Smith R L, Banks J L, Snavely M D, Maguire M E
Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106-4965.
J Biol Chem. 1993 Jul 5;268(19):14071-80.
The CorA Mg2+ transport systems of Salmonella typhimurium and Escherichia coli mediate both influx and efflux of Mg2+. The product of the CorA locus is sufficient for mediation of Mg2+ influx while product(s) of the unlinked CorBCD loci allow CorA to mediate efflux in addition to influx. The nucleotide sequences of the S. typhimurium and E. coli CorA loci have been determined. The locus in each species consists of a single gene expressing a protein with gel molecular masses of 42 kDa (S. typhimurium) and 39 kDa (E. coli). The predicted amino acid sequences of these proteins are each 316 amino acids in length, are 98% identical, and lack homology to any known protein. Although CorA is an integral membrane protein by biochemical criteria, its predicted amino acid sequence contains 28% charged amino acid. Membrane localization of CorA was shown to be dependent on the Sec pathway in E. coli. Hydropathy analysis predicts two C-terminal hydrophobic sequences of sufficient length to span the membrane bilayer. The membrane topology of CorA was determined by constructing deletion derivatives of CorA and genetically fusing them to BlaM or LacZ cassettes. The enzymatic activities of these hybrid proteins indicate that the N-terminal 235 amino acid residues of the CorA protein are located within the periplasmic space, comprising a single periplasmic domain. The C-terminal region of CorA is composed of three membrane-spanning segments rather than the two suggested by hydropathy plots, thus depositing the C terminus within the cytoplasm. This topology suggests that CorA functions as an oligomer since three membrane loops are most likely insufficient for any sort of membrane pore or channel. Its lack of homology to known proteins and its topology indicate that the CorA Mg2+ transporter represents a new class of membrane transport system.
鼠伤寒沙门氏菌和大肠杆菌的CorA镁离子转运系统介导镁离子的流入和流出。CorA基因座的产物足以介导镁离子的流入，而不连锁的CorBCD基因座的产物则使CorA除了能介导流入外，还能介导流出。已经确定了鼠伤寒沙门氏菌和大肠杆菌CorA基因座的核苷酸序列。每个物种的基因座都由一个单一基因组成，该基因表达的蛋白质的凝胶分子量分别为42 kDa（鼠伤寒沙门氏菌）和39 kDa（大肠杆菌）。这些蛋白质的预测氨基酸序列长度均为316个氨基酸，同一性为98%，且与任何已知蛋白质均无同源性。尽管根据生化标准CorA是一种整合膜蛋白，但其预测的氨基酸序列含有28%的带电荷氨基酸。在大肠杆菌中，CorA的膜定位被证明依赖于Sec途径。亲水性分析预测了两个C端疏水序列，其长度足以跨越膜双层。通过构建CorA的缺失衍生物并将其与BlaM或LacZ盒进行基因融合，确定了CorA的膜拓扑结构。这些杂合蛋白的酶活性表明，CorA蛋白的N端235个氨基酸残基位于周质空间内，构成一个单一的周质结构域。CorA的C端区域由三个跨膜片段组成，而不是亲水性图谱所显示的两个，因此C端位于细胞质内。这种拓扑结构表明CorA作为一种寡聚体发挥作用，因为三个膜环对于任何类型的膜孔或通道来说可能都不够。它与已知蛋白质缺乏同源性以及其拓扑结构表明，CorA镁离子转运蛋白代表了一类新的膜转运系统。
