Selective Nutrient Transport in Bacteria: Multicomponent Transporter Systems Reign Supreme

Abstract

Multicomponent transporters are used by bacteria to transport a wide range of nutrients. These systems use a substrate-binding protein to bind the nutrient with high affinity and then deliver it to a membrane-bound transporter for uptake. Nutrient uptake pathways are linked to the colonisation potential and pathogenicity of bacteria in humans and may be candidates for antimicrobial targeting. Here we review current research into bacterial multicomponent transport systems, with an emphasis on the interaction at the membrane, as well as new perspectives on the role of lipids and higher oligomers in these complex systems.

Keywords: ABC transporter; TRAP transporter; membrane proteins; protein-protein interaction; transport mechanism.

FIGURE 1

Multicomponent transporters. Left, an experimentally determined structure of the type I ABC importer ModBC-A from A. fulgidus (PDB ID: 2ONK). Middle, an experimentally determined structure of the type II ABC importer BtuCD-F from E. coli (PDB ID: 2QI9) (Hollenstein et al., 2007; Hvorup et al., 2007). Right, a hypothetical model of the TRAP transporter YiaMN-O from E. coli that was generated by Ovchinnikov et al. (2014) using comparative modelling in RosettaCM. The substrate-binding proteins (SBPs) dock to the transmembrane domains (TMDs) at the periplasmic surface of the inner membrane. ABC transporter nucleotide-binding domains (NBDs) situated in the cytoplasm catalyse the hydrolysis of ATP. The TRAP transporter facilitates the movement of sodium ions across the membrane.

FIGURE 2

Co-evolution analysis as a tool to explore SBP:TMD interactions in multicomponent systems. The predicted MetIQ structure (Ovchinnikov et al., 2014) maps well on to the MetNIQ crystal structure (MetN not displayed) (Nguyen et al., 2018). The top co-evolved residues between the two components are in similar positions, with the predicted complex correctly orienting the SBP.