Smith D L, Maguire M E
Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106-4965.
Miner Electrolyte Metab. 1993;19(4-5):266-76.
The gram-negative bacterium Salmonella typhimurium possesses three distinct Mg2+ transport systems, encoded by the CorA, MgtA, and MgtB loci. The CorA transport system is the constitutive Mg2+ influx system but can also mediate efflux at high extracellular Mg2+ concentrations. The CorA protein lacks homology to any known protein, is an integral membrane protein containing 28% percent-charged amino acids, and has three carboxyl terminal membrane-spanning segments. Its properties indicate that it is a new class of membrane transport protein, likely found in all gram-negative bacteria and possibly other organisms. In contrast, the MgtA and MgtB Mg2+ transport systems are normally expressed only at low extracellular Mg2+ concentrations and can mediate only the influx of Mg2+. Both MgtA and MgtB system are P-type ATPases; they have relatively poor homology to other known prokaryotic P-type ATPases but are highly homologous to mammalian P-type ATPases, particularly reticular Ca(2+)-ATPases. Expression of both MgtA and MgtB is highly regulated by the concentration of extracellular Mg2+. Transcription of MgtB is increased about 1,000-fold by lowering Mg2+ from 1 mM to 1 microM, and, under growth conditions of limiting Mg2+, MgtB becomes the dominant Mg2+ influx system. However, it is unclear why the cells require the use of ATP to mediate the influx of Mg2+ down its electrochemical gradient. Study of these Mg2+ transport systems should lead to further understanding of cellular Mg2+ homeostasis and eventually to characterization of eukaryotic Mg2+ transport systems.
革兰氏阴性菌鼠伤寒沙门氏菌拥有三种不同的镁离子转运系统，分别由CorA、MgtA和MgtB基因座编码。CorA转运系统是组成型镁离子内流系统，但在高细胞外镁离子浓度下也能介导外流。CorA蛋白与任何已知蛋白均无同源性，是一种整合膜蛋白，含有28%的带电荷氨基酸，并有三个羧基末端跨膜片段。其特性表明它是一类新的膜转运蛋白，可能存在于所有革兰氏阴性菌以及其他可能的生物体中。相比之下，MgtA和MgtB镁离子转运系统通常仅在低细胞外镁离子浓度下表达，且只能介导镁离子的内流。MgtA和MgtB系统都是P型ATP酶；它们与其他已知的原核P型ATP酶的同源性相对较差，但与哺乳动物P型ATP酶高度同源，尤其是网状Ca(2+)-ATP酶。MgtA和MgtB的表达都受到细胞外镁离子浓度的高度调节。将镁离子浓度从1 mM降至1 microM可使MgtB的转录增加约1000倍，并且在镁离子限制的生长条件下，MgtB成为主要的镁离子内流系统。然而，尚不清楚细胞为何需要利用ATP来介导镁离子沿其电化学梯度的内流。对这些镁离子转运系统的研究应能进一步了解细胞内镁离子的稳态，并最终实现对真核镁离子转运系统的表征。
