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Can independent origins be established for two families of transport systems having no sequence or motif similarities without three-dimensional structural data?


Many integral membrane transport proteins contain 6 or 12 TMSs. For example, both mitochondrial carrier (MC) family members and the aquaporins and glycerol facilitators within the major intrinsic protein (MIP) family contain six TMSs per polypeptide chain. Sequence analyses reveal that MIP family members arose by duplication of a three-TMS-encoding genetic element, while members of the MC family arose by triplication of a two-TMS-encoding element.
The MIP family arose before the three domains of life (bacteria, archaea, and eukaryotes) diverged from each other, whereas the MC family arose late within eukaryotes, after endosymbiotic proteobacteria became permanent denizens of eukaryotic cells. The advent of mitochondria evidently required a new mode of communication between the mitochondrial matrix and the cytoplasmic compartment of such cells. Thus, mitochondrial carriers depend on a distinctive solute. Solute exchange mechanism rather than on the cation symport mechanisms that bacteria use for ingesting nutrients.

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Did integral membrane transport proteins arise as an independent protein class or from other types of proteins?Did bacteria, archaea, and eukaryotes exchange transporter genes appreciably during the past two billion years?