Membrane steroid-binding protein 1 (MSBP1) negatively regulates brassinosteroid signaling by enhancing the endocytosis of BAK1

Abstract

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 transmembrane 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 MSBP1 specifically interacts with the extracellular 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 intracellular kinase domain, suggesting that MSBP1 may suppress BR signaling through interacting with BAK1. Subcellular localization studies revealed that both MSBP1 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 BAK1 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.

Response to metal stress of Nicotiana langsdorffii plants wild-type and transgenic for the rat glucocorticoid receptor gene

“…This behavior is similar to the negative regulation of brassinosteroid signaling in the presence of over expression of the Membrane Steroid-Binding Protein 1 (MSBP1) shown by Yang et al (2005) and Song et al (2009) in Arabidopsis.…”

Immunophilin-like FKBP42/TWISTED DWARF1 Interacts with the Receptor Kinase BRI1 to Regulate Brassinosteroid Signaling in Arabidopsis

“…When BR levels are high, BR binding to the extracellular domain of BRI1 recruits its co-receptor kinase BAK1, which leads receptor kinase activation through transphosphorylation between their kinase domains. BRI1 and BAK1 are mainly localized at the plasma membrane, but also undergo endocytosis and cycle between the endosome and plasma membrane (Geldner et al, 2007; Russinova et al, 2004; Song et al, 2009). Activation of the BRI1/BAK1 receptor complex then leads to BRI1 phosphorylation of BR-signaling kinases (BSKs) (Tang et al, 2008), BSK1 activation of the PP1-like phosphatase BSU1, BSU1-mediated dephosphorylation and inactivation of BIN2 kinase (Kim et al, 2009), and PP2A-mediated dephosphorylation and activation of the BZR1 transcription factor (Tang et al, 2011).…”

Overexpression of a SNARE protein AtBS14b alters BR response in Arabidopsis

“…In addition, the interaction triggered BAK1 endocytosis, inhibiting BR signaling (Song et al [2009]). MSBP1 is localized at both plasma membrane and endosome and is able to bind BR molecules (Song et al [2009]).…”

“…In addition, the interaction triggered BAK1 endocytosis, inhibiting BR signaling (Song et al [2009]). MSBP1 is localized at both plasma membrane and endosome and is able to bind BR molecules (Song et al [2009]). We detected YFP-AtBS14b fusion protein localization at vesicular compartments like the Golgi, which is important for protein endocytosis, and AtBS14b-MSBP1 interactions were mainly detected in vesicular compartments in the cells (Figure 6A).…”

“…Ligand-independent BRI1 trafficking happens between the plasma membrane and the trans-Golgi network/early endosome, followed by degradation in the vacuole (Geldner et al [2007]). Membrane steroid-binding protein 1 (MSBP1) interacts with and enhances endocytosis of BAK1 to negatively regulate BR signaling (Song et al [2009]). However, the mechanism and regulatory molecules involved in BRI1, BAK1 and MSBP1 endocytosis are unclear.…”