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Title: INTERACTION OF ARABIDOPSIS BRASSINOSTEROID-INSENSITIVE BETA RECEPTOR KINASE WITH A HOMOLOG OF MAMMALIAN TGF RECEPTOR INTERACTING PROTEIN.

Author
item EHSAN, H - NC STATE UNIVERSITY
item RAY, W - MI STATE UNIVERISTY
item PHINNEY, B - MI STATE UNIVERSITY
item WANG, X - NC STATE UNIVERSITY
item Huber, Steven
item CLOUSE, S - NC STATE UNIVERSITY

Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/28/2005
Publication Date: 7/1/2005
Citation: Ehsan, H., Ray, W.K., Phinney, B., Wang, X., Huber, S.C., Clouse, S.D. 2005. Interaction of Arabidopsis BRASSINOSTEROID-INSENSITIVE 1 receptor kinase with a homolog of mammalian TGF receptor interacting protein. Plant Journal. 43:251-261.

Interpretive Summary: Brassinosteroids (BRs) are important plant growth hormones that are required for many aspects of normal plant growth and development, including cell elongation, vascular differentiation, seed germination, and senescence. Current thinking is that BR perception occurs when it binds to a ‘receptor-like kinase’ known as BRI1, a protein in the plasma membrane that has an extracellular domain that interacts with the hormone and an intracellular protein kinase domain that transmits the hormone signal. Several downstream components of BR signaling have been identified by genetic studies, but a serious gap in knowledge remains identification of cellular proteins that might be direct substrates of the BRI1 kinase. In the present study, a protein known as TRIP-1 is shown to be phosphorylated at three specific sites by BRI1. Moreover, TRIP-1 was shown to physically interact with the BRI1 receptor-like kinase both in vitro and in vivo, strongly suggesting that TRIP-1 may be the first identified cytoplasmic substrate for this receptor kinase and an early step in BR signaling. Because TRIP-1 is known to play a role in initiation of translation, phosphorylation of this protein could be an important early event in BR signaling. Identifying signal transduction components and understanding interactions among components is necessary in order to elucidate, and perhaps one day manipulate, BR-regulated growth and development.

Technical Abstract: Brassinosteroids (BRs) regulate multiple aspects of plant growth and development and require an active Brassinosteroid Insensitive 1 (BRI1) receptor serine/threonine kinase for hormone perception and signal transduction. In mammals, the transforming growth factor-beta (TGF - beta) family of polypeptides modulate numerous aspects of development and are perceived at the cell surface by a complex of Type I and Type II TGF - beta receptor serine/threonine kinases. TGF - beta Receptor Interacting Protein (TRIP-1) is a cytoplasmic substrate of the TGF - beta Type II receptor kinase and plays a role in TGF - beta signaling. TRIP-1 is a WD domain protein that also functions as an essential subunit of the eIF3 eukaryotic translation initiation factor in animals, yeast and plants. We previously cloned putative TRIP-1 homologs from bean and Arabidopsis and found that transgenic Arabidopsis plants expressing anti-sense TRIP-1 RNA exhibited a broad range of developmental defects including some morphological characteristics that resemble the phenotype of BR-deficient and –insensitive mutants. We now show that the BRI1 kinase domain phosphorylates Arabidopsis TRIP-1 on three specific sites in vitro (Thr-14, Thr-89 and either Thr-197 or Ser-198). Co-immunoprecipitation experiments using antibodies against TRIP-1, BRI1 and various fusion proteins strongly suggest that TRIP-1 and BRI1 also interact directly in vivo. These findings support a role for TRIP-1 in the molecular mechanisms of BR-regulated plant growth and development, possibly as a cytoplasmic substrate of the BRI1 receptor kinase.