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In Arabidopsis thaliana brassinosteroid (BR), perception is mediated by two Leu-rich repeat receptor-like kinases, BRASSlNOSTEROID INSENSITIVE1 (BRI1) and BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1) (Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR-like KINASE3 [AtSERK3]). Genetic, biochemical, and yeast (Saccharomyces cerevisiae) interaction studies suggested that the BRI1-BAK1 receptor complex initiates BR signaling, but the role of the BAK1 receptor is still not clear. Using transient expression in protoplasts of BRI1 and AtSERK3 fused to cyan and yellow fluorescent green fluorescent protein variants allowed us to localize each receptor independently in vivo. We show that BRI1, but not AtSERK3, homodimerizes in the plasma membrane, whereas BRI1 and AtSERK3 preferentially heterodimerize in the endosomes. Coexpression of BRI1 and AtSERK3 results in a change of the steady state distribution of both receptors because of accelerated endocytosis. Endocytic vesicles contain either BRI1 or AtSERK3 alone or both. We propose that the AtSERK3 protein is involved in changing the equilibrium between plasma membrane-located BRI1 homodimers and endocytosed BRI1-AtSERK3 heterodimers. [ABSTRACT FROM AUTHOR]