摘要
Brassinosteroids (BR ) 被 transmembrane 受体和戏察觉在植物生长和开发的重要角色，以及响应环境刺激的房间。transmembrane 受体 BRI1 能直接绑在 brassinolide (BL ) ，并且 BAK1 与 BRI1 交往提高发信号的调停 BRI1 的 BR。我们的以前的研究显示了那膜类固醇绑定蛋白质(MSBP1 ) 1 能在 vitro 绑在 BL 并且否定地涉及 BR 发信号。进一步阐明内在的机制，我们这里证明 MSBP1 明确地以一种 BL 独立的方式在 vivo 与 BAK1 的细胞外的领域交往。由 MSBP1 的增加的表示的压制的房间扩大和 BR 回答能被 overexpressing BAK1 或它的细胞内部的 kinase 领域恢复，建议 MSBP1 可以压制通过与 BAK1 交往发信号的 BR。Subcellular 本地化研究表明 MSBP1 和 BAK1 对血浆膜和 endocytic 泡局部性， MSBP1 加速 BAK1 endocytosis，它导致由向内涵体转移 BAK1 的平衡发信号的压制的 BR。确实，提高了 MSBP1 表示还原剂在在 vivo 的 BRI1 和 BAK1 之间的相互作用，表明那 MSBP1 在发信号的 BR 的早步充当一个否定因素小径。
Brassinosteroids （BRs） are perceived by transmembrane receptors and play vital roles in plant growth and development, as well as cell in responses to environmental stimuli. The transmemhrane receptor BRI1 can directly bind to brassinolide （BL）, and BAK1 interacts with BRI1 to enhance the BRI1-mediated BR signaling. Our previous studies indicated that a membrane steroid-binding protein 1 （MSBP1） could bind to BL in vitro and is negatively involved in BR signaling. To further elucidate the underlying mechanism, we here show that MSBPI specifically interacts with the extraeellular domain of BAK1 in vivo in a BL-independent manner. Suppressed cell expansion and BR responses by increased expression of MSBP1 can be recovered by overexpressing BAK1 or its intracellnlar kinase domain, sug- gesting that MSBP1 may suppress BR signaling through interacting with BAK1. Subcellular localization studies re- vealed that both MSBPI and BAK1 are localized to plasma membrane and endocytic vesicles and MSBP1 accelerates BAK1 endocytosis, which results in suppressed BR signaling by shifting the equilibrium of BAKI toward endosomes. Indeed, enhanced MSBP1 expression reduces the interaction between BRI1 and BAK1 in vivo, demonstrating that MSBP1 acts as a negative factor at an early step of the BR signaling pathway.
基金
Acknowledgments This study was supported by the Chinese Academy of Sciences and National Natural Science Foundation of China （Grants 30425029, 30421001, 90717001）. We greatly thank Prof Hong Ma （Penn. State University, USA） for critical reading and writing improvement and Prof Nam-Hai Chua （The Rockefeller University, USA） for helpful comments. We thank the Salk Institute Genomic Analysis Laboratory for providing the sequence-indexed Arabidopsis T-DNA insertion mutants, and Prof Sheng Luan （University of California, Berkeley, USA） for providing the construct pATC940. We thank Prof Hong-Quan Yang （SIPPE, CAS） for providing LexA yeast two-hybrid system and Prof Zhi-Yong Wang （The Stanford University, USA） for providing the BRI1 antibody. We thank Mr Xiao-Shu Gao for the help on Confocal Laser Scanning Microscopy.
