Figure 5.

Topology, structure, and conformations of distinct states of type II ABC importers. (a) The topology of the BtuC subunit (TMD) is shown, with TM helices numbered and critical regions indicated. (b) Ribbon diagram of the BtuCD-F complex probably reflecting a post-translocation intermediate. (c) Comparison of the conformations of the TM helices 5 and 5a in distinct crystal structures. The conformation of these helices in BtuCD-F is a hybrid between those of BtuCD (outward facing) and those of HIF (inward facing).