A direct docking mechanism for a plant GSK3-like kinase to phosphorylate its substrates.

Glycogen synthase kinase 3 (GSK3) is a highly conserved serine/threonine protein kinase that plays important roles in a variety of physiological and developmental processes in animals. It is well known that the GSK3 kinase-catalyzed protein phosphorylation often requires a stable kinase-substrate docking interaction, which is achieved mainly by two mechanisms as follows: priming phosphorylation of a substrate by a distinct kinase to create a docking phosphate group and scaffold protein-mediated protein complex formation. Brassinosteroid-INsensitive 2 (BIN2) is an Arabidopsis GSK3-like kinase that negatively regulates brassinosteroid (BR) signaling by phosphorylating BES1 (bri1 EMS suppressor 1) and BZR1 (brassinazole-resistant 1), two highly similar transcription factors critical for bringing about characteristic BR responses. However, little is known about the biochemical mechanism by which BIN2 phosphorylates its substrates. Here, we show that BIN2 interacts directly with BZR1 through a 12-amino acid BIN2-docking motif adjacent to the C terminus of BZR1. Interestingly, this 12-amino acid motif is sufficient to allow a Drosophila GSK3 substrate Armadillo to be phosphorylated by BIN2 in vitro. Deletion of this motif inhibits the phosphorylation and subsequent degradation of BZR1 in vivo, resulting in phenotypic suppression of a hypermorphic bin2 mutation and enhanced resistance to a BR biosynthesis inhibitor. We thus concluded that BIN2 utilizes a direct kinase-substrate docking mechanism to phosphorylate BZR1 and regulate its protein stability.