Sequence and topology of the CorA magnesium transport systems of Salmonella typhimurium and Escherichia coli. Identification of a new class of transport protein

Abstract

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.