Evidence has been presented that 5+5 TMS and 7+7 TMS inverted repeat fold transporters are members of a single superfamily named the Amino acid-Polyamine-organoCation (APC) superfamily. However, the evolutionary relationship between the 5+5 and the 7+7 topological types has not been established. We have identified a common fold, consisting of a spiny membrane helix/sheet, followed by a U-like structure and a V-like structure that is recurrent between domain duplicated units of 5+5 and 7+7 inverted repeat folds. This fold is found in the following protein structures: AdiC, ApcT, LeuT, Mhp1, BetP, CaiT, and SglT (all 5+5 TMS repeats), as well as UraA and SulP (7+7 TMS repeats). AdiC, LeuT and Mhp1 have two extra TMSs after the second duplicated domain, SglT has four extra C-terminal TMSs, and BetP has two extra TMSs before the first duplicated domain. UraA and SulP on the other hand have two extra TMSs at the N-terminus of each duplicated domain unit. These observations imply that multiple hairpin and domain duplication events occurred during the evolution of the APC superfamily. We suggest that the five TMS architecture was primordial and that families gained two TMSs on either side of this basic structure via dissimilar hairpin duplications either before or after intragenic duplication. Evidence for homology between TMSs 1-2 of AdiC and TMSs 1-2 and 3-4 of UraA suggests that the 7+7 topology arose via an internal duplication of the N-terminal hairpin loop within the five TMS repeat unit followed by duplication of the 7 TMS domain.
Keywords: AdiC; UraA; amino acid-polyamine-organocation; APC; duplication; hairpin; superfamily.
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