Laboratory of Biochemistry , Department of Agrotechnology and Food Sciences, 6703 HA Wageningen, The Netherlands.
Plant Physiol. 2013 Aug;162(4):1911-25. doi: 10.1104/pp.113.220152. Epub 2013 Jun 24.
The leucine-rich repeat receptor-like kinase BRASSINOSTEROID-INSENSITIVE1 (BRI1) is the main ligand-perceiving receptor for brassinosteroids (BRs) in Arabidopsis (Arabidopsis thaliana). Binding of BRs to the ectodomain of plasma membrane (PM)-located BRI1 receptors initiates an intracellular signal transduction cascade that influences various aspects of plant growth and development. Even though the major components of BR signaling have been revealed and the PM was identified as the main site of BRI1 signaling activity, the very first steps of signal transmission are still elusive. Recently, it was shown that the initiation of BR signal transduction requires the interaction of BRI1 with its SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) coreceptors. In addition, the resolved structure of the BRI1 ectodomain suggested that BRI1-ASSOCIATED KINASE1 BAK1 may constitute a component of the ligand-perceiving receptor complex. Therefore, we investigated the spatial correlation between BRI1 and BAK1(SERK3) in the natural habitat of both leucine-rich repeat receptor-like kinases using comparative colocalization analysis and fluorescence lifetime imaging microscopy. We show that activation of BR signaling by exogenous ligand application resulted in both elevated colocalization between BRI1 and BAK1(SERK3) and an about 50% increase of receptor heterooligomerization in the PM of live Arabidopsis root epidermal cells. However, large populations of BRI1 and BAK1(SERK3) colocalized independently of BRs. Moreover, we could visualize that approximately 7% of the BRI1 PM pool constitutively heterooligomerizes with BAK1(SERK3) in live root cells. We propose that only small populations of PM-located BRI1 and BAK1(SERK3) receptors participate in active BR signaling and that the initiation of downstream signal transduction involves preassembled BRI1-BAK1(SERK3) heterooligomers.
富含亮氨酸重复受体样激酶 BRASSINOSTEROID-INSENSITIVE1（BRI1）是拟南芥（Arabidopsis thaliana）中油菜素内酯（BRs）的主要配体感知受体。BRs 与质膜（PM）定位的 BRI1 受体的细胞外结构域结合，启动影响植物生长和发育各个方面的细胞内信号转导级联反应。尽管已经揭示了 BR 信号的主要成分，并且 PM 被鉴定为 BRI1 信号活性的主要位点，但信号转导的最初步骤仍然难以捉摸。最近，研究表明 BR 信号转导的启动需要 BRI1 与其 SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE（SERK）共受体相互作用。此外，BRI1 细胞外结构域的解析结构表明，BRI1-ASSOCIATED KINASE1 [BAK1]（SERK3）可能构成配体感知受体复合物的一个组成部分。因此，我们使用比较共定位分析和荧光寿命成像显微镜研究了天然环境中富含亮氨酸重复受体样激酶 BRI1 和 BAK1（SERK3）之间的空间相关性。我们发现，外源性配体应用激活 BR 信号导致 BR1 和 BAK1（SERK3）之间的共定位增加，并且在活体拟南芥根表皮细胞的 PM 中受体异源寡聚化增加约 50%。然而，大量的 BRI1 和 BAK1（SERK3）在没有 BR 的情况下独立共定位。此外，我们可以在活根细胞中观察到大约 7%的 BRI1 PM 池与 BAK1（SERK3）组成性异源寡聚化。我们提出，只有 PM 定位的 BRI1 和 BAK1（SERK3）的小群体参与活性 BR 信号，并且下游信号转导的启动涉及预先组装的 BRI1-BAK1（SERK3）异源寡聚体。
