Table 2 Description of secretion systems

1*

- Consists of three protein subunits: the ATP-binding cassette (ABC) transporters, membrane fusion proteins, and outer membrane proteins.

- Transports various proteins, e.g., RTX toxins and the lipases, as well as non-proteinaceous substrates, e.g., cyclic β-glucans and polysaccharides.

2*

- Represents a Sec/Tat-dependent system, as proteins that pass through this system must first reach the periplasm via either the general secretion route (Sec pathway) or the twin-arginine translocation pathway (Tat-pathway).

- Sometimes used by Gram-negative bacteria type IV pili for their biogenesis.

3*

- Consists of machinery proteins (called injectisomes) and proteins that are secreted into a host cell (called effectors).

- Sometimes consists of two or more gene clusters (pathogenicity islands).

- Found in Gram-negative bacteria that interact with both plant and animal hosts.

4

- Can be divided into three types: 1) a type IVA secretion system resembling the archetypal VirB/VirD4 system and consisting of conjugative plasmids F and RP4 (IncF and IncP); 2) a type IVB secretion system also known as the intracellular multiplication/defect in organelle trafficking genes (icm/dot) system, consisting of conjugative plasmid R64; and 3) a GI type that is, so far, associated exclusively with genomic islands.

- Evolutionarily related to bacterial conjugation systems and capable of transporting both proteins and nucleic acids into host cells, as well as into other bacteria.

5*

- Also known as the autotransporter system.

- Can be divided into three types: 1) the archetypal bacterial proteins exported into the periplasm via the Sec system; 2) trimeric proteins with a single beta barrel domain; and 3) pairs of proteins in which one partner carries the beta barrel domain and the other partner is the secreted protein.

6*

- Consists of machinery proteins (called injectisomes) and proteins that are secreted into a host cell (called effectors).

- Sometimes consists of two or more gene clusters (pathogenicity islands).

- Nearly universally secretes two proteins: Hcp and VgrG.

7

- Used for the transport of extracellular proteins across the Gram-positive bacteria cell wall.

- Often encoded in two or more gene clusters (pathogenicity islands).