title: Dimerization and blue light regulation of PIF1 interacting bHLH proteins in Arabidopsis.
abstract: Phytochrome Interacting Factor 1 (PIF1), a basic helix-loop-helix (bHLH) protein, functions as a negative regulator of various facets of photomorphogenesis. To indentify PIF1-interacting proteins, we performed yeast two-hybrid screening using PIF1 as a bait and identified a group of proteins including PIF1 itself, PIF3 and long hypocotyl in far-red 1 (HFR1), an atypical HLH protein. Directed yeast two-hybrid interaction assays showed that PIF1 can form heterodimers with all other PIFs as well as with HFR1. PIF1 and PIF3 interacted with each other in both in vitro and in vivo co-immunoprecipitation assays. PIF1-PIF3 heterodimer also bound to a G-box DNA sequence element in vitro. To understand the biological significance of these interactions, a pif1pif3 double mutant was obtained and characterized. Analyses of the single and double mutants showed that PIF3 plays a prominent role in repressing photomorphogenesis under continuous blue light conditions. pif1 and pif3 showed additive phenotypes more prominently under discontinuous blue light conditions. Similar to PIF1, PIF3 was also rapidly phosphorylated, poly-ubiquitylated and degraded in response to blue light. PIF3 also interacted with phytochromes in response to blue light. A PIF3 mutant defective in interaction with both phyA and phyB displayed reduced degradation under blue light, suggesting that phy-interaction was necessary for the blue light-induced degradation of PIF3. Taken together, these data suggest a combinatorial control of photomorphogenesis by bHLH proteins in response to light in Arabidopsis.
authors.affiliation: Biological Laboratories, Section of Molecular Cell and Developmental Biology, The Institute for Cellular and Molecular Biology, The University of Texas at Austin, 205 W. 24th St., Austin, TX 78712, USA.
authors.forename: Qingyun
authors.initials: Q
authors.lastname: Bu
authors.forename: Alicia
authors.initials: A
authors.lastname: Castillon
authors.forename: Fulu
authors.initials: F
authors.lastname: Chen
authors.forename: Ling
authors.initials: L
authors.lastname: Zhu
authors.forename: Enamul
authors.initials: E
authors.lastname: Huq
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D051792:Basic Helix-Loop-Helix Proteins / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000378:metabolism; D019281:Dimerization; D008027:Light; D009687:Nuclear Proteins / Q000378:metabolism; D010641:Phenotype; D010766:Phosphorylation; D025941:Protein Interaction Mapping; D054875:Ubiquitination; D001616:beta-Galactosidase / Q000032:analysis
chemical_list: D029681:Arabidopsis Proteins; D051792:Basic Helix-Loop-Helix Proteins; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D001616:beta-Galactosidase; C414438:At1g02340 protein, Arabidopsis; C508068:At2g20180 protein, Arabidopsis; C116162:PIF3 protein, Arabidopsis