T4SS

Type IV secretion systems (T4SS) are multiprotein complexes found in many Gram-negative bacteria that transport DNA and/or proteins across the cell envelope. They span inner and outer membranes and are powered by ATPases, enabling translocation into recipient cells or into host cells.

T4SSs are classified into several subclasses, most notably type IVA and type IVB, with distinct phylogenetic lineages. They participate in two broad modes: conjugative transfer of DNA, often plasmid-borne, and delivery of effector proteins to eukaryotic or prokaryotic cells, which can manipulate host processes or promote interbacterial interactions. Some T4SSs specialize in DNA transfer, while others primarily deliver protein effectors.

The core apparatus comprises a set of conserved proteins, including VirB1-11 and the VirD4 coupling protein, which recruits substrates. An ATPase such as VirB4, and often a second ATPase VirB11, powers translocation and substrate handling. A pilus built from VirB2 subunits extends from the cell surface to contact recipient cells. The outer membrane core complex is formed by components such as VirB7, VirB9 and VirB10, which create a channel through which substrates pass.

Notable examples include the VirB/VirD4 system of Agrobacterium tumefaciens, which transfers T-DNA into plant cells, and the Cag-type IV secretion system of Helicobacter pylori, which delivers virulence factors into gastric epithelial cells. Other bacteria, such as Legionella pneumophila, use related systems to translocate effectors that modulate host processes. T4SSs also mediate plasmid conjugation, enabling horizontal gene transfer and the spread of antibiotic resistance determinants.

Research on T4SSs informs understanding of bacterial pathogenesis, symbiosis, and genetic exchange, and these systems are explored as potential targets for antimicrobial intervention or tools for genetic engineering.