Auxin Receptor Hides in Plain Sight

Breaking an Animal Paradigm

?The auxin story has been very difficult to understand,? says Leyser, ?because it?s one simple molecule and yet appears to have dozens of different effects.? Auxin affects tropisms to light and gravity. It can affect cell elongation or cell division depending on the tissue. Auxin also acts as a morphogen during embryogenesis and causes rapid changes in gene expression.

In the 1980s, the goal was to untangle auxin?s complexity through identifying Arabidopsis auxin mutants. TIR1 was identified in such a genetic screen in the 1990s and quickly recognized as an F-box protein ? a conserved component of the ubiquitin protein ligase complex called SCF ? important for protein degradation. 3 Other components also started turning up, including transcription factors called Aux/IAA proteins that were degraded by high auxin levels. In 2001, Estelle and Leyser?s groups put a piece of puzzle together, showing that auxin stimulated SCFTIR1 to degrade Aux/IAA proteins. 4

Signaling paradigms from animals predict that Aux/IAAs were likely modified by the auxin signal, through a phosphorylation cascade, for example. The analogy was always to the human system, says Judy Callis, from University of California, Davis, in that you should be able to find some tight binding protein that would interact with auxin and start a signaling cascade. But, ?TIR1 being an auxin receptor is highly unusual,? says Callis, ?and that?s likely why it took so long to uncover.?

Indeed, TIR1 doesn?t need a signaling cascade. Using pull-down assays with epitope-tagged components of the pathway, Estelle and Leyser?s groups independently found that auxin binds directly to TIR1 and promotes the Aux/IAA-SCFTIR1 interaction. 1,2 This makes the pathway surprisingly short ? auxin to TIR1 to Aux/IAA ubiquitination ? and suggests that TIR1 is the elusive receptor that controls auxin-induced changes in gene expression. ?This is a landmark finding for understanding how auxin works,? says Napier.

Auxin Unwinds

Auxin transport is becoming clearer with the recent implication of plant-specific pin-formed (PIN) proteins in the movement of auxin between cells.5 While, the murky question of how auxin is produced was recently shown to involve YUCCA flavin monooxygenases.6

More recently, a crystal structure of TIR1 complexed with auxin was published by Ning Zheng?s group at the University of Washington in collaboration with Estelle?s group. It reveals a thick ?spring washer? shape for the leucine-rich-repeat domain that contains an unexpected inositol hexakisphosphate co-factor, InsP6 or phytate.7 The work is beginning to show how a small molecule like auxin might regulate the activity of an ubiquitin ligase.

Though such signaling might be valuable in human biology, major questions remain. Is TIR1?s ligand-mediated proteolysis unique or simply the first of many to be discovered? Arabidopsis contains approximately 700 proteins with F-box domains, as well as a family of partially redundant TIR1 genes, suggesting there is much more to be discovered. ?There?s still a lot to understand,? says Napier, ?in particular whether the TIR1 family controls all the responses to auxin.?

For Leyser, despite the new discoveries, the question remains the same as when she first started working on auxin as a post-doc in Estelle?s lab: ?how do you get so many different outputs from such a simple input as auxin?? The answer just might start with TIR1.

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