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phate cofactor loHORMONAL COMPLEX cated near auxin in Auxin (green and red space-filling molecule), the TIRi binding with help from inositol hexakisphosphate (orange pocket also lends a CELLULAR SIGNALING: Crystal structure), acts as a molecular glue for the protein sticky hand. structure shows how plant TIRI (light green) and a peptide (yellow) involved in Auxin can turn hormone interacts with proteins blocking gene expression. on such a variety of genes because the peptide that the TIRi-auxin RUIT RIPENING, root branching, and even the combo binds is flowering of some species are all nurtured by auxin, the plant world's most legendary hormone. present in many different tranNow researchers have finally figured out how this inscription represdole activates such a wide variety of plant processes sors. By targeting (Nature 2007,446,640). the degradation "It's amazing that plants all over the world are using of diverse represauxin at this very moment," says research leader Ning sors, auxin actiZheng, a biochemist at the University of Washington, vates diverse plant Seattle. "And now we know the mechanism." processes. Charles Darwin was the first to be captivated by the action of this hormone, when he studied how plants It's a beautiful" reorient their leaves to grow toward light—yet anstudy, comments other plant behavior that can be chalked up to auxin. Bonnie Bartel, a Although auxin's molecular structure was identified geneticist at Rice in the 1930s, it took until 2005 before its biological University. "Not receptor was finally pinpointed (G&EN, May 30,2005, only is it the first page 11). That receptor is a protein called TIRi, which plant hormone receptor-ligand structure solved, it is an shuttles proteins to degradation centers in the cell. entirely new type of receptor for protein degradation." But this discovery left researchers scratching their Typically, proteins are targeted for degradation by heads about how TIRi's association with auxin could undergoing a cascade of phosphorylations. These lead to a myriad of plant processes. The new X-ray crys- chemical signals prompt the covalent attachment of tal structure, solved by Zheng's graduate student Xu ubiquitin proteins that then destine the protein to Tan in collaboration with Mark Estelle's group at Indithe cell's landfill site. The use of a noncovalent ligand ana University, provides a surprisingly simple answer. such as auxin as the destruction signal is novel, notes Judy Callis, a biochemist at the University of CaliforAt high enough concentrations, auxin nestles into nia, Davis. a groove on TIRi. The presence of auxin dramatically enhances the association of TIRi with a short peptide In fact, this new mechanism to target proteins for found in many proteins that block gene transcription. degradation maybe operative in all eukaryotic cells, from plants to humans, Callis says. "Researchers in The auxin-TIRi duo pulls these gene transcription other systems should be aware of this possibility."— repressors away from their normal business and allows blocked genes to be turned on. An inositol hexakisphos- SARAH EVERTS
AUXIN IN ACTION
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EMBARGO ACS is forced to drop Iranian members because of U.S. regulations The American Chemical Society has confirmed that, in January, it was forced to drop 36 members of the society who live in Iran because of the terms of a U.S. embargo against Iran and other nations. ACS officials say they came to the decision after careful legal review indicated that the society was running afoul of the regulations, which are administered by the Treasury Department. "We regretted having to do this, and we will get it sorted out," says ACS Executive Director and CEO Madeleine Jacobs.
She says the society is working to reinstate the Iranian members by applying for a license from the U.S. government. Jacobs adds that the breadth of services that come with ACS membership "distinguish us in important ways" from other scientific societies with regard to the Treasury Department rules. Nonetheless, she says she plans to be in touch with her counterparts at other scientific societies to explain the ACS decision. The regulations do not permit ACS to provide members in Iran with services or
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benefits beyond the receipt of journals and other publications, says ACS Assistant General Counsel David T. Smorodin. Even providing publications becomes problematic, he says, if the society offers them at a discounted member rate. "Chemists around the world network through ACS," says Ali Banijamali, chair of the board of directors of the Iranian Chemists' Association of ACS. With the Treasury Department rules, he says, "politics is prohibiting this collaboration."-WILLIAM SCHULZ
