J Membr Biol. 2013 Dec;246(12):915-26. doi: 10.1007/s00232-013-9593-0.
Mammalian glycosylated rhesus (Rh) proteins include the erythroid RhAG and the nonerythroid RhBG and RhCG. RhBG and RhCG are expressed in multiple tissues, including hepatocytes and the collecting duct (CD) of the kidney. Here, we expressed human RhAG, RhBG and RhCG in Xenopus oocytes (vs. H2O-injected control oocytes) and used microelectrodes to monitor the maximum transient change in surface pH (DpHS) caused by exposing the same oocyte to 5 % CO₂/33 mM HCO₃⁻ (an increase) or 0.5 mM NH₃/NH₄⁺ (a decrease). Subtracting the respective values for day-matched, H₂O-injected control oocytes yielded channel-specific values (). (ΔpH(S))(CO₂) and (-ΔpH*(S))(NH₃) were each significantly >0 for all channels, indicating that RhBG and RhCG--like RhAG--can carry CO₂ and NH₃. We also investigated the role of a conserved aspartate residue, which was reported to inhibit NH₃ transport. However, surface biotinylation experiments indicate the mutants RhBG(D178N) and RhCG(D177N) have at most a very low abundance in the oocyte plasma membrane. We demonstrate for the first time that RhBG and RhCG--like RhAG--have significant CO₂ permeability, and we confirm that RhAG, RhBG and RhCG all have significant NH₃ permeability. However, as evidenced by (ΔpH*(S))(CO₂)/ (-ΔpH*(S))(NH₃) values, we could not distinguish among the CO₂/ NH₃ permeability ratios for RhAG, RhBG and RhCG. Finally, we propose a mechanism whereby RhBG and RhCG contribute to acid secretion in the CD by enhancing the transport of not only NH₃ but also CO₂ across the membranes of CD cells.
哺乳动物糖基化的恒河猴 (Rh) 蛋白包括红系 RhAG 和非红系 RhBG 和 RhCG。RhBG 和 RhCG 在多种组织中表达，包括肝细胞和肾脏的集合管 (CD)。在这里，我们在非洲爪蟾卵母细胞中表达了人 RhAG、RhBG 和 RhCG（与注射 H2O 的对照卵母细胞相比），并使用微电极监测将相同卵母细胞暴露于 5% CO₂/33 mM HCO₃⁻（增加）或 0.5 mM NH₃/NH₄⁺（减少）时引起的表面 pH 最大瞬时变化 (DpHS)。从与天匹配的、注射 H2O 的对照卵母细胞中减去各自的值得到通道特异性值 ()。(ΔpH(S))(CO₂) 和 (-ΔpH*(S))(NH₃) 对于所有通道均显著大于 0，表明 RhBG 和 RhCG-与 RhAG 一样-可以携带 CO₂ 和 NH₃。我们还研究了一个保守天冬氨酸残基的作用，据报道该残基抑制 NH₃ 转运。然而，表面生物素化实验表明，突变体 RhBG(D178N) 和 RhCG(D177N) 在卵母细胞膜中最多只有非常低的丰度。我们首次证明 RhBG 和 RhCG-与 RhAG 一样-具有显著的 CO₂ 通透性，并且我们证实 RhAG、RhBG 和 RhCG 均具有显著的 NH₃ 通透性。然而，正如 (ΔpH*(S))(CO₂)/(-ΔpH*(S))(NH₃) 值所示，我们无法区分 RhAG、RhBG 和 RhCG 的 CO₂/NH₃ 通透性比值。最后，我们提出了一种机制，即 RhBG 和 RhCG 通过增强不仅 NH₃ 而且 CO₂ 穿过 CD 细胞的膜的转运，从而有助于 CD 中的酸分泌。
