Allele-specific suppression of a defective brassinosteroid receptor reveals a physiological role of UGGT in ER quality control

Regulation of the Arabidopsis GSK3-like kinase BRASSINOSTEROID-INSENSITIVE 2 through proteasome-mediated protein degradation

Glycogen synthase kinase 3 (GSK3) is a unique serine/threonine kinase that is implicated in a variety of cellular processes and is regulated by phosphorylation or protein-protein interaction in animal cells. BIN2 is an Arabidopsis GSK3-like kinase that negatively regulates brassinosteroid (BR) signaling. Genetic studies suggested that BIN2 is inhibited in response to BR perception at the cell surface to relieve its inhibitory effects on downstream targets; however, little is known about biochemical mechanisms of its inhibition. Here, we show that BIN2 is regulated by proteasome-mediated protein degradation. Exogenous application of a BR biosynthesis inhibitor and an active BR increased and decreased the amount of BIN2 proteins, respectively. Interestingly, the gain-of-function bin2-1 mutation significantly stabilizes BIN2, making it unresponsive to BR-induced BIN2 depletion. Exogenous application of different plant growth hormones revealed that BIN2 depletion is specifically induced by BR through a functional BR receptor, while treatment of a proteasome inhibitor, MG132, not only prevented the BR-induced BIN2 depletion but also nullified the inhibitory effect of BR on the BIN2 kinase activity. Taken together, our results strongly suggest that proteasome-mediated protein degradation constitutes an important regulatory mechanism for restricting the BIN2 activity.

Multiple mechanisms modulate brassinosteroid signaling

Brassinosteroids are essential hormones for plant growth and development. Genetic studies have identified key components of the BR signaling pathway, including the cell-surface receptor kinases that perceive BR, an intracellular kinase and a phosphatase, and nuclear transcription factors. Subsequent biochemical studies have revealed many details about signaling events from BR perception at the cell surface to gene expression in the nucleus. Recent studies have identified the 14-3-3 proteins as BR signaling components and elucidated multiple mechanisms by which phosphorylation modulates the BR transcription factors. In addition, BRI1 signaling from the endosomes and BR-independent functions of BAK1 have been observed. However, a major gap still exists in the current BR signaling pathway between the receptor complex and downstream signaling events.

How sugars convey information on protein conformation in the endoplasmic reticulum

The N-glycan-dependent quality control of glycoprotein folding prevents endoplasmic reticulum to Golgi exit of folding intermediates, irreparably misfolded glycoproteins and not completely assembled multimeric complexes. It also enhances folding efficiency by preventing aggregation and facilitating formation of proper disulfide bonds. The control mechanism essentially involves four components, resident lectin-chaperones that recognize monoglucosylated polymannose glycans, a lectin-associated oxidoreductase acting on monoglucosylated glycoproteins, a glucosyltransferase and a glucosidase that creates monoglucosylated epitopes in glycans transferred in protein N-glycosylation or removes the glucose units added by the glucosyltransferase. This last enzyme is the only mechanism component sensing glycoprotein conformations as it creates monoglucosylated glycans exclusively in not properly folded species or in not completely assembled complexes. The purpose of the review is to describe the most significant recent findings on the mechanism of glycoprotein folding and assembly quality control and to discuss the main still unanswered questions.