RESUMO
Cytokinin receptors play a key role in cytokinin-dependent processes regulating plant growth, development, and adaptation; therefore, the functional properties of these receptors are of great importance. Previously the properties of cytokinin receptors were investigated in heterologous assay systems using unicellular microorganisms, mainly bacteria, expressing receptor proteins. However, within microorganisms receptors reside in an alien environment that might distort the receptor properties. Therefore, a new assay system has been developed allowing studies of individual receptors within plant membranes (i.e. closer to their natural environment). The main ligand-binding characteristics of receptors from Arabidopsis [AHK2, AHK3, and AHK4] and maize (ZmHK1) were refined in this new system, and the properties of full-length Arabidopsis receptor AHK2 were characterized for the first time. Ligand specificity profiles of receptors towards cytokinin bases were comparable with the profiles retrieved in bacterial assay systems. In contrast, cytokinin-9-ribosides displayed a strongly reduced affinity for receptors in the plant assay system, indicating that ribosides as the common transport form of cytokinins have no or very weak cytokinin activity. This underpins the central role of free bases as the sole biologically active cytokinin compounds. According to molecular modelling and docking studies, N (9)-ribosylation alters the bonding pattern in cytokinin-receptor interaction and prevents ß6-ß7 loop movement important for tight hormone binding. A common feature of all receptors was a greatly reduced ligand binding at low (5.0-5.5) pH. The particularly high sensitivity of ZmHK1 to pH changes leads to the suggestion that some cytokinin receptors may play an additional role as pH sensors in the lumen of the endoplasmic reticulum.
Assuntos
Proteínas de Arabidopsis/metabolismo , Arabidopsis/metabolismo , Citocininas/metabolismo , Modelos Moleculares , Proteínas Quinases/metabolismo , Receptores de Superfície Celular/metabolismo , Arabidopsis/genética , Proteínas de Arabidopsis/genética , Membrana Celular/metabolismo , Retículo Endoplasmático/metabolismo , Expressão Gênica , Histidina Quinase , Concentração de Íons de Hidrogênio , Ligantes , Simulação de Acoplamento Molecular , Proteínas de Plantas/genética , Proteínas de Plantas/metabolismo , Ligação Proteica , Proteínas Quinases/genética , Receptores de Superfície Celular/genética , Transdução de Sinais , Nicotiana/genética , Nicotiana/metabolismo , Zea mays/genética , Zea mays/metabolismoRESUMO
Cytokinins are ubiquitous plant hormones; their signal is perceived by sensor histidine kinases-cytokinin receptors. This review focuses on recent advances on cytokinin receptor structure, in particular sensing module and adjacent domains which play an important role in hormone recognition, signal transduction and receptor subcellular localization. Principles of cytokinin binding site organization and point mutations affecting signaling are discussed. To date, more than 100 putative cytokinin receptor genes from different plant species were revealed due to the total genome sequencing. This allowed us to employ an evolutionary and bioinformatics approaches to clarify some new aspects of receptor structure and function. Non-transmembrane areas adjacent to the ligand-binding CHASE domain were characterized in detail and new conserved protein motifs were recovered. Putative mechanisms for cytokinin-triggered receptor activation were suggested.
Assuntos
Arabidopsis/genética , Citocininas/metabolismo , Reguladores de Crescimento de Plantas/metabolismo , Plantas/genética , Receptores de Superfície Celular/química , Transdução de Sinais , Motivos de Aminoácidos , Sequência de Aminoácidos , Arabidopsis/metabolismo , Biologia Computacional , Evolução Molecular , Modelos Moleculares , Dados de Sequência Molecular , Filogenia , Proteínas de Plantas/química , Proteínas de Plantas/genética , Plantas/metabolismo , Estrutura Terciária de Proteína , Transporte Proteico , Receptores de Superfície Celular/genéticaRESUMO
Bromination of 6-benzylaminopurine (1) with Br(2) in AcOH in the presence of AcONa afforded 6-benzylamino-8-bromopurine (2) in 59% yield. The position of bromination was confirmed by direct transformation of bromide 2 by reaction with NaN(3) in dimethyl sulfoxide to 8-azido-6-benzylaminopurine (3) in a yield of 70% and comparison of its properties with the known compound 2-azido-6-benzylaminopurine (11). Compounds 3 and 11 were checked for their biological activity in specific biotests based on the primary cytokinin effects in living plants. Both synthesized compounds displayed effects similar to the typical cytokinin 6-benzylaminopurine (1).