How do brassinosteroids activate their receptors

At the origin of growth

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08/11/13 Function of the plant receptor BRI1 clarified

Biologists from Tübingen observe plant hormones at work

If you want to understand how plants grow, you don't just have to go down to the molecular level, but also to the atomic level. Michael Hothorn from the Friedrich Miescher Laboratory of the Max Planck Society in Tübingen does this: Together with his working group, he is investigating down to the atomic level how plant hormones and their receptors interact. In the current issue of the journal Science, the researchers now report that plant membrane receptors need a helper protein in order to recognize an important growth signal and guide it across the cell membrane.

Fig. 1: Activation of the plant brassinosteroid signaling pathway. (Left) X-ray structural model of the LRR domain of the BRI1 receptor (in blue). In the first step, BRI1 binds the steroid hormone (in yellow) in a pocket on its surface. Then the smaller LRR domain of the auxiliary receptor (in orange) can be bound, the hormone holds the two proteins together like an adhesive (right).
Source: Julia Santiago, Max Planck Institute for Developmental Biology, Tübingen

The plant steroid receptor BRI1 belongs to the large family of LRR receptor kinases and is therefore an example of an active principle that is widespread throughout the plant world. "The majority of plant membrane receptors belong to this family - in algae as well as in higher plants," explains Michael Hothorn. Like all membrane receptors, the LRR receptor kinases fulfill three tasks: They pick up molecular signals that reach the cell from outside; they transmit the information through the cell membrane; and ultimately they cause the inside of the cell to respond to the external signal. Compared to animal or bacterial receptors, which have a completely different structure, the plant "receiver molecules" are still very little researched.

As Michael Hothorn and his colleagues have now found out, BRI1 cannot recognize the steroid signal on its own and transmit it across the membrane. "A second membrane receptor is already involved when the hormone signal is received," explains the Tübingen biologist. It was already known that this protein, known as SERK1, is essential for growth hormone to work - the researchers were surprised, however, that SERK1 comes into play at the very beginning of the signaling pathway and functions as a kind of helper protein. As the X-ray structural images by Julia Santiago, the first author of the publication, show, the steroid hormone lies like a double-sided adhesive tape between the two receptors, bringing them into close proximity to one another. As a result, the parts of the receptors on the inside of the membrane can interact with one another and switch on an already known signal cascade.

The construction kit of nature

The analyzes so far indicate that BRI1 can work together with at least three different auxiliary receptors to control the growth and development of the plant. Conversely, the helping protein SERK1 can come together with several different receptors, recognize different ligands and then develop different effects. The mode of action of the LRR receptor kinases thus follows the modular principle: Simply the new combination of existing structures is sufficient to cover a large variety of functions.

As the X-ray structure images show, only around a tenth of the accessible area of ​​SERK1 is required for contact with BRI1 and steroid hormones - the rest is available for interaction with other receptors. "It must have been an advantage for the plants to bundle these various functions in one auxiliary receptor," Michael Hothorn is convinced. From an evolutionary point of view, it would have been much easier to duplicate the gene that codes for the auxiliary receptor over and over again, to further develop the copies independently of one another and thus to keep the various functions separate. Hothorn therefore expects that there is a reason for the bundling: "The various signal paths influence each other in this way," he suspects. This is because an auxiliary receptor that is currently triggering a signal path cannot activate a second signal path at the same time. Such interactions have so far been known mainly at the level of transcription - right at the end of the signal chain. In further research projects, Hothorn would like to clarify whether the interaction at the receptor level is actually used to control growth or metabolic processes.

Alternative to genetic manipulation of crops

The atomic models that the Tübingen researchers create of the receptor interactions could, in the long term, also show new ways for plant breeding. "We can now predict which mutations have which effect on the receptor function," says Michael Hothorn, explaining an aspect of his work that is also echoed in the Science article. The models also make it possible to design active ingredients that imitate or block the effect of the respective hormone. Such designer molecules are urgently needed for basic research on plants, but in the long term they could also be used to control the growth of crops.


Molecular mechanism for plant steroid receptor activation by somatic embryogenesis co-receptor kinases
J. Santiago, et. al., Science2013. DOI: 10.1126 / science.1242468

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Function of the plant receptor BRI1 clarified
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