They are widespread in bacteria and archaea, especially in marine environments, but absent in eukaryotic organisms.
Tripartite ATP-independent periplasmic (TRAP) transporters represent a structural- and functional mix of the well-studied ATP-binding cassette (ABC) transporters and secondary active transporters, by functioning as substrate-binding protein (SBP) dependent secondary transporters 1, 2, 3, 4. Furthermore, we characterize high-affinity single variable domains on heavy chain (VHH) antibodies that bind to the periplasmic side of HiSiaQM and inhibit sialic acid uptake, providing insight into how TRAP transporter function might be inhibited in vivo. We use single-molecule total internal reflection fluorescence (TIRF) microscopy in solid-supported lipid bilayers and surface plasmon resonance to study the formation of the tripartite complex and to investigate the impact of interface mutants. A model of the tripartite PQM complex is experimentally validated and reveals the coupling of the substrate-binding protein to the transporter domains. The idiosyncratic Q-domain of TRAP transporters enables the formation of a monomeric elevator architecture. It is composed of 16 transmembrane helices that are unexpectedly structurally related to multimeric elevator-type transporters. Here, we reconstitute HiSiaQM into lipid nanodiscs and use cryo-EM to reveal the structure of a TRAP transporter. HiSiaPQM and its homologs are TRAP transporters for sialic acid and are essential for host colonization by pathogenic bacteria. Tripartite ATP-independent periplasmic (TRAP) transporters are found widely in bacteria and archaea and consist of three structural domains, a soluble substrate-binding protein (P-domain), and two transmembrane domains (Q- and M-domains). Nature Communications volume 13, Article number: 4471 ( 2022)
Structural and mechanistic analysis of a tripartite ATP-independent periplasmic TRAP transporter
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