Department of Biology, University of North Carolina, CB #3280, Chapel Hill, NC 27599-3280, USA.
Plant J. 2010 May;62(3):473-82. doi: 10.1111/j.1365-313X.2010.04165.x. Epub 2010 Feb 3.
Cytokinins are a class of mitogenic plant hormones that play an important role in most aspects of plant development, including shoot and root growth, vascular and photomorphogenic development and leaf senescence. A model for cytokinin perception and signaling has emerged that is similar to bacterial two-component phosphorelays. In this model, binding of cytokinin to the extracellular domain of the Arabidopsis histidine kinase (AHKs) receptors induces autophosphorylation within the intracellular histidine-kinase domain. The phosphoryl group is subsequently transferred to cytosolic Arabidopsis histidine phosphotransfer proteins (AHPs), which have been suggested to translocate to the nucleus in response to cytokinin treatment, where they then transfer the phosphoryl group to nuclear-localized response regulators (Type-A and Type-B ARRs). We examined the effects of cytokinin on AHP subcellular localization in Arabidopsis and, contrary to expectations, the AHPs maintained a constant nuclear/cytosolic distribution following cytokinin treatment. Furthermore, mutation of the conserved phosphoacceptor histidine residue of the AHP, as well as disruption of multiple cytokinin signaling elements, did not affect the subcellular localization of the AHP proteins. Finally, we present data indicating that AHPs maintain a nuclear/cytosolic distribution by balancing active transport into and out of the nucleus. Our findings suggest that the current models indicating relocalization of AHP protein into the nucleus in response to cytokinin are incorrect. Rather, AHPs actively maintain a consistent nuclear/cytosolic distribution regardless of the status of the cytokinin response pathway.
细胞分裂素是一类促分裂植物激素，在植物发育的大多数方面都起着重要作用，包括芽和根的生长、维管和光形态建成发育以及叶片衰老。细胞分裂素感知和信号转导的模型已经出现，类似于细菌双组分磷酸传递。在这个模型中，细胞分裂素与拟南芥组氨酸激酶（AHKs）受体的细胞外结构域结合，诱导细胞内组氨酸激酶结构域内的自磷酸化。磷酸基团随后被转移到细胞质拟南芥组氨酸磷酸转移蛋白（AHPs）上，据推测，这些蛋白在细胞分裂素处理后会向核内转移，然后将磷酸基团转移到核内定位的应答调节因子（A型和 B 型 ARRs）上。我们研究了细胞分裂素对拟南芥中 AHP 亚细胞定位的影响，与预期相反，在细胞分裂素处理后，AHPs 保持恒定的核/细胞质分布。此外，突变 AHP 中保守的磷酸受体组氨酸残基以及破坏多个细胞分裂素信号元件，都不会影响 AHP 蛋白的亚细胞定位。最后，我们提供的数据表明，AHPs 通过平衡主动运输进出细胞核来保持核/细胞质分布。我们的发现表明，当前表明 AHP 蛋白在细胞分裂素响应时重新定位到核内的模型是不正确的。相反，AHPs 主动维持一致的核/细胞质分布，而不管细胞分裂素反应途径的状态如何。
