A divergent external loop confers antagonistic activity on floral regulators FT and TFL1

Abstract

The Arabidopsis genes FT and TERMINAL FLOWER1 (TFL1) encode related proteins with similarity to human Raf kinase inhibitor protein. FT, and likely also TFL1, is recruited to the promoters of floral genes through interaction with FD, a bZIP transcription factor. FT, however, induces flowering, while TFL1 represses flowering. Residues responsible for the opposite activities of FT and TFL1 were mapped by examining plants that overexpress chimeric proteins. A region important in vivo localizes to a 14-amino-acid segment that evolves very rapidly in TFL1 orthologs, but is almost invariant in FT orthologs. Crystal structures show that this segment forms an external loop of variable conformation. The only residue unambiguously distinguishing the FT and TFL1 loops makes a hydrogen bond with a residue near the entrance of a potential ligand-binding pocket in TFL1, but not in FT. This pocket is contacted by a C-terminal peptide, which also contributes to the opposite FT and TFL1 activities. In combination, these results identify a molecular surface likely to be recognized by FT-and/or TFL1-specific interactors.

Raf Kinase Inhibitor Protein (RKIP) Dimer Formation Controls Its Target Switch from Raf1 to G Protein-coupled Receptor Kinase (GRK) 2

“…Indeed, loop 127-150 contains eight charged and eight polar amino acids, as well as two glycines and two prolines of 24 amino acids; Banfield et al (8) have generated crystals of dimeric hRKIP and suggested this loop as a putative dimer interface. Others have reported that differences in this loop region participate in opposing effects on flowering of two proteins in Arabidopsis thaliana with high similarity to RKIP (48). Our results, these observations, and the fact that the loop is highly conserved among mammalian RKIP isoforms (12) all suggest that this loop plays an important role in the regulation of protein interactions of RKIP.…”

Contributions of Flowering Time Genes to Sunflower Domestication and Improvement

“…Genetic redundancy, their similar positions to HaFT1 in the flowering time regulatory network, and their molecular mode of action all may have primed HaFT2, HaFT4, and HaTFL1 to respond most appropriately. Both FT and TFL1 homologs interact with FD in other organisms (Pnueli et al 2001;Abe et al 2005;Wigge et al 2005), and it has been hypothesized that phenotypic effects of these genes on flowering and growth depend on the outcome of competitive interactions between members of this family (Ahn et al 2006;Shalit et al 2009;Krieger et al 2010). Increased expression of HaFT2 and HaFT4 are consistent with this scenario, particularly since there is evidence that HaFT2's elevated expression level is controlled by differences in cis-regulatory elements (Blackman et al 2010).…”

The Multiple Signals That Control Tuber Formation

“…It then moves as a protein from the leaf to the shoot apical meristem through plasmodesmata and the sieve element system (for review, see Turck et al, 2008). FT is a member of the phosphatidylethanolamine-binding protein (PEBP) family with a tertiary structure similar to that of mammalian PEBPs (Ahn et al, 2006). FT is a coregulator of transcription and likely does not function without a TF partner.…”