Currently, it is premature to speculate on metal import mechanism of the SsTroABCD transporter, but it is nevertheless very tempting to hypothesize a scenario (Fig. 7) . BtuF is a periplasmic binding protein (PBP) for the vitamin B12 transporter BtuCD. BtuF and TroA are classified in the same class of SBPs, which both exhibit very little domain movement upon ligand binding [20] . The complex structure of BtuCD-BtuF revealed that the conserved glutamate residues Glu 74 and Glu 202 from BtuF form salt bridges with Arg56 residues from two BtuC subunits [55] , which are also found in other ABC importers [56] . Moreover, these salt bridges are critical for molecular interaction in vivo [56] . Intriguingly, we found that SsTroA also has two conserved glutamate residues Glu 107 and Glu 230 that are positioned on the surface of the N-terminal and C-terminal domains (Fig. 5A) . Based on the findings described above, we predicted that SsTroA docks to the periplasmic face of SsTroCD, and charged amino acid residues probably are likely involved in salt bridge formation and contribute to the interface.