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 antisense 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.