Kehres D G, Lawyer C H, Maguire M E
Department of Pharmacology, Case Western Reserve University, Cleveland, Ohio, USA.
Microb Comp Genomics. 1998;3(3):151-69. doi: 10.1089/omi.1.1998.3.151.
The CorA transport system is the primary Mg2+ influx system of Salmonella typhimurium and Escherichia coli. The CorA protein has no homology to any other known family of proteins. It has an unusual membrane topology, with a large, soluble, highly charged periplasmic N-terminal domain with three transmembrane segments in a shorter, hydrophobic C-terminal domain. Previous phenotypic and molecular data had suggested that this transport system was widespread in the Bacteria. In this report we show that CorA is virtually ubiquitous in the Bacteria and Archaea, forming a distinct family of transport proteins. Genomic sequences to date have revealed at least 22 members of the CorA family in the Bacteria and the Archaea, with 6 more distant members in the yeasts. Only three of the smallest bacterial genomes lack a CorA homologue. Strikingly, phylogenetic analysis does not show clustering by related species or even within kingdom. Several species of Bacteria contain two or even three CorA paralogues. Within species, these paralogues are not closely related, however, and we suggest that they might have distinct transport functions. A multiple alignment suggests three extended consensus regions within the N-terminal soluble domain of CorA, which is predicted to be virtually all alpha-helical. A fourth consensus region includes the last 20 residues of the soluble domain and continues through the entire membrane domain. The first half of this last consensus domain may form an amphipathic alpha-helix that extends from the soluble domain into the first transmembrane segment. The degree of charge in the first transmembrane segment is quite variable, and we suggest that this transport family may include members with only two rather than three transmembrane segments. If so, this would place the N-terminal soluble domain on different sides of the membrane in different members of the family. We suggest that the CorA Mg2+ transport system forms the major Mg2+ uptake system in the Bacteria and Archaea but that some family members may have a function other than Mg2+ transport.
CorA转运系统是鼠伤寒沙门氏菌和大肠杆菌主要的Mg2+内流系统。CorA蛋白与其他任何已知的蛋白家族都没有同源性。它具有不寻常的膜拓扑结构，有一个大的、可溶的、带高电荷的周质N端结构域，在较短的疏水C端结构域中有三个跨膜片段。先前的表型和分子数据表明，这种转运系统在细菌中广泛存在。在本报告中，我们表明CorA在细菌和古细菌中几乎无处不在，形成了一个独特的转运蛋白家族。迄今为止的基因组序列显示，细菌和古细菌中至少有22个CorA家族成员，酵母中有6个亲缘关系较远的成员。只有三个最小的细菌基因组缺乏CorA同源物。引人注目的是，系统发育分析并未显示按相关物种甚至在界内进行聚类。几种细菌含有两个甚至三个CorA旁系同源物。然而，在物种内部，这些旁系同源物关系并不密切，我们认为它们可能具有不同的转运功能。多重比对表明，CorA的N端可溶结构域内有三个延伸的共有区域，预计几乎全部为α螺旋。第四个共有区域包括可溶结构域的最后20个残基，并贯穿整个膜结构域。最后一个共有结构域的前半部分可能形成一个两亲性α螺旋，从可溶结构域延伸到第一个跨膜片段。第一个跨膜片段中的电荷程度变化很大，我们认为这个转运家族可能包括只有两个而不是三个跨膜片段的成员。如果是这样，这将使N端可溶结构域在该家族的不同成员中位于膜的不同侧。我们认为，CorA Mg2+转运系统是细菌和古细菌中主要的Mg2+摄取系统，但该家族的一些成员可能具有除Mg2+转运以外的功能。
