title: Regulation of Arabidopsis brassinosteroid signaling by atypical basic helix-loop-helix proteins.
abstract: Basic helix-loop-helix (bHLH) proteins are highly conserved transcription factors critical for cell proliferation and differentiation. Recent studies have implicated bHLH proteins in many plant signaling processes, including brassinosteroid (BR) signaling. Here, we report identification of two families of atypical bHLH proteins capable of modulating BR signaling. We found that activation-tagged bri1 suppressor 1-Dominant (atbs1-D), previously identified as a dominant suppressor of a weak BR receptor mutant bri1-301, was caused by overexpression of a 93-amino acid atypical bHLH protein lacking amino acids critical for DNA binding. Interestingly, atbs1-D only suppresses weak BR mutants, while overexpression of a truncated ATBS1 lacking the basic motif also rescues bri1-301, suggesting that ATBS1 likely stimulates BR signaling by sequestering negative BR signaling components. A yeast two-hybrid screen using ATBS1 as bait discovered four ATBS1-Interacting Factors (AIFs) that are members of another atypical bHLH protein subfamily. AIF1 exhibits an overlapping expression pattern with ATBS1 and its homologs and interacts with ATBS1 in vitro and in vivo. AIF1 overexpression nullifies the suppressive effect of atbs1-D on bri1-301 and results in dwarf transgenic plants resembling BR mutants. By contrast, silencing of AIF1 partially suppressed the bri1-301 phenotype. Our results suggested that plants use these atypical bHLH proteins to regulate BR signaling.
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, University of Michigan, An Arbor, Michigan 48109-1048, USA.
authors.forename: Hao
authors.initials: H
authors.lastname: Wang
authors.forename: Yongyou
authors.initials: Y
authors.lastname: Zhu
authors.forename: Shozo
authors.initials: S
authors.lastname: Fujioka
authors.forename: Tadao
authors.initials: T
authors.lastname: Asami
authors.forename: Jiayang
authors.initials: J
authors.lastname: Li
authors.forename: Jianming
authors.initials: J
authors.lastname: Li
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D051792:Basic Helix-Loop-Helix Proteins / Q000235:genetics / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant; D020868:Gene Silencing; D008969:Molecular Sequence Data; D010937:Plant Growth Regulators / Q000378:metabolism*; D030821:Plants, Genetically Modified / Q000235:genetics / Q000378:metabolism; D018749:RNA, Plant / Q000235:genetics; D016415:Sequence Alignment; D015398:Signal Transduction; D013256:Steroids / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D051792:Basic Helix-Loop-Helix Proteins; D010937:Plant Growth Regulators; D018749:RNA, Plant; D013256:Steroids