6.8: Gram-negative Bacterial Protein Secretion Systems

Gram-negative bacteria utilize sophisticated protein secretion systems to transport proteins across their double-membrane envelope into the extracellular environment or host cells. Based on their mechanism of action, these systems are classified into one-step and two-step pathways.

One-Step Secretion Systems (Types I, III, IV, and VI)One-step secretion systems bypass the periplasm entirely, forming a continuous channel that spans both the inner and outer membranes:

• Type I Secretion System (T1SS): T1SS directly exports substrates, including toxins and proteases, from the cytoplasm to the extracellular environment. This system requires an ATP-binding cassette (ABC) transporter for energy-dependent translocation.
• Type III Secretion System (T3SS): Also known as the injectisome, T3SS delivers effector proteins directly into host cells. These effectors modulate host cellular processes to benefit the bacteria, often playing critical roles in virulence during infection.
• Type IV Secretion System (T4SS): T4SS facilitates the transfer of macromolecules, including DNA and proteins, during conjugation and horizontal gene transfer. Some pathogens also use it to inject virulence factors into host cells.
• Type VI Secretion System (T6SS): T6SS employs a phage-like mechanism, using a contraction-based system to inject toxic effector proteins into competing bacteria or host cells. This system plays a role in bacterial competition and pathogenesis.

Two-Step Secretion Systems (Types II and V)Two-step secretion systems rely on the Sec or Tat pathways to translocate proteins across the inner membrane into the periplasm. From there, additional components facilitate transport across the outer membrane:

• Type II Secretion System (T2SS): This system uses a pseudopilus, a dynamic pilus-like structure, to push proteins such as toxins and hydrolytic enzymes through a specialized pore in the outer membrane. These proteins often play roles in nutrient acquisition and pathogenesis.
• Type V Secretion System (T5SS): Proteins secreted via this system carry an autotransporter domain that aids their translocation across the outer membrane. Some proteins are released extracellularly, while others remain anchored as adhesins, contributing to bacterial attachment and biofilm formation.