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  • Review Article
  • Published:

Protein export through the bacterial Sec pathway

Nature Reviews Microbiology volume 15, pages 21–36 (2017)Cite this article

Subjects

Key Points

  • More than one-third of the bacterial proteome is destined for the cell envelope. The general secretory (Sec) pathway is the ubiquitous, central and essential protein export pathway into, and through, the plasma membrane.

  • Nascent exported chains that emerge from the ribosome are scanned by chaperones and export-specific proteins that select them from cytoplasmic residents and direct them for either co-translational or post-translational export through the SecYEG channel.

  • Kinetic competition between the scanning factors that recognize exported protein signals selects for the export route.

  • SecA, the ATPase motor of post-translational export, 'proof-reads' its substrates, which are possibly already at the ribosome, and assembles with SecYEG into the translocase holoenzyme.

  • SecA is allosterically activated by amino-terminal signals that are present on exported proteins and uses its highly regulated quaternary and intraprotomeric dynamics for chemo–mechanical coupling that drives protein translocation.

  • SecYEG is regulated by its cytoplasmic partners, ribosomes and SecA, and is activated by exported proteins for either vectorial or lateral translocation.

Abstract

The general secretory (Sec) pathway comprises an essential, ubiquitous and universal export machinery for most proteins that integrate into, or translocate through, the plasma membrane. Sec exportome polypeptides are synthesized as pre-proteins that have cleavable signal peptides fused to the exported mature domains. Recent advances have re-evaluated the interaction networks of pre-proteins with chaperones that are involved in pre-protein targeting from the ribosome to the SecYEG channel and have identified conformational signals as checkpoints for high-fidelity targeting and translocation. The recent structural and mechanistic insights into the channel and its ATPase motor SecA are important steps towards the elucidation of the allosteric crosstalk that mediates secretion. In this Review, we discuss recent biochemical, structural and mechanistic insights into the consecutive steps of the Sec pathway — sorting and targeting, translocation and release — in both co-translational and post-translational modes of export. The architecture and conformational dynamics of the SecYEG channel and its regulation by ribosomes, SecA and pre-proteins are highlighted. Moreover, we present conceptual models of the mechanisms and energetics of the Sec-pathway dependent secretion process in bacteria.

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Figure 1: Bacterial protein export.
Figure 2: Structures of the sorting and targeting factors.
Figure 3: Architecture and dynamics of the Sec translocase components.
Figure 4: Conformational states of the SecYEG channel.
Figure 5: Model of Sec-dependent post-translational translocation and pre-protein maturation and release factors.

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Acknowledgements

The authors thank G. Gouridis, A. Kuhn, R. Dalbey, S. White and B. Berks for fruitful discussions in relation to this manuscript, and C. Huang and B. Kalodimos for the structure of the SecB pre-protein. Research in the authors' laboratory is funded by grants: KUL-Spa (Onderzoekstoelagen 2013; Bijzonder Onderzoeksfonds; Katholieke Universiteit (KU) Leuven) RiMembR (Vlaanderen Onderzoeksprojecten; #G0C6814N; Research Foundation — Flanders (FWO)), StrepSynth (FP7 KBBE.2013.3.6-02: Synthetic Biology towards applications; #613877; European Union), T3RecS (#G002516N; FWO) and DIP-BiD (#AKUL/15/40 - G0H2116N; Hercules/FWO) to A.E and grant #G0B4915N from the FWO to S.K. J.D.G. is an FWO doctoral fellow.

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  1. Department of Microbiology and Immunology, Laboratory of Molecular Bacteriology, Rega Institute for Medical Research, Katholieke Universiteit Leuven (KU Leuven), B-3000, Leuven, Belgium

    Alexandra Tsirigotaki, Jozefien De Geyter, Nikolina Šoštaric´, Anastassios Economou & Spyridoula Karamanou

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Supplementary information

Supplementary information S1 (table)

Protein export through the bacterial Sec pathway (DOCX 136 kb)

Supplementary information S2 (table)

Cell envelope concentrations of the secretory proteins that employ the Sec secretion pathway in E. coli. (XLS 346 kb)

Supplementary information S3 (table)

Protein export through the bacterial Sec pathway (DOCX 206 kb)

Glossary

Membranome

The portion of the bacterial exportome that is integrated into the plasma membrane (approximately 22% of the total proteome of Escherichia coli K12).

Secretome

The portion of the bacterial exportome that is exported beyond the plasma membrane (that is, to the periplasm, outer membrane, extracellular milieu or a host cell; approximately 13% of the total proteome of Escherichia coli K12).

Chemo–mechanical conversion

The conversion of chemical energy, such as the energy that is produced through breaking chemical bonds during ATP hydrolysis, to mechanical work, such as the movement of protein domains or whole proteins.

Exportome

The non-cytoplasmic portion of the bacterial proteome that is integrated into the plasma membrane or exported beyond the plasma membrane (that is, to the periplasm, outer membrane, extra-cellular milieu or a host cell).

Sortases

Cysteine transpeptidases, found mainly in Gram-positive bacteria, that recognize carboxy-terminal signals (cell wall sorting signals) with an LPXTG motif on their substrate proteins (surface proteins), cleave them at the threonyl residue of the motif and then attach them on the cell surface.

Pre-proteins

Exported proteins that are synthesized as pre-forms, that is, containing an amino-terminal signal peptide (also known as a signal sequence or leader peptide) extension that is proteolytically removed during, or after, export.

Chaperones

Proteins (ATP-dependent or independent) that assist in de novo protein folding or refolding, or disaggregation or the prevention of aggregation under stress or physiological conditions.

Proton motive force

(PMF). The potential energy stored in the plasma membrane, due to proton and voltage gradients across the membrane, that becomes liberated during proton movement through the membrane plane towards achieving electrochemical equilibrium.

Holdase

A chaperone that does not promote protein folding, but 'holds' onto substrates to prevent misfolding and/or aggregation.

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Tsirigotaki, A., De Geyter, J., Šoštaric´, N. et al. Protein export through the bacterial Sec pathway. Nat Rev Microbiol 15, 21–36 (2017). https://doi.org/10.1038/nrmicro.2016.161

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