Maeda Shin-Ichi, Sugita Chieko, Sugita Mamoru, Omata Tatsuo
Laboratory of Molecular Plant Physiology, Graduate School of Bioagricultural Sciences Nagoya University, Furocho, Chikusaku, Nagoya 464-8601, Japan
J Biol Chem. 2006 Dec 8;281(49):37868-76. doi: 10.1074/jbc.M608680200. Epub 2006 Oct 12.
Nitrate transport activity of the LtnT permease of the cyanobacterium Synechococcus elongatus is activated when LtnA, a response regulator without an effector domain, is phosphorylated by LtnB, a hybrid histidine kinase. We identified a protein (LtnC) that is required for activation of LtnT. LtnC consists of an N-terminal histidine-containing phosphoacceptor (HisKA) domain, a receiver domain, and a unique C-terminal domain found in some cyanobacterial proteins. Because LtnC lacks an ATP-binding kinase domain of a histidine kinase, it is incapable of autophosphorylation, but LtnC is phosphorylated by LtnA. The histidine residue in the HisKA domain but not the aspartate residue in the receiver domain is essential for phosphorylation of LtnC and activation of LtnT. LtnC phosphorylation leads to oligomerization of the protein. Fusion of the C-terminal domain of LtnC to glutathione S-transferase, which forms oligomers, also activates LtnT, suggesting that oligomerization of the LtnC C-terminal domain causes LtnT activation. These results indicate that the C-terminal domain of LtnC acts as an effector domain that directs the output of the signal from the phosphorelay system. The two-step (His-Asp-His) phosphorelay system, composed of the LtnB, LtnA, and LtnC proteins, is distinct from the known phosphorelay systems, namely, the typical two-component system (His-Asp) and the multistep phosphorelay system (His-Asp-His-Asp), because the HisKA domain of LtnC is the terminal phosphoacceptor that determines the signal output. LtnC is a new class of signal transducer in His-Asp phosphorelay systems that contains a HisKA domain and an effector domain.
当细长聚球藻的LtnT通透酶的硝酸盐转运活性被LtnB（一种杂合组氨酸激酶）磷酸化时，无效应结构域的响应调节蛋白LtnA会被激活。我们鉴定出一种LtnT激活所需的蛋白质（LtnC）。LtnC由一个含N端组氨酸的磷酸受体（HisKA）结构域、一个接收结构域和一些蓝细菌蛋白中发现的独特C端结构域组成。由于LtnC缺乏组氨酸激酶的ATP结合激酶结构域，它不能进行自磷酸化，但LtnC可被LtnA磷酸化。HisKA结构域中的组氨酸残基而非接收结构域中的天冬氨酸残基对于LtnC的磷酸化和LtnT的激活至关重要。LtnC磷酸化导致该蛋白寡聚化。将LtnC的C端结构域与形成寡聚体的谷胱甘肽S-转移酶融合也能激活LtnT，这表明LtnC C端结构域的寡聚化会导致LtnT激活。这些结果表明，LtnC的C端结构域作为效应结构域，指导来自磷酸转移系统的信号输出。由LtnB、LtnA和LtnC蛋白组成的两步（His-Asp-His）磷酸转移系统不同于已知的磷酸转移系统，即典型的双组分系统（His-Asp）和多步磷酸转移系统（His-Asp-His-Asp），因为LtnC的HisKA结构域是决定信号输出的末端磷酸受体。LtnC是His-Asp磷酸转移系统中的一类新型信号转导蛋白，含有一个HisKA结构域和一个效应结构域。
