Escherichia coli Cotranslational Targeting and Translocation


Targeting of proteins to their proper location inside the cell, in the membrane or in the extracellular space is crucial to the cell. The targeting information is provided in form of a signal sequence by the protein itself. In Escherichia coli, mostly membrane proteins are targeted cotranslationally via the signal recognition particle (SRP) to the membrane. The SRP and its receptor are both guanosine triphosphatases (GTPases) which handover the ribosome‐nascent chain complex to the protein‐conducting channel. Subsequently, the nascent polypeptide is translocated into or across the membrane. To date, a host of biochemical data and a number of structures of important complexes along the pathway are available which shed light on the molecular mechanism of the targeting and translocation process.

Key Concepts:

  • Nascent polypeptides with signal sequences are recognised by the SRP.

  • The hydrophobicity of the signal sequence determines whether a protein is targeted co‐ or posttranslationally.

  • Cotranslational protein targeting is mediated by the SRP and the SRP receptor. They deliver the translating ribosome to the translocation machinery in the membrane.

  • The SecYEG translocon is a channel that allows hydrophilic polypeptides to cross the membrane. In addition, the channel can open laterally and release transmembrane helices into the lipid bilayer.

  • YidC acts as a membrane protein insertase for small membrane proteins in a Sec‐independent manner. In a Sec‐associated form, YidC is suggested to assist in membrane protein folding.

  • YidC, SecD, SecF and YajC can associate with the SecYEG protein‐conducting channel. The resulting complex is called the ‘holotranslocon’, which is suggested to be responsible for membrane protein integration, folding and complex assembly.

Keywords: membrane protein integration; signal recognition particle; FtsY; Sec translocon; YidC integrase; holo‐translocon; SecA ATPase; signal sequence; ribosome; protein export

Figure 1.

Protein export and membrane protein insertion. Exported proteins are targeted posttranslationally via the SecB chaperone and the SecA ATPase to the translocation machinery in the membrane (left). SecA provides the energy for translocation across the membrane. The signal recognition particle (SRP) and its receptor (FtsY) target nascent membrane proteins cotranslationally to the SecYEG protein‐conducting channel (right). Additional proteins (SecD, SecF, YajC and YidC) can associate with SecYEG and assist in the translocation across or into the membrane.

Figure 2.

Cotranslational targeting. (a) Scheme of the domain architecture of E. coliFfh (the SRP protein) and FtsY (SRP receptor). (b) Atomic model of the E. coliSRP in the extended conformation which is observed when the SRP binds the translating ribosome (2J28.pdb) (Halic et al., ). The Ffh N domain is depicted in red, the GTPase domain in orange, the M domain in yellow, the signal sequence in green and the 4.5S RNA in dark red. (c) Co‐crystal structure of the NG domains of Ffh and FtsY from Thermus aquaticus (1OKK.pdb) (Egea et al., ; Focia et al., ). The two GTPase domains form a composite active site. The two GTP analogues are shown as green spheres. The FtsY NG domain is depicted in blue. (d) Model of the SRP and FtsY conformational states during cotranslational targeting (adapted from Zhang et al., ). First, the ribosome nascent chain complex (cargo) is recognised and tightly bound by the SRP. The cryo‐EM structure of the RNC‐SRP (filtered to 15 Å resolution, Halic et al., ) is depicted with the same colour coding as the cartoon drawings of the other complexes. Subsequently, the SRP and FtsY form a GTP‐independent early intermediate. In the presence of GTP, the early intermediate rearranges into the closed conformation. Rearrangements in the catalytic loops of the GTPase domains activate GTP hydrolysis (activated state). This leads to cargo‐release, that is, handover of the RNC to the SecYEG translocon and dissociation of the SRP–FtsY complex.

Figure 3.

The translocation machinery. The crystal structure of the archaeal SecYEβ (top, 1RHZ.pdb) (Van den Berg et al., ) shown from the cytoplasm (a) and as a side view (cut in the middle of the protein‐conducting channel) (b). SecY is depicted in grey; TM2 and TM7 of SecY that line the lateral gate are highlighted in blue and green, respectively. The inactive translocation pore is sealed by a small helix on the periplasmic site (the plug, depicted in red). The ring of hydrophobic amino acids that seals the translocation pore is highlighted in yellow. (c) Topology of the additional translocation factors SecD, SecF, YajC and YidC. The periplasmic domain of E. coliYidC (3BLC.pdb) (Oliver and Paetzel, ) is the only high‐resolution structural information of these proteins currently available.



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Further Reading

Cross BC, Sinning I, Luirink J and High S (2009) Delivering proteins for export from the cytosol. Nature Reviews Molecular Cell Biology 10(4): 255–264.

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Rapoport TA (2008) Protein transport across the endoplasmic reticulum membrane. FEBS Journal 275(18): 4471–4478.

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von Loeffelholz, Ottilie, Botte, Matthieu, and Schaffitzel, Christiane(Jan 2011) Escherichia coli Cotranslational Targeting and Translocation. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0023170]