The internal repeats in the Na+/Ca2+ exchanger-related Escherichia coli protein YrbG have opposite membrane topologies.

We have determined the topology of the Escherichia coli inner membrane protein YrbG, a putative Na(+)/Ca(2+) exchanger with homology to a family of eukaryotic ion exchangers. Our results show that the two homologous halves of YrbG both have five transmembrane segments but opposite membrane orientations. This has implications for our understanding of the function of Na(+)/Ca(2+) exchangers and provides an example of "divergent" evolution of membrane protein topology.