BT2, a BTB Scaffold Protein, Mediates Responses to Multiple Biotic and Abiotic Signals in Arabidopsis

Abstract

We previously described BT2, a BTB/POZ domain containing protein, as an activator of telomerase in Arabidopsis thaliana. In the current study, I present evidence of its interesting roles in mediating multiple hormone, stress and metabolic responses in plants. Steady-state expression of BT2 mRNA was regulated diurnally and was under the control of circadian clock, with a maximum expression in the dark. BT2 mRNA was responsive to nutrient status and to multiple biotic and abiotic stress signals. Using bt2 loss-of-function and BT2 over-expressing lines, I show that BT2 suppresses sugar and ABA-mediated responses during germination. BT2 is also essential for transcriptional gene activation mediated by CaMV 35S enhancers in Arabidopsis. Loss of BT2 in several well-characterized 35S enhancer activation-tagged lines such as yucca1d, pap1d, jaw1d etc., resulted in suppression of the activation phenotypes. The suppression of the phenotypes was due to decreased transcription of the activation-tagged genes. I further demonstrate that BT2 genetically interacts with CULLIN3. I propose that BT2 and CULLIN3 are components of a ubiquitin ligase complex. Together with associated proteins BET9 and BET10, the BT2 complex is required for CaMV 35S enhancer-mediated activation of gene expression and may regulate expression of target genes involved in multiple responses to fluctuating biotic and abiotic conditions. I also found that BT2 protein levels are tightly regulated in plants. BT2 protein was primarily localized in the nucleus and was developmentally regulated. BT2 turn-over was regulated in part by the 26S-proteosome, and rare codons present in its open reading frame affected BT2 protein accumulation. In addition to BT2, its orthologs, BT1, BT3, BT4 and BT5, also responded to light, clock and nutrients, with some differences. Moreover, BT1, BT3 and BT4 were also required for 35S enhancer-mediated activation of gene expression. I propose that BT family proteins assemble into multi-protein complexes to mediate multiple responses to changing environmental and nutritional conditions.