Last week’s issue of Science had an interesting paper and commentary on biochemical evolution. The study, by Rapp et al. (2007), elucidates yet another plausible evolutionary pathway for an oddity of the cell – in short, the orientation of an unusual bacterial membrane protein suggests how the particular topology of other membrane proteins may have evolved.
How do integral membrane proteins evolve in size and complexity? Using the small multidrug-resistance protein EmrE from Escherichia coli as a model, we experimentally demonstrated that the evolution of membrane proteins composed of two homologous but oppositely oriented domains can occur in a small number of steps: An original dual-topology protein evolves, through a gene-duplication event, to a heterodimer formed by two oppositely oriented monomers. This simple evolutionary pathway can explain the frequent occurrence of membrane proteins with an internal pseudo–two-fold symmetry axis in the plane of the membrane.