Putative E3 ligases as candidates controlling BRASSINOSTEROID INSENSITIVE 2 (BIN2) kinase in Arabidopsis

The Glycogen synthase kinase 3 (GSK3)-like kinase BRASSINOSTEROID-INSENSITIVE2 (BIN2), a major negative regulator in the Brassinosteroids (BRs) signaling, is involved in a variety of plant signaling pathways by interacting with novel substrates and plays a major role in cellular, growth and developmental regulation. Despite BIN2 functional studies including BR signaling related proteasome-mediated BIN2 degradation, the molecular regulating mechanisms and it's related the protein component for regulating BIN2 degradation has not been completely known. This study aimed to i) identify BIN2 protein and its interacting partner from HA-Immunoprecipitation (IP) of the BL-treated BIN2-HA and bin2-6D-HA lines, using liquid chromatography tandem mass spectrometry (LC-MS/MS) and matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF MS) and ii) characterize relationships between BIN2 and interacting partner proteins. We generated transgenic plants constitutively expressing BIN2-HA, bin2-6D (BIN2(E264K))-HA, and BIN2KD (BIN2(K69R))-HA construct. IP of the HA-tagged bin2-6D/BIN2 protein followed by mass spectrometry identified F-box protein, BRASSINOSTEROID F-BOX 1 (BRF1) and BRF2 containing the LRR and FBD domain, as interacting proteins with BIN2. Validation in vitro by yeast two-hybrid analyses further confirmed the interacting protein. These results, together with phylogeny and sequence alignments of BRF1 and BRF2 homologs multiple methods, suggest that f-box protein BRF1 and BRF2 play redundant or overlapping roles in regulating BIN2. It is likely that the BIN2 protein stability is controlled by BRF1 and BRF2.