Positive brassinosteroid-signaling protein. Transcription factor that bind specifically to the DNA sequence 5'-CANNTG-3'(E box). Can bind individually to the promoter as a homodimer or synergistically as a heterodimer with BZR2/BES1. Does not itself activate transcription but enhances BZR2/BES1-mediated target gene activation. Homodimer (Probable). Interacts with BZR2/BES1 through both C-terminal and bHLH domains. Interacts also with LHW. Expressed constitutively in roots. Repressed by cold treatment.