Mechanistic basis for the activation of plant membrane receptor kinases by SERK-family coreceptors

Abstract

SignificancePlants contain a unique family of membrane receptors, which are different from the ones found in bacteria and animals. These proteins are able to sense very different signals, such as steroid molecules, peptides, and proteins at the cell surface using a spiral-shaped ligand binding domain. Ligand binding allows the receptor to engage with a smaller coreceptor kinase, which is shared among different receptors. Here it is analyzed how one coreceptor protein can contribute to the sensing of two different ligands involved in plant growth and organ abscission and to activation of their cognate receptors.

How do brassinosteroids activate their receptors?

“…This suggests a positive ∆G of association at relevant concentrations, in contrast to our ∆G • of −2.120 ± 0.380 kcal·mol −1 . Previously reported experimental values of ∆G • for BAK1 association with BL-bound BRI1 are -8.44 kcal·mol −1 from grating-coupled interferometry and -9.20 kcal·mol −1 from isothermal titration calorimetry [19] (see Supplementary Information for details), both far larger in magnitude than our result of −3.630 ± 0.453 kcal·mol −1 .…”

“…Both experimental association free energies were determined in solutions with pH 5 [19], whereas our protein models were constructed in the most likely protonation states at pH 7. BRI1 and BAK1 are known to interact more strongly in acidic conditions [6], possibly due to double protonation of the BAK1 H61 sidechain.…”

“…An atomistic picture of brassinosteroid signaling initiation has begun to emerge from a number of structural and computational studies [2,3,5,6,7,12,13,14,15,16,17,18,19]. The predominant hypothesis concerning the mechanism of BL-induced BR1-BAK1 association is that BL acts as a "molecular glue", mediating interactions between the BRI1 and BAK1 extracellular domains (ECDs), as observed in crystal structures [5,6].…”

“…However, the exact molecular events that unfold upon binding of BL to BRI1 remain unclear, especially in light of evidence that BRI1 and BAK1 preform dimers in vivo in the absence of BL [21,22]. While recent work has provided quantitative thermodynamic data concerning BL binding to BRI1 and BRI1-BAK1 association [19], the nanoscale details of how hormone binding causes the BRI1-BAK1 complex to assemble and activate remain largely unknown.…”

Molecular mechanism for the recognition of sequence-divergent CIF peptides by the plant receptor kinases GSO1/SGN3 and GSO2

“…Many of the currently known LRR-RKs require the interaction with a shape-complementary coreceptor kinase for high affinity ligand binding and for receptor activation (1,21). In contrast to, for example, the peptide hormone IDA, CIF1-4 bind to GSO1/SGN3 with nanomolar affinity already in the absence of a co-receptor kinase (Figs.…”