The tighter the binding of a ligand to its receptor,the lower the concentration of ligand needed to achieve binding to 50 percent of the receptors.

TLS Online TPP Program

#Id: 10228

#Unit 6. System Physiology – Plant

brassinolide binds to brassinosteroid receptor kinase BRASSINOSTEROID-INSENSITIVE1 (BRI1) on the plasma membrane triggers a phosphorylation cascade that causes a repressor protein, BRASSINOSTEROID-INSENSITIVE2 (BIN2), to become inactivated.

TLS Online TPP Program

#Id: 10229

#Unit 6. System Physiology – Plant

TLS Online TPP Program

#Id: 10230

#Unit 6. System Physiology – Plant

Upon binding brassinolide, homodimers of BRI1 are activated and heterooligomerize with the RLK BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1)

TLS Online TPP Program

#Id: 10231

#Unit 6. System Physiology – Plant

BIN2 is a serine/threonine protein kinase that, in the absence of brassinolide, negatively regulates the closely related transcription factors BES1 and BZR1 by phosphorylation.

TLS Online TPP Program

#Id: 10232

#Unit 6. System Physiology – Plant

In the presence of brassinolide, the activated phosphatase BSU1 dephosphorylates BIN2 and promotes its degradation by the 26S proteasome system, thus blocking its activity. BES1 and BZR1

are then dephosphorylated by PROTEIN PHOSPHATASE2A

(PP2A), and the active forms of BES1 and BZR1 move into the nucleus where they regulate the expression of brassinolide response genes

TLS Online TPP Program

#Id: 10233

#Unit 6. System Physiology – Plant

BES1 & BZR1 give BR response

Phosphorylated BIN2 inhibits BES1 & BZR1 

Phosphorylated BIN2 is degraded by BSU1 

BSK proteins activate BSU1 phosphatase.

BRI1–BAK1-BR complex activates BSK