news of the week D ECE M B E R 1 , 20 08 E D I T E D BY L AU R E N K . WO L F & K E N N E T H J. M O O R E
which then interact with a hydrophobic portion of a DELLA protein. This association with the gibberellin receptor causes DELLA proteins to be chopped up, CROP SCIENCE: Structure of receptor thereby allowing gene expression. of key plant hormone gibberellin “Our results reveal a completely different mechanism of plant hormone perception” than that of auxin, the reveals how molecule works only other plant hormone that’s had its receptor’s structure solved, Hakoshima says. Although both hormones activate gene expression by targeting transcription AKING LEMONS and grapes plumper, perblockers for destruction, the two hormones achieve this suading asparagus to send out more shoots, end differently. and waking up a wide variety of seeds from When auxin binds dormancy are all part of gibberellin’s job description, to its receptor, the but how this plant hormone gets down to business has long been a mystery. Now, two teams of researchers are hormone acts as a “molecular glue” reporting the first X-ray crystal structures of this imthat pulls transcripportant hormone’s receptor. tion blockers into “It’s very exciting,” says Neil E. Olszewski, a plant bithe same receptor ologist at the University of Minnesota. “I’ve been waiting pocket as the horfor the structure. We’ve known what the gibberellin remone (C&EN, April ceptor is, but its structure will provide the framework for understanding how the hormone’s signal process works.” 9, 2007, page 11). But gibberellin binding The receptor’s structure “could help in designing initiates a conformamore effective and potentially cheaper gibberellin-like tional change of the growth regulators for agriculture,” Peter Hedden, a receptor, creating a plant biochemist at Rothamsted Research, a crop science research institute in England, notes in a published hydrophobic platform that entices commentary. the transcription Japanese scientists studying rice germination first blocker to bind. discovered gibberellin, a diterpenoid carboxylic acid, In another paper as a hormone more than 50 years ago. Since then, some in the same issue 130 different kinds of structurally similar gibberellins BOXED IN Binding of gibberellin (gray spaceof Nature (2008, have been identified, although not all are biologically filling structure) within its receptor (green) 456, 520), a second active as hormones in plants. causes the receptor’s N-terminus (purple) team of Japanese In recent years, researchers figured out that when a to close over the hormone like a lid. Closing gibberellin slips into its receptor, proteins called DELLA, researchers report the lid provides a platform for binding gene the structure of which interfere with gene expression, are targeted for transcription blockers called DELLA (blue). gibberellin and its degradation. Destroying DELLA proteins consequently receptor but without activates genes related to germination, stem elongation, the DELLA transcription blocker. The structure reand flowering. But the mechanism for how all this hapported by Hiroaki Kato, a structural biologist at Kyoto pens was previously unknown, Olszewski says. University, and Makoto Matsuoka, a plant scientist at The structure of the gibberellin receptor reported in Nagoya University, reveals a hormone-in-a-box strucNature (2008, 456, 459) reveals a deep pocket that can ture similar to that in the first paper. accommodate the hormone. It was solved by Toshio The second team also mutated sections of the gibHakoshima, a crystallographer at the Nara Institute of berellin-binding pocket, “which proScience & Technology, in Japan; Taivides insight about which functional ping Sun, a plant biologist from Duke O groups on the gibberellin are important University; and their colleagues. for binding in the receptor pocket and Gibberellin binding causes an unOC why some gibberellins are biologically structured section of the receptor’s HO inactive,” Olszewski says. “In the long N-terminus to collapse into a helical O term, we may be able to develop new bundle, which then closes over the gibOH gibberellin mimics or manipulate the berellin molecule like a lid, Hakoshima plant receptor to recognize synthetic explains. The top of the closed lid Gibberellin 4 gibberellins.”—SARAH EVERTS contains several hydrophobic residues,
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DECE M B E R 1 , 2008
TOSHIO HAKOSHIMA
PLANT PUBERTY
