The cellular mechanics of auxin perception and signaling have been well studied over the past two decades. The pivotal interaction that controls this activity involves the auxin-dependent contact between the TIR1 receptor and a family of transcriptional regulators called AuxIAA proteins. This interaction has been characterised at a structural level with the auxin indole-3-acetic acid (IAA) shown to act as a ‘molecular glue’, stabilising the interaction between TIR1 and the AuxIAA. This subsequently causes the degradation of the AuxIAA protein, setting off a cascade of auxin-dependent transcriptional responses.
Revealing the precise kinetics of this interaction is complicated by the fact that TIR1 belongs to a family of six related receptors and the AuxIAA family comprises 29 members. Although IAA is able to mediate the interaction between each of these family members, the TIR1 auxin binding pocket is somewhat promiscuous, with a wide range of auxin analogues being able to illicit a similar responses.
In order to develop a synthetic auxin signaling complex that was free from the complexities of varying protein family interactions, Keiko Torii and co-workers from the University of Washington and Nagoya University employed a bump-and-hole strategy. This technique sits at the interface of chemistry, biology and engineering and in this case was able to create a functional synthetic receptor-substrate interaction that did not interfere with the endogenous activity of TIR1-AuxIAA. This research has been published in Nature Chemical Biology.
Using the bump-and-hole strategy the authors interrogated the TIR1 auxin binding-pocket, predicting that removal of a bulky phenylalanine would result in a ‘hole’ whose space could be filled by a version of IAA that included an aryl-ring ‘bump’.
The authors showed that this ‘concave (ccv) TIR1’ was able to interact with the ‘convex (cvx) IAA’ and remarkably be able to elicit a biological relevant response in vivo. Generation of transgenic plants expressing ccvTIR1 or the application of exogenous cvxIAA has little effect on plant growth. However in the presence of cvxIAA, these ccvTIR1 transgenic plants show alterations in primary root elongation, lateral root development and gene expression changes characteristic of an auxin response. Therefore this paper synthetically replicated the auxin signaling module, whose function absolutely relies upon the presence of both components.
This research is a superb example for the use of modeling and synthetic chemistry to facilitate the study of a complex biological system. There is no doubt that the ccvTIR1-cvxIAA system is an important tool for study of the cellular auxin response as well for the tissue-specific activities of this do-it-all phytohormone. We await the development of an engineered enzyme that can produce cvxIAA in vivo so that the system will not need to rely on any external additions!!!
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