acid. The structure, available in PDB entry 1y7i, shows that SABP2 is one of a class of alpha/beta hydrolase enzymes that cleave small esters and other molecules. The active site completely surrounds the molecule, recognizing both the distinctive aromatic ring and the acidic group. A catalytic triad reminiscent of the digestive serine proteases performs the cleavage reaction. Based on this structure, researchers have designed analogues of salicylic acid to probe signaling methods in other plants.

References

1. Forouhar, F., Yang, Y., Kumar, D., Chen, Y., Fridman, E., Park, S. W., Chiang, Y., Acton, T. B., Montelione, G. T., Pichersky, E., Klessig, D. F. and Tong, L. (2005) Structural and biochemical studies identify tobacco SABP2 as a methyl salicylate esterase and implicate it in plant innate immunity. PNAS 102, 1773-1778.
2. Park, S. W., Liu, P. P., Forouhar, F., Vlot, A. C., Tong, L., Tietjen, K. and Klessig, D. F. (2009) Use of a synthetic salicylic acid analog to investigate the roles of methyl salicylate and its esterases in plant disease resistance. J. Biol. Chem. 284, 7307-7317.
3. Park, S. W., Kaimoyo, E., Kumar, D., Mosher, S. and Klessig, D. F. (2007) Methyl salicylate is a critical mobile signal for plant systemic acquired resistance. Science 318, 113-116.
4. Loake, G. and Grant, M. (2007) Salicylic acid in plant defence--the players and protagonists. Curr. Op. Plant Biol. 10, 456-472.
5. Durrant, W. E. and Dong, X. (2004) System acquired resistance. Annu. Rev. Phytopathol. 42, 185-209.