Transmembrane subunit (TM) of Periplasmic Binding Protein (PBP)-dependent ATP-Binding Cassette (ABC) transporters which are involved in the uptake of branched-chain amino acids (AAs), as well as TMs of transporters involved in the uptake of monosaccharides including ribose, galactose, and arabinose. These transporters generally bind type 1 PBPs. 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 two ABCs bind and hydrolyze ATP and drive the transport reaction. This group includes Escherichia coli LivM and LivH, two TMs which heterodimerize to form the translocation pathway of the E. coli branched-chain AA LIV-1/LS transporter. This transporter is comprised of two TMs (LivM and LivH), two ABCs (LivG and LivF), and one of two alternative PBPs, LivJ (LIV-BP) and LivK (LS-BP). In addition to transporting branched-chain AAs including leucine, isoleucine and valine, the E. coli LIV-1/LS transporter is involved in the uptake of the aromatic AA, phenylalanine. Included in this group are proteins from transport systems that contain a single TM which homodimerizes to generate the transmembrane pore; for example E. coli RbsC, AlsC, and MglC, the TMs of the high affinity ribose transporter, the D-allose transporter and the galactose transporter, respectively. The D-allose transporter may also to be involved in low affinity ribose transport.
Jiyao W et al.(2023). "The conserved domain database in 2023.",