Chapter 2 : SecA-Mediated Protein Translocation through the SecYEG Channel

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Protein transport occurs in all domains of life ( ). Proteins that function outside the cytosol are translocated across membranes. The general system for protein translocation is formed by the Sec translocase at its core the translocon: SecYEG in bacteria ( ), SecYEβ in archaea ( ), and Sec61αβγ in the endoplasmic reticulum of eukaryotes ( ). The translocon forms a protein conducting channel in the membrane for unfolded preproteins ( ) but also mediates cotranslational insertion of nascent membrane proteins into the membrane ( Fig. 1 ).

Citation: Komarudin A, Driessen A. 2019. SecA-Mediated Protein Translocation through the SecYEG Channel, p 13-28. In Sandkvist M, Cascales E, Christie P (ed), Protein Secretion in Bacteria. ASM Press, Washington, DC. doi: 10.1128/microbiolspec.PSIB-0028-2019
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Figure 1

The Sec pathway. Posttranslational pathway: after complete synthesis at the ribosome, the unfolded preprotein is recognized by the molecular chaperone SecB (blue) and targeted to SecA (green). SecA guides the preprotein through the SecYEG pore (lime), employing the energy from ATP binding and hydrolysis. The signal peptide is cleaved by the signal peptidase (SPase [yellow]). SecDF (pink) pulls the preprotein across the membrane at the expense of the PMF. Cotranslational pathway: once a hydrophobic transmembrane domain of a nascent membrane protein emerges from the ribosomes, signal recognition particle (SRP) (brown) binds to the ribosome nascent chain (RNC) and guides the complex to the SR receptor FtsY (dark brown) at the membrane. Upon the binding of GTP to the SRP:FtsY heterodimer, the RNC is released from SRP and transferred to the SecYEG channel, where chain elongation at the ribosome is directly coupled to membrane insertion of the nascent membrane protein.

Citation: Komarudin A, Driessen A. 2019. SecA-Mediated Protein Translocation through the SecYEG Channel, p 13-28. In Sandkvist M, Cascales E, Christie P (ed), Protein Secretion in Bacteria. ASM Press, Washington, DC. doi: 10.1128/microbiolspec.PSIB-0028-2019
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Figure 2

Structural stages of the translocation channel. The SecYEG/β crystal structures viewed from the membrane: SecY TMS 1 to 5 (blue), TMS 6 to 10 (green), plug domain (red), SecE (yellow), and SecG/β (orange). Cartoon illustration of SecYEG/β. The illustrations depict the opening of the constriction and movement of the plug domain depending on the state of the translocon. SecYEβ (PDB entry 1RH5), known as the closed or resting conformation. SecYEG cocrystallized with SecA (not shown) in an Mg-ADP-BeFx-bound transition state (PDB entry 3DIN) as a preopen conformation. SecYEG cocrystallized with SecA (not shown) and a signal sequence (magenta) latched into the lateral gate (PDB entry 5EUL), resembling an actively engaged translocation channel.

Citation: Komarudin A, Driessen A. 2019. SecA-Mediated Protein Translocation through the SecYEG Channel, p 13-28. In Sandkvist M, Cascales E, Christie P (ed), Protein Secretion in Bacteria. ASM Press, Washington, DC. doi: 10.1128/microbiolspec.PSIB-0028-2019
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Figure 3

Conformational states of SecA. Structures of SecA from (PDB entry 1M6N) , Mg-ADP-BeFx-bound SecA cocrystallized with SecYEG (not shown) from (PDB 3DIN) , and Mg-ADP-BeFx-bound SecA from engaged with the SecYEG and a signal sequence (not shown) (PDB entry 5EUL) . The locations of the PPXD domain (yellow), NBD1 (red), NBD2 (blue), HWD (green), HSD (purple), and 2HF (cyan) are indicated. A large movement of the PPXD domain (yellow) suggests a closed or open conformation of SecA.

Citation: Komarudin A, Driessen A. 2019. SecA-Mediated Protein Translocation through the SecYEG Channel, p 13-28. In Sandkvist M, Cascales E, Christie P (ed), Protein Secretion in Bacteria. ASM Press, Washington, DC. doi: 10.1128/microbiolspec.PSIB-0028-2019
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Figure 4

Structure of SecA-SecYEG complex. SecA penetrates into the SecYEG channel (red) via the so-called two-helix finger (2HF [light blue]). The SecA PPXD domain (yellow) also binds to TMS6/7 loop of SecYEG. The conserved tyrosine 794 is depicted in green.

Citation: Komarudin A, Driessen A. 2019. SecA-Mediated Protein Translocation through the SecYEG Channel, p 13-28. In Sandkvist M, Cascales E, Christie P (ed), Protein Secretion in Bacteria. ASM Press, Washington, DC. doi: 10.1128/microbiolspec.PSIB-0028-2019
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Figure 5

Proposed models of SecA-mediated protein translocation. Power stroke: ATP binding and hydrolysis induce conformational changes of SecA that result in a mechanical force on the preprotein, pushing it through the SecYEG channel. In this model, oligomerization of SecA is required to prevent backsliding of the preprotein. Brownian ratchet: SecA regulates the SecYEG channel opening via the 2HF of SecA, allowing the protein translocation via diffusion. Trapping and release of the translocating preprotein at the -side result in translocation, while SecA may fulfill an additional function by opening the translocation channel. The oligomeric state of SecA is not explicitly shown in this model. Push and slide: this model uses both SecA-dependent pushing and Brownian motion. The oligomeric state of SecA is not explicitly shown in this model. Reciprocating piston: this model is a combination of a power stroke mechanism with the conversion of dimeric-monomeric SecA. Repeated cycles of SecA monomerization-rebinding and ATP binding-hydrolysis yield a stepwise translocation process. In none of these models is the exact role of the PMF and SecDF included, but they contribute to efficient translocation.

Citation: Komarudin A, Driessen A. 2019. SecA-Mediated Protein Translocation through the SecYEG Channel, p 13-28. In Sandkvist M, Cascales E, Christie P (ed), Protein Secretion in Bacteria. ASM Press, Washington, DC. doi: 10.1128/microbiolspec.PSIB-0028-2019
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