Figure 1.

Four distinct folds of ABC transporters. All share a similar general architecture: two NBDs (blue and sky blue) are attached to two TMDs (orange and yellow). In some transporters, additional domains are present (green), which often have a regulatory function (C-terminal regulatory domain [CRD]). In Type I and II importers, the transported compounds are delivered to TMDs by SBPs (or SBDs; magenta) located in periplasm (Gram-negative bacteria) or external space (Gram-positive bacteria and Archaea). ECF, energy coupling factor.