Phosphorylation-mediated regulation of a rice ABA responsive element binding factor.

OREB1 is a rice ABRE binding factor characterized by the presence of multiple highly-conserved phosphorylation domains (C1, C2, C3, and C4) and two kinase recognition motifs, RXXS/T and S/TXXE/D, within different functional domains. An in vitro kinase assay showed that OREB1 is phosphorylated not only by the SnRK2 kinase, but also by other Ser/Thr protein kinases, such as CaMKII, CKII, and SnRK3. Furthermore, the N-terminal phosphorylation domain C1 was found to be differentially phosphorylated by the SnRK2/SnRK3 kinase and by hyperosmotic/cold stress, suggesting that the C1 domain may function in decoding different signals. The phosphorylation-mediated regulation of OREB1 activity was investigated through mutation of the SnRK2 recognition motif RXXS/T within each phosphorylation module. OREB1 contains a crucial nine-amino acid transactivation domain located near the phosphorylation module C1. Deletion of the C1 domain increased OREB1 activity, whereas mutation of Ser 44, Ser 45, and Ser 48 of the C1 domain to aspartates decreased OREB1 activity. In the C2 domain, a double mutation of Ser 118 and Ser 120 to alanines suppressed OREB1 activity. These findings strongly suggest that selective phosphorylation of the C1 or C2 modules may positively or negatively regulate OREB1 transactivation. In addition, mutation of Ser 385 of the C4 domain to alanines completely abolished the interaction between OREB1 and a rice 14-3-3 protein, GF14d, suggesting that SnRK2-mediated phosphorylation may regulate this interaction. These results indicate that phosphorylation domains of OREB1 are not functionally redundant and regulate at least three different functions, including transactivation activity, DNA binding, and protein interactions. The multisite phosphorylation of OREB1 is likely a key for the fine control of its activity and signal integration in the complex stress signaling network of plant cells.

Chemical inhibitors destabilize HuR binding to the AU-rich element of TNF-alpha mRNA.

Hu protein R (HuR) binds to the AU-rich element (ARE) in the 3UTR to stabilize TNF-alpha mRNA. Here, we identified chemical inhibitors of the interaction between HuR and the ARE of TNF-alpha mRNA using RNA electrophoretic mobility gel shift assay (EMSA) and filter binding assay. Of 179 chemicals screened, we identified three with a half-maximal inhibitory concentration (IC(50)) below 10 microM. The IC(50) of quercetin, b-40, and b-41 were 1.4, 0.38, and 6.21 microM, respectively, for binding of HuR protein to TNF-alpha mRNA. Quercetin and b-40 did not inhibit binding of tristetraprolin to the ARE of TNF-alpha mRNA. When LPS-treated RAW264.7 cells were treated with quercetin and b-40, we observed decreased stability of TNF-alpha mRNA and decreased levels of secreted TNF-alpha. From these results, we could find inhibitors for the TNF-alpha mRNA stability, which might be used advantageously for both the study for post-transcriptional regulation and the discovery of new anti-inflammation drugs.

Flavonoids inhibit the AU-rich element binding of HuC.

Post-transcriptional regulation of mRNA stability by Hu proteins is an important mechanism for tumorigenesis. We focused on the molecular interactions between the HuC protein and AU-rich elements (AREs) to find chemical inhibitors of RNA-protein interactions using RNA electrophoretic mobility shift assay with non-radioactive probes. Screening of 52 natural compounds identified 14 candidate compounds that displayed potent inhibitory activity. Six (quercetin, myricetin, (-)-epigallocatechin gallate, ellagic acid, (-)-epicatechin gallate, and rhamnetin) were categorized as phytochemicals, and their IC(50) values were low (0.2-1.8 microM). [BMB reports 2009; 42(1): 41-46].

A rice dehydration-inducible SNF1-related protein kinase 2 phosphorylates an abscisic acid responsive element-binding factor and associates with ABA signaling.

By a differential cDNA screening technique, we have isolated a dehydration-inducible gene (designated OSRK1) that encodes a 41.8 kD protein kinase of SnRK2 family from Oryza sativa. The OSRK1 transcript level was undetectable in vegetative tissues, but significantly increased by hyperosmotic stress and Abscisic acid (ABA). To determine its biochemical properties, we expressed and isolated OSRK1 and its mutants as glutathione S-transferase fusion proteins in Escherichia coli. In vitro kinase assay showed that OSRK1 can phosphorylate itself and generic substrates as well. Interestingly, OSRK1 showed strong substrate preference for rice bZIP transcription factors and uncommon cofactor requirement for Mn(2+) over Mg(2+). By deletion of C-terminus 73 amino acids or mutations of Ser-158 and Thr-159 to aspartic acids (Asp) in the activation loop, the activity of OSRK1 was dramatically decreased. OSRK1 can transphosphorylate the inactive deletion protein. A rice family of abscisic acid-responsive element (ABRE) binding factor, OREB1 was phosphorylated in vitro by OSRK1 at multiple sites of different functional domains. MALDI-TOF analysis identified a phosphorylation site at Ser44 of OREB1 and mutation of the residue greatly decreased the substrate specificity for OSRK1. The recognition motif for OSRK1, RQSS is highly similar to the consensus substrate sequence of AMPK/SNF1 kinase family. We further showed that OSRK1 interacts with OREB1 in a yeast two-hybrid system and co-localized to nuclei by transient expression analysis of GFP-fused protein in onion epidermis. Finally, ectopic expression of OSRK1 in transgenic tobacco resulted in a reduced sensitivity to ABA in seed germination and root elongation. These findings suggest that OSRK1 is associated with ABA signaling, possibly through the phosphorylation of ABF family in vivo. The interaction between SnRK2 family kinases and ABF transcription factors may constitute an important part of cross-talk mechanism in the stress signaling networks in plants.