Membrane proteins commonly evolve in size and complexity by gene duplication. A particular intriguing mode of gene-duplication based evolution is when a so-called dual topology protein undergoes duplication to yield two oppositely orientated, homologous proteins. These two proteins may even fuse into a single molecule with an approximate internal symmetry in its 3D structure. We have focused on the small multidrug-transporter family of proteins that provides many examples of dual topology, oppositely orientated, and fused, internally symmetric proteins.
Daley, D.O., Rapp, M., Granseth, E., Melén, K., Drew, D., and von Heijne, G. (2005) Global topology analysis of the Escherichia coli inner membrane proteome. Science 308, 1321-1323.
Rapp, M., Seppälä, S., Granseth, E., and von Heijne, G. (2006) Identification and evolution of dual topology membrane proteins. Nature Struct.Mol.Biol. 13, 112-116.
Rapp, M., Susanna Seppälä, S., Granseth, E., and von Heijne, G. (2007) Emulating membrane protein evolution by rational design. Science 315, 1282-1284.