Protein Secretion Mechanisms in Gram-Negative Bacteria

Gram-negative bacteria deploy a suite of dedicated secretion pathways to traverse their dual-membrane envelope and deliver proteins critical for nutrient acquisition, environmental adaptation and interaction with host tissues. General secretory (Sec) and twin-arginine translocation (Tat) pathways translocate unfolded or folded polypeptides, respectively, across the inner membrane into the periplasm. From there, specialised systems (Types I–VI) carry substrates to the extracellular milieu or directly into target cells. Type I secretion systems (T1SS) constitute a one-step conduit spanning both membranes, driven by an ATP-binding cassette (ABC) transporter, a membrane fusion protein and an outer membrane porin. Type II and V routes rely on Sec or Tat for periplasmic delivery before engaging specific outer-membrane complexes. Type III and IV injectisomes form syringe-like apparatuses for effector injection, while Type VI resembles a contractile phage tail to mediate intercellular antagonism. Recent structural and mechanistic studies have elucidated the modular interplay of secretion components, the vectorial emergence of C-terminal secretion signals and the energetic principles underpinning substrate recognition and translocation. Elucidation of these pathways informs the design of novel antimicrobials, vaccines and biomanufacturing platforms that exploit bacterial secretion for recombinant protein export.

Research from Nature Portfolio

Recent studies have dissected the modular architecture of a prototype T1SS, revealing that the N-terminal peptidase-like domain of the ABC transporter is the principal determinant for substrate engagement, whereas the transmembrane and nucleotide-binding domains act cooperatively to maintain optimal secretion efficiency. In parallel, investigations into the translocation mechanism have demonstrated that the C-terminal secretion signal of substrate proteins is extruded first, confirming a strictly vectorial transport process that enforces unidirectional passage across the bacterial envelope.

Protein Secretion Mechanisms in Gram-Negative Bacteria publication trend

The graph below shows the total number of articles in protein secretion mechanisms in gram-negative bacteria across all publications each year (not limited to Nature Index journals).

Technical terms

ATP-binding cassette (ABC) transporter: Inner-membrane protein that hydrolyses ATP to power substrate translocation.

Type I secretion system (T1SS): A one-step pathway spanning both membranes, comprising an ABC transporter, membrane fusion protein and outer membrane porin.

C-terminal secretion signal: A short peptide motif at the substrate’s C-terminus recognised by T1SS machinery to initiate export.

Membrane fusion protein: Periplasmic adaptor that connects the ABC transporter to the outer membrane porin.

Outer membrane porin: Channel in the outer membrane forming the exit duct for secreted substrates.

Directed evolution: Iterative laboratory technique to generate protein variants with enhanced traits by successive rounds of mutation and selection.

Linear charge density: Distribution of charged residues along a polypeptide chain that influences passage through charged transporter pores.

References

1. Bacterial Secretion Systems: An Overview. Microbiology Spectrum (2016).
2. Type 1 secretion necessitates a tight interplay between all domains of the ABC transporter. Scientific Reports (2024).
3. Directionality of substrate translocation of the hemolysin A Type I secretion system. Scientific Reports (2015).
4. A step forward to the optimized HlyA type 1 secretion system through directed evolution. Applied Microbiology and Biotechnology (2023).
5. The hemolysin A secretion system is a multi-engine pump containing three ABC transporters. Cell (2022).
6. PySupercharge: a python algorithm for enabling ABC transporter bacterial secretion of all proteins through amino acid mutation. Microbial Cell Factories (2024).