Department of Physiology, David Geffen School of Medicine at UCLA, 405 Hilgard Ave., Los Angeles, California 90024, USA.
J Bacteriol. 2010 Jan;192(1):94-103. doi: 10.1128/JB.00848-09.
Helicobacter pylori colonizes the normal human stomach by maintaining both periplasmic and cytoplasmic pH close to neutral in the presence of gastric acidity. Urease activity, urea flux through the pH-gated urea channel, UreI, and periplasmic alpha-carbonic anhydrase are essential for colonization. Exposure to pH 4.5 for up to 180 min activates total bacterial urease threefold. Within 30 min at pH 4.5, the urease structural subunits, UreA and UreB, and the Ni(2+) insertion protein, UreE, are recruited to UreI at the inner membrane. Formation of this complex and urease activation depend on expression of the cytoplasmic sensor histidine kinase, HP0244. Its deletion abolishes urease activation and assembly, impairs cytoplasmic and periplasmic pH homeostasis, and depolarizes the cells, with an approximately 7-log loss of survival at pH 2.5, even in 10 mM urea. Associated with this assembly, UreI is able to transport NH(3), NH(4)(+), and CO(2), as shown by changes in cytoplasmic pH following exposure to NH(4)Cl or CO(2). To be able to colonize cells in the presence of the highly variable pH of the stomach, the organism expresses two pH-sensor histidine kinases, one, HP0165, responding to a moderate fall in periplasmic pH and the other, HP0244, responding to cytoplasmic acidification at a more acidic medium pH. Assembly of a pH-regulatory complex of active urease with UreI provides an advantage for periplasmic buffering.
幽门螺杆菌通过在胃酸存在的情况下将周质和细胞质的 pH 值维持在接近中性的水平来定植于正常的人类胃中。脲酶活性、尿素通过 pH 门控尿素通道 UreI 的通量以及周质α-碳酸酐酶对于定植是必需的。暴露于 pH 4.5 长达 180 分钟可使总细菌脲酶激活三倍。在 pH 4.5 下 30 分钟内，脲酶结构亚基 UreA 和 UreB 以及 Ni(2+)插入蛋白 UreE 被募集到内膜上的 UreI。该复合物的形成和脲酶的激活依赖于细胞质传感器组氨酸激酶 HP0244 的表达。其缺失会消除脲酶的激活和组装，损害细胞质和周质的 pH 稳态，并使细胞去极化，在 pH 2.5 下的存活率约降低 7 个对数级，即使在 10 mM 尿素中也是如此。与这种组装相关，UreI 能够转运 NH(3)、NH(4)(+)和 CO(2)，如暴露于 NH(4)Cl 或 CO(2)后细胞质 pH 的变化所示。为了能够在胃中高度变化的 pH 存在下定植于细胞，该生物体表达了两种 pH 传感器组氨酸激酶，一种是 HP0165，对周质 pH 的适度下降作出反应，另一种是 HP0244，对更酸性介质 pH 下的细胞质酸化作出反应。具有活性脲酶的 pH 调节复合物与 UreI 的组装为周质缓冲提供了优势。
