3.A.1.12.17
OpuFB of 505 aas with 6 or 7 N-terminal TMSs in a 3 + 3 or 4 TMS arrangement.  It is an osmoprotectant ABC transporter/substrate-binding subunit OpuFB.  The B. subtilis ortholog a compatible solute ABC transporter with a substrate-binding protein fused to the transmembrane domain (Teichmann et al. 2018).  There are five transport systems (OpuA, OpuB, OpuC, OpuD, and OpuE) for compatible solutes in B. subtilis (Teichmann et al. 2018). The new system is called OpuF (OpuFA-OpuFB). OpuF is not present in B. subtilis but is widely distributed in members of the genus Bacillus. OpuF mediates the import of glycine betaine, proline betaine, homobetaine, and the marine osmolyte dimethylsulfoniopropionate (DMSP).  It has an aromatic cage, a structural feature commonly present in ligand-binding sites of compatible solute importers (Teichmann et al. 2018).