Figure 3. Molecular architecture of the cell envelope of Treponema pallidum.
Shown in the outer membrane are TP0751 (as known as pallilysin)79,81 and Tpp17 (also known as TP0435)82,241 — two surface-exposed lipoproteins; TP0453, a lipoprotein attached to the inner leaflet of the outer membrane83; BamA (also known as TP0326)84,94; a full-length T. pallidum repeat (Tpr) attached by its N-terminal portion to the peptidoglycan93,94; and a generic β-barrel that represents other non-Tpr outer-membrane proteins identified by computational mining of the T. pallidum genome112. Substrates and nutrients present in high concentration in the extracellular milieu (such as, glucose) traverse the outer membrane through porins, such as TprC. At the cytoplasmic membrane, prototypic ABC-like transporters (such as RfuABCD, a riboflavin transporter) use a periplasmic substrate-binding protein (SBP), usually lipoproteins, and components with transmembrane and ATP-binding domains to bind nutrients that have traversed the outer membrane for transport across the cytoplasmic membrane. The energy coupling factor (ECF)-type ABC transporters use a transmembrane ligand-binding protein in place of a separate periplasmic SBP for binding of ligands (BioMNY is thought to transport biotin)242. Symporter permeases (for example, TP0265) use the chemiosmotic or electrochemical gradient across the cytoplasmic membrane to drive substrate transport243. The tripartite ATP-independent periplasmic (TRAP)-type transporters also use transmembrane electrochemical gradients to drive substrate transport; the periplasmic component protein TatT (also known as TP0956) likely associates with the SBP TatP (also known as TP0957) that binds ligands (perhaps hydrophobic molecules, such as long chain fatty acids), uptake of which is probably is facilitated by the permease TatQ-M (also known as TP0958) 244,245. Figure adapted from Ref.63 with permission.