Brassinosteroid signal transduction from cell-surface receptor kinases to nuclear transcription factors

Abstract

Brassinosteroid (BR) regulates gene expression and plant development through a receptor kinase-mediated signal transduction pathway1. Despite many components of the pathway identified, how the BR signal is transduced from the cell surface to the nucleus remains unclear2. Here we describe a complete BR signaling pathway by elucidating the key missing steps of the pathway. We show that phosphorylation of BSK1 by the BR receptor kinase BRI1 promotes BSK1 binding to the BSU1 phosphatase, and BSU1 inactivates the GSK3-like kinase BIN2 by dephosphorylating a conserved phospho-tyrosine residue (pTyr200). Mutations that affect phosphorylation/dephosphorylation of BIN2 pTyr200 (bin2-1, bin2-Y200F, and quadruple loss-of-function of BSU1-related phosphatases) support an essential role for BSU1-mediated BIN2 dephosphorylation in BR-dependent plant growth. These results demonstrate direct sequential BR activation of BRI1, BSK1, and BSU1, and inactivation of BIN2, leading to accumulation of unphosphorylated BZR transcription factors in the nucleus. This study establishes a fully connected BR signaling pathway and provides new insight into the mechanism of GSK3 regulation.

Structural and Functional Characterization of Arabidopsis GSK3-like Kinase AtSK12

“…We could not detect dominant nuclear localization of E297K-AtSK12 compared with Ndel-AtSK12 although transgenic plants overexpressing E297K-AtSK12 showed more severe phenotypes. It seems that a strong phenotype of E297K-AtSK12 is mainly due to escaping BSU1 regulation (Kim et al, 2009). It remains unknown how subcellular localization of AtSKs is regulated.…”

“…So far, Arabidopsis Shaggy/GSK3-like kinases (AtSK) appear to have diverse functions in the regulation of growth, responses for abiotic and biotic stress, flower and stomatal development (Gudesblat et al, 2012;Jonak and Hirt, 2002;Saidi et al, 2012). Of these, the molecular action of plant GSK3-like kinases was best understood in brassinosteroid (BR) signaling (Choe et al, 2002;He et al, 2002;Kim et al, 2009;Li and Nam, 2002;Peng et al, 2008;Perez-Perez et al, 2002;Rozhon et al, 2010).…”

“…BIN2 phosphorylation of BZR1/2 causes the inhibition of BZR1/2 activity through multiple mechanisms such as cytoplasmic retention mediated by 14-3-3 protein, loss of DNA binding activity, and degradation by 26S proteasome (Gampala et al, 2007;He et al, 2002;Ryu et al, 2007;Vert and Chory, 2006;Zhao et al, 2002). Of ten GSK3-like kinases in Arabidopsis, recent studies suggest that seven members including BIN2 are involved in BR signaling (Kim et al, 2009;Rozhon et al, 2010). In particular, AtSK12 was shown to interact with and phosphorylate BZR1, like BIN2.…”

“…Overexpression of AtSK12 in Arabidopsis causes a typical dwarf phenotype shown in BR-insensitive mutant. Fur-thermore, AtSK12 protein level was regulated by brassinolide (a most active BR) and BSU1 phosphatase, suggesting that AtSK12 also acts as a negative regulator in BR signal transduction pathway (Kim et al, 2009). Although many AtSKs appear to be involved in BR signaling, their specificities for substrates and interacting proteins in BR signaling are poorly understood.…”

“…Instead, it was shown that dephosphorylation of a Tyr residue equivalent to Tyr216/279 of human GSK3β/α is a key mechanism to inhibit plant GSK3-like kinase activity. BSU1 phosphatase inhibits BIN2 through direct dephosphorylation Tyr200 of BIN2 (Kim et al, 2009;2011).…”