First published online October 15, 2002; 10.1104/pp.102.010918
Plant Physiol, November 2002, Vol. 130, pp. 1221-1229
Two Putative BIN2 Substrates Are Nuclear Components of
Brassinosteroid Signaling1
Jun
Zhao,2
Peng
Peng,2
Robert J.
Schmitz,
Adria D.
Decker,
Frans E.
Tax, and
Jianming
Li*
Department of Molecular, Cellular, and Developmental Biology,
University of Michigan, Ann Arbor, Michigan 48109-1048 (J.Z., P.P.,
J.L.); and Department of Molecular and Cell Biology, University of
Arizona, Tucson, Arizona 85721-0106 (R.J.S., A.D.D., F.E.T.)
GSK3 is a highly conserved kinase that negatively
regulates many cellular processes by phosphorylating a variety of
protein substrates. BIN2 is a GSK3-like kinase in Arabidopsis that
functions as a negative regulator of brassinosteroid (BR) signaling. It was proposed that BR signals, perceived by a membrane BR receptor complex that contains the leucine (Leu)-rich repeat receptor-like kinase BRI1, inactivate BIN2 to relieve its inhibitory effect on
unknown downstream BR-signaling components. Using a yeast
(Saccharomyces cerevisiae) two-hybrid approach,
we discovered a potential BIN2 substrate that is identical to a
recently identified BR-signaling protein, BES1. BES1 and its closest
homolog, BZR1, which was also uncovered as a potential BR-signaling
protein, display specific interactions with BIN2 in yeast. Both BES1
and BZR1 contain many copies of a conserved GSK3 phosphorylation site
and can be phosphorylated by BIN2 in vitro via a novel GSK3
phosphorylation mechanism that is independent of a priming
phosphorylation or a scaffold protein. Five independent
bes1 alleles containing the same proline-233-Leu mutation were identified as semidominant suppressors of two different bri1 mutations. Over-expression of the wild-type
BZR1 gene partially complemented
bin2/+ mutants and resulted in a
BRI1 overexpression phenotype in a
BIN2+ background, whereas
overexpression of a mutated BZR1 gene containing the
corresponding proline-234-Leu mutation rescued a weak
bri1 mutation and led to a bes1-like
phenotype. Confocal microscopic analysis indicated that both BES1 and
BZR1 proteins were mainly localized in the nucleus. We propose that
BES1/BZR1 are two nuclear components of BR signaling that are
negatively regulated by BIN2 through a phosphorylation-initiated
process.
1
This work was supported in part by a
University of Michigan Start-up Fund (to J.L.), by an Overseas
Outstanding Young Investigator Award from the Chinese Natural Science
Foundation (to J.L.), and by the National Institutes of Health (grant
no. GM60519 to J.L.). R.J.S. and A.D.D. were supported by the
Undergraduate Biology Research Program and the University of Arizona
Honors Program.
2
These authors contributed equally to this paper.
*
Corresponding author; e-mail jian{at}umich.edu; fax
734-647-0884.
© 2002 American Society of Plant Biologists
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