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Fig. 4 | BMC Biology

Fig. 4

From: Outer membrane protein folding from an energy landscape perspective

Fig. 4

OMP assembly pathway across the periplasm. OMPs are secreted into the periplasm by SecYEG (purple, 5ABB [239]), where they are recognised by chaperones, of which the most important in E. coli are SurA (red, 1M5Y [148], with missing residues built using MODELLER [238]), and Skp (blue, 1U2M [240], missing residues built using PyMOL [241]). OMP sequences must also contain information for targeting to the BAM complex (5LJ0 [79]), which catalyses the final folding and insertion step into the OM. Misfolded OMPs trigger stress responses (e.g. DegS/RseA pathway [242, 243] (not shown)), and are degraded by the periplasmic protease DegP (shown with multiple colours highlighting the subunits of this dodecameric complex (2ZLE [244])). An example of a folded OMP in the OM (FhuA, 1BY3 [231]) is shown in orange. Note that LPS and the peptidoglycan layer are omitted from this schematic. The inner and outer membranes are shown approximated to the width of a di-oleoylphosphatidylethanolamine (diC18:1PE, DOPE) bilayer (generated in VMD [245]). The distance between the inner and the outer membranes can be inferred from the structures of machineries that span the periplasm, giving estimates in the range ~ 165–170 Å [179, 246] to ~ 190–210 Å [247]. Here, the latter distance (210 Å) is used, consistent with the dimensions of the periplasm observed by cryo-electron microscopy [248]. The question mark highlights the fact that whether Skp can deliver OMPs to BAM in vivo remains unknown, although BamA-containing membranes can promote folding of OMPs from their complexes with Skp in vitro [65, 126]

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