Minisymposium 14: Protein Modification

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Abs # M1403: Novel transcription factors and E3 ubiquitin ligases involved in brassinosteroid signal transduction in Arabidopsis

Presenter:	Yin, Yanhai       Contact Presenter
Authors	Yin, Yanhai  (A)   Guo, Michelle  (A)   Li, Li  (A)   Li, Lei  (A)   Tao, Yi  (B)   Yoshida, Shigeo  (C)   Asami, Tadao  (C)   Chory, Joanne  (B)
Affiliations:	(A): Department of Genetics, Development and Cell Biology and Plant Sciece Institute, Iowa State University (B): Howard Hughes Medical Institute and Plant Biology Laboratory, The Salk Institute for Biological Studies (C): Plant Functions Lab, RIKEN (The Institute of Physical and Chemical Research), Japan

Plant steroid hormone brassinosteroids (BRs) play important roles throughout growth and development. Unlike animal steroids that bind nuclear receptor superfamily transcription factors to directly regulate target gene expression, BRs are perceived by membrane-localized receptor kinase BRI1. Multidisciplinary approaches have been used to study how the BR signal is transduced from the cell surface receptor to nuclear target genes. By screening for bri1 mutant suppressors, we identified a nuclear protein BES1 and found that BES1 protein level is regulated by BR signaling. To understand how BES1 protein level is regulated, we identified two BES1-interacting E3 ubiquitin ligases that potentially target BES1 for degradation. Consistent with the hypothesis, suppression of these ubiquitin ligase genes leads to BR-response phenotypes including increased BES1 protein level, excessive cell elongation and partial resistance to BR biosynthesis inhibitor brassinazole. Although BES1 does not have significant homology to known DNA-binding domains, its amino-terminal domain is involved in DNA binding and can potentially form a novel basic-helix-loop-helix (bHLH) motif. Indeed, BES1 is a transcription activator that binds to and activates BR-target gene promoters both in vitro and in vivo. In addition, BES1 interacts with a typical bHLH protein, BIM1, to synergistically bind to E-box sequences present in many BR-induced promoters. Our recent genetic and molecular studies identified several new transcription factors that interact with BES1 and regulate BR target gene expression. Loss-of-function and gain-of-function mutants of these BES1 partners display various BR-response phenotypes. Our results therefore establish that BRs signal to regulate the protein level of transcription activator BES1 and BES1 functions with other transcription factors to regulate plant growth and development.