Auxins are small molecules with big effects, which they achieve via intermediaries. At EMBL Grenoble, the mode of action of those middlemen is coming to light.
They were among the first plant hormones to be identified, and play important roles in guiding the growth of plants from the earliest stages of development. The reason auxins have such powerful effects is that they’re able to turn on the expression of genes within cells, with profound consequences for the way cells function. Auxins do not directly interact with DNA to activate genes; this is the job of auxin response factors (ARFs), a kind of transcription factor that binds to auxin response elements in DNA and initiates reading (or transcription) of auxin-responsive genes. At low auxin concentrations, ARFs become associated with specialised transcriptional repressors called AUX/IAA proteins, which block their ability to turn genes on. When auxin levels rise, these repressors are broken down and the ARFs again become able to switch auxin-responsive genes on.
Knowing how ARFs and their repressors interact is crucial to a full understanding of how auxins regulate gene expression. So Max Nanao, a staff scientist at EMBL Grenoble, teamed up with an international group of researchers to work out the atomic structure of the parts of ARF that interact with the repressors. “There were hypotheses about what these might look like, but there were no structural data”, says Nanao.
There were hypotheses about what these might look like, but there were no structural data
Now there is, as Nanao and colleagues report in a recent paper in Nature Communications, and another published in PNAS. They found that the parts of ARF that bind to the repressors have two faces, one positively charged, the other negatively. These allow ARFs to form chains linked head-to-tail.
Breaking the chain
Notably, AUX/IAA proteins can also bind to both the positive and negative faces of ARF proteins, thereby competing with ARFs for binding to these faces — and this competition is likely to be involved in the regulation of gene activation. Although it’s common for gene-activating proteins to need to form pairs, Nanao and colleagues believe that the capacity of ARFs to form larger complexes involving many ARF subunits may be important for carrying out their biological functions. “This is a question for future research,” says Nanao.