The Bromodomain Proteins GTE9 and GTE11 Associate with BT2-based E3 Ligase Complex and Mediate Responses to Multiple Signals in Arabidopsis thaliana.

Abstract

BT2 is an Arabidopsis thaliana protein with N-terminal BTB, central TAZ and a C-terminal calmodulin binding domain and associates with Cullin3 to form an E3 ubiquitin ligase. We have shown previously that BT2 regulates telomerase activity in mature vegetative organs and controls a variety of hormone, stress and metabolic responses in Arabidopsis thaliana. Loss of BT2 results in plants that are hypersensitive to inhibition of germination by ABA and sugars. Conversely, overexpression of BT2 results in resistance to ABA and sugars, suggesting that BT2 is a negative regulator of ABA and sugar responses. Here, we report the roles of BT2-interacting partners GTE9 and GTE11, bromodomain and extraterminal-domain proteins of Global Transcription Factor Group E, in BT2-mediated responses to sugars and hormones. Loss-of-function mutants gte9-1 and gte11-1 phenocopy the bt2-1-null mutant responses; germination in all three mutants is hypersensitive to inhibition by glucose and ABA. Loss of either GTE9 or GTE11 in a BT2 over-expressing background blocks resistance to sugars and ABA, indicating that both GTE9 and GTE11 are required for BT2 function. Additionally, loss of GTE9 or GTE11, similar to loss of BT2, suppresses transcriptional gene activation mediated by CaMV 35S enhancers in Arabidopsis. The suppressed phenotype was accompanied by decreased transcription and hypermethylation of the 35S enhancers in the activation-tagged lines. We showed that BT2 and GTE9 co-immunoprecipitate and physically interact in vivo to mediate diverse responses to biotic and abiotic signals and 35S enhancer activity. Our working model is that the GTE9 and GTE11 function as chromatin adaptors that localize the BT2-CULLIN3 E3 ubiquitin ligase complex to acetylated chromatin on transcriptionally competent promoters in response to calcium signals detected by BT2?s calmodulin-binding domain.