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 Mg 2+ -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.

Members of a recently discovered subfamily of cytokinin receptors display differences and similarities to their classical counterparts

“…This subcellular localization is similar to the one described for classical cytokinin receptors previously. [15][16][17] In summary, the additional data presented here corroborate the previously published analyses and thus further emphasize the role of the members of the newly described subfamily as functional cytokinin receptors. 9 However, their biological role is currently unclear.…”

Cytokinins - recent news and views of evolutionally old molecules

“…Three very recent studies reported the localisation of the Arabidopsis and maize CK receptors in the ER (Fig. 4; Wulfetange et al 2011;Lomin et al 2011;Caesar et al 2011). Although the general concept of CK signalling assumed that CK receptors are located in the plasma membrane (Kim et al 2006) and sense the extracellular hormone (Inoue et al 2001;Ueguchi et al 2001a), data on the pH dependence of CK binding by AHK3 and CRE1/AHK4 support the function of the receptors inside the plant cell (Romanov et al 2006).…”

“…Model of CK signalling and cellular distribution of biosynthetic CKs (AtITPs, in green rectangles) and degradation CKs (AtCKXs, in red rectangles) enzymes in Arabidopsis thaliana. CK signalling involves AHK receptors (AHK2, AHK3 and CRE1/AHK4) localised in the plasma membrane (Kim et al 2006) and the endoplasmatic reticulum (ER; Lomin et al 2011;Wulfetange et al 2011;Caesar et al 2011). CKI1 is involved in CK signalling but does not perceive CKs (Hejátko et al 2009;Yamada et al 2001).…”

Crossing paths: cytokinin signalling and crosstalk

“…The receptors show differing sensitivity to different cytokinin species, suggesting there may be some specificity in the signalling pathway (Spíchal et al, 2004;Romanov et al, 2006). Recent research has found that a large majority of cytokinin receptors are present on the ER membrane, suggesting that this compartment may play an important role in cytokinin signal transduction (Caesar et al, 2011;Wulfetange et al, 2011). For an overview of our current understanding of cytokinin receptors, the reader is referred to a recent review by Heyl et al .…”