The Arabidopsis AHK4 histidine kinase is a cytokinin-binding receptor that transduces cytokinin signals across the membrane.
Common histidine-to-aspartate (His-->Asp) phosphorelay is a paradigm of signal transduction in both prokaryotes and eukaryotes for the propagation of certain environmental stimuli, in which histidine (His)-kinases play central roles as sensors for environmental signals. For the higher plant, Arabidopsis thaliana, it was recently suggested that the His-kinase (AHK4 / CRE1 / WOL) is a sensor for cytokinins, which are a class of plant hormones important for the regulation of cell division and differentiation. Interestingly, AHK4 is capable of functioning as a cytokinin sensor in the eubacterium, Escherichia coli (Suzuki et al. 2001, Plant Cell Physiol. 42: 107). Here we further show that AHK4 is a primary receptor that directly binds a variety of natural and synthetic cytokinins (e.g. not only N(6)-substituted aminopurines such as isopentenyl-adenine, trans-zeatin, benzyl-adenine, but also diphenylurea derivatives such as thidiazuron), in a highly specific manner (K(d) = 4.55+/-0.48x10(-9) M). AHK4 has a presumed extracellular domain, within which a single amino acid substitution (Thr-301 to Ile) was shown to result in loss of its ability to bind cytokinins. This particular mutation corresponds to the previously reported wol allele (wooden leg) that causes a striking phenotype defective in vascular morphogenesis. Collectively, evidence is presented that AHK4 and its homologues (AHK3 and possibly AHK2) are receptor kinases that can transduce cytokinin signals across the plasma membrane of A. thaliana.
Yamada
H
H
Laboratory of Molecular Microbiology, School of Agriculture, Nagoya University, Chikusa-ku, Nagoya, 464-8601 Japan.
Suzuki
T
T
Terada
K
K
Takei
K
K
Ishikawa
K
K
Miwa
K
K
Yamashino
T
T
Mizuno
T
T
D000225:Adenine / Q000031:analogs & derivatives* / Q000378:metabolism; D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins; D002454:Cell Differentiation; D002455:Cell Division; D002462:Cell Membrane / Q000378:metabolism; D003583:Cytokinins / Q000378:metabolism*; D000071677:Histidine Kinase; D007541:Isopentenyladenosine / Q000378:metabolism; D008969:Molecular Sequence Data; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction; D014018:Tissue Distribution; D015026:Zeatin / Q000378:metabolism; D001616:beta-Galactosidase / Q000378:metabolism
D029681:Arabidopsis Proteins; D003583:Cytokinins; C001478:N(6)-(delta(2)-isopentenyl)adenine; C079839:3-benzyladenine; D007541:Isopentenyladenosine; D015026:Zeatin; D011494:Protein Kinases; D000071677:Histidine Kinase; C000605492:AHK2 protein, Arabidopsis; C000605493:AHK3 protein, Arabidopsis; D001616:beta-Galactosidase; D000225:Adenine