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Molecular Plant 9, 178–181, January 2016

Oligomerization between BSU1 Family Members Potentiates Brassinosteroid Signaling in Arabidopsis (Letter to the Editor)
Eun-Ji Kim, Ji-Hyun Youn, Chan-Ho Park, Tae-Woo Kim, Shenheng Guan, Shouling Xu, Alma L Burlingame, Young-Pil Kim, Seong-Ki Kim, Zhi-Yong Wang, Tae-Wuk Kim

Dear Editor,

Brassinosteroid (BR) regulates diverse physiological and developmental processes in plants (Choudhary et al., 2012). BR binds to the receptor kinase BRI1 to trigger a phosphorylation/ dephosphorylation cascade, which includes phosphorylation of Brassinosteroid Signaling Kinase 1 (BSK1) and Constitutive Differential Growth 1 (CDG1) by BRI1, phosphorylation of BSU1 by CDG1, and dephosphorylation of BIN2 (a GSK3-like kinase) by BSU1 (Kim et al., 2009, 2011; Wang et al., 2014). Almost all the downstream components of BR signaling are encoded by multiple genes of a family. The Arabidopsis BSU1 family (BSUf; BSU1, BSL1, BSL2, and BSL3) is classified into a unique phosphatase family, the PPKL (Protein Phosphatases with Kelch-Like repeat domains), which is not found in animals (Kutuzov and Andreeva, 2002; Mora-Garcia et al., 2004). BSUf members contain an N-terminal Kelch-repeat domain and a C-terminal phosphatase domain that is closely related to protein phosphatase 1. Knockdown of BSL2 and BSL3 by artificial micro-RNA in the bsu1bsl1 mutant (amiBSL2,3/bsu1bsl1) causes pleiotropic defects, including severe dwarfism similar to strong BR-deficient or BR-insensitive mutants and abnormal stomatal development (Kim et al., 2009, 2012; Youn et al., 2013). A recent study suggested that BSUf members exhibit specific/non-overlapping functions in addition to BR signaling, although all four members are involved in BR signaling (Maselli et al., 2014). However, the biochemical characteristics underlying the functional redundancy and diversity of PPKL family members are largely unknown....