title: The cytokinin receptors of Arabidopsis are located mainly to the endoplasmic reticulum.
abstract: The plant hormone cytokinin is perceived by membrane-located sensor histidine kinases. Arabidopsis (Arabidopsis thaliana) possesses three cytokinin receptors: ARABIDOPSIS HISTIDINE KINASE2 (AHK2), AHK3, and CYTOKININ RESPONSE1/AHK4. The current model predicts perception of the cytokinin signal at the plasma membrane. However, cytokinin-binding studies with membrane fractions separated by two-phase partitioning showed that in the wild type, as well as in mutants retaining only single cytokinin receptors, the major part of specific cytokinin binding was associated with endomembranes. Leaf epidermal cells of tobacco (Nicotiana benthamiana) expressing receptor-green fluorescent protein fusion proteins and bimolecular fluorescence complementation analysis showed strong fluorescence of the endoplasmic reticulum (ER) network for all three receptors. Furthermore, separation of the microsomal fraction of Arabidopsis plants expressing Myc-tagged AHK2 and AHK3 receptors by sucrose gradient centrifugation followed by immunoblotting displayed the Mg²⁺-dependent density shift typical of ER membrane proteins. Cytokinin-binding assays, fluorescent fusion proteins, and biochemical fractionation all showed that the large majority of cytokinin receptors are localized to the ER, suggesting a central role of this compartment in cytokinin signaling. A modified model for cytokinin signaling is proposed.
authors.affiliation: Institute of Biology/Applied Genetics, Dahlem Centre of Plant Sciences, Freie Universität Berlin, Berlin D-14195, Germany.
authors.forename: Klaas
authors.initials: K
authors.lastname: Wulfetange
authors.forename: Sergey N
authors.initials: SN
authors.lastname: Lomin
authors.forename: Georgy A
authors.initials: GA
authors.lastname: Romanov
authors.forename: Andrea
authors.initials: A
authors.lastname: Stolz
authors.forename: Alexander
authors.initials: A
authors.lastname: Heyl
authors.forename: Thomas
authors.initials: T
authors.lastname: Schmülling
mesh_terms: D017360:Arabidopsis / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D005591:Chemical Fractionation; D003583:Cytokinins / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005456:Fluorescent Dyes / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D019441:Plant Epidermis / Q000166:cytology / Q000378:metabolism; D018520:Plant Shoots / Q000378:metabolism; D021381:Protein Transport; D011956:Receptors, Cell Surface / Q000378:metabolism*; D036226:Seedlings / Q000378:metabolism; D014026:Nicotiana / Q000166:cytology / Q000378:metabolism
chemical_list: D003583:Cytokinins; D005456:Fluorescent Dyes; D011956:Receptors, Cell Surface