Fig. 1.

Three topological types of ABC exporters, illustrating the types of internal repeats present in each one. In all cases, vertical dashed lines separate the repeat units. a ABC1: a six-TMS topology resulting from intragenic triplication of a primordial two-TMS-encoding genetic element. The three hairpin repeats have the same orientation in the membrane. b ABC2: a six-TMS topology resulting from intragenic duplication of a primordial three-TMS-encoding genetic element. The two three-TMS repeats have opposite orientation in the membrane. c ABC3: an eight-TMS topology resulting from intragenic duplication of a primordial four-TMS-encoding genetic element (Khwaja et al. 2005). The two four-TMS repeats have the same orientation in the membrane