Figure 1

Structure of TatB translocase component. (a) Ribbon diagram of the solution structure of TatB^1–101 adapted from Zhang et al.³¹. QCS substitutions isolated by Rocco et al.²⁶ are marked as red balls (except for H109N which is marked with asterisk). Locations of truncation sites in 10-residue increments from the C-terminus are labeled in gray. Histidines at positions 109, 112, and 123 are shown as black circles. (b) Model of possible TatB-substrate interactions adapted from Ulfig et al.³⁵. The framed light gray box represents the lipid bilayer, while a TatC monomer is depicted by six transmembrane helices, and a TatB monomer is depicted with a membrane-embedded transmembrane helix (TMH α1), a strongly amphipathic helix (α2), and two highly hydrophilic and flexible helices (α3 and α4). The cytosolic α3 and α4 helices encapsulate a folded substrate protein (diagonal hatched oval), with possible movement of the helices depicted by black arrow and dashed line cylinders. It should also be pointed out that the TatBC complex functions as a receptor for the N-terminal signal peptide, which is defined by a twin-arginine motif (RR) and h-region α-helix (diagonal hatched cylinder). For clarity, the cartoon does not account for the well-known signal peptide-TatBC interaction. Also, for clarity, only one TatB monomer (dark gray) and one TatC monomer (light gray) are shown, but the model could easily accommodate higher order oligomeric structures such as the tetrameric TatBC complex described by Lee et al.³⁶. One possible scenario for QC could be that upon binding of the signal peptide and N-terminal part of the substrate, the C-terminal domain of TatB dynamically wraps around the substrate and performs conformational proofreading. (c) Multiple sequence alignment of TatB proteins from γ-proteobacteria E. coli and Y. pseudotuberculosis generated using CLUSTALW. Asterisks indicate identical amino acids, colons indicate conservation between amino acids with strongly similar properties, and periods indicate conservation between amino acids with weakly similar properties. The α-helical regions were determined previously by solution NMR spectroscopy³¹ and are represented here as cylinders. The truncation point after residue 91 is marked with red arrow/dashed line and the histidines at positions 109, 112, and 123 are shaded red.