Transmembrane subunit (TM), of Periplasmic Binding Protein (PBP)-dependent ATP-Binding Cassette (ABC) transporters involved in the uptake of siderophores, heme, vitamin B12, or the divalent cations Mg2+ and Zn2+. PBP-dependent ABC transporters consist of a PBP, two TMs, and two cytoplasmic ABCs, and are mainly involved in importing solutes from the environment. The solute is captured by the PBP which delivers it to a gated translocation pathway formed by the two TMs. The TMs are bundles of alpha helices that transverse the cytoplasmic membrane multiple times. The two ABCs bind and hydrolyze ATP and drive the transport reaction. Each TM has a prominent cytoplasmic loop which contacts an ABC and represents a conserved motif. The two TMs form either a homodimer (e.g. in the case of the BtuC subunits of the Escherichia coli BtuCD vitamin B12 transporter), a heterodimer (e.g. the TroC and TroD subunits of the Treponema pallidum general transition metal transporter, TroBCD), or a pseudo-heterodimer (e.g. the FhuB protein of the E. coli ferrichrome transporter, FhuBC). FhuB contains two tandem TMs which associate to form the pseudo-heterodimer. Both FhuB TMs are found in this hierarchy.