The Versatile Type VI Secretion System

Abstract

Bacterial type VI secretion systems (T6SSs) function as contractile nanomachines to puncture target cells and deliver lethal effectors. In the 10 years since the discovery of the T6SS, much has been learned about the structure and function of this versatile protein secretion apparatus. Most of the conserved protein components that comprise the T6SS apparatus itself have been identified and ascribed specific functions. In addition, numerous effector proteins that are translocated by the T6SS have been identified and characterized. These protein effectors usually represent toxic cargoes that are delivered by the attacker cell to a target cell. Researchers in the field are beginning to better understand the lifestyle or physiology that dictates when bacteria normally express their T6SS. In this article, we consider what is known about the structure and regulation of the T6SS, the numerous classes of antibacterial effector T6SS substrates, and how the action of the T6SS relates to a given lifestyle or behavior in certain bacteria.

Figure 1. Comparison between bacteriophage and the T6SS

(A) Bacteriophage possess tail fibers that attach to lipopolysaccharides of target bacterial cells. During reversible binding, the tail fibers bring the base plate in contact with the cell surface. Once in contact, irreversible binding is initiated, the tail sheath contracts and the rigid tail tube and spike are forced though the outer membrane (OM), proteins of the spike degrade the peptidoglycan, and final interaction with the inner membrane (IM) initiates translocation of viral DNA into the cell. (B) The T6SS functions much like a bacteriophage with several proteins being structurally similar, depicted here in the same color. Formation of the T6SS base plate complex, which spans the IM, peptidoglycan, and OM initiates Hcp tube polymerization and sheath formation of VipA and VipB heterodimers. Upon contact with a target bacterial cell, this ‘ready to fire’ state is triggered causing the sheath to contract and deployment of the Hcp tube and VgrG spike into the target cell. Effector proteins are delivered into the periplasm, possibly degrading peptidoglycan for cytoplasmic effectors to gain further access into the cell. OM, outer membrane; IM, inner membrane.

Figure 2. Infiltration of resistant and sensitive opposing swarms by P. mirabilis HI4320 expressing VipA::sfGFP

Agar plate inoculated with P. mirabilis HI4320 VipA::sfGFP opposing strains HI4320 or mutant 9C1 (mutation in immunity protein PefE) expressing dsRED. Infiltrating Proteus mirabilis HI4320 expressing VipA::sfGFP opposing strains HI4320 or mutant 9C1 expressing dsRED. Elapsed time (T) is indicated in seconds (reprinted with permission from Alteri CJ et al. 2013. PLoS Pathog 9: e1003608).

Figure 3. Conservation of the Type 6 Secretion System effector operons among Proteus mirabilis, Vibrio cholerae, and Pseudomonas aeruginosa

(A) The 17 genes that encode the Proteus mirabilis HI4320 T6SS are highly conserved with the well characterized T6SS genes from V. cholerae N16961 and have identical gene order in their respective chromosome and (B) comparison to the three known T6SSs (HIS-1, HIS-2, HIS-3) encoded by the genome of P. aeruginosa PA01. The arrows are color-coded based upon known or predicted gene function. Black arrows represent genes that are not found in either P. mirabilis HI4320 or V. cholerae N16961 T6SS gene locus (reprinted with permission from Alteri CJ et al. 2013. PLoS Pathog 9: e1003608).

Figure 4. Contact-dependent preemptive antagonism is dependent on the T6SS in Proteus mirabilis

A Dienes line (black arrows) forms between two different wild-type isolates, HI4320 and BB2000 (strain A and B kill each other). Loss of the T6SS (ΔT6) in either isolate by disruption of PMI0742 does not affect the discriminatory Dienes line (strain A kills strain B or strain B kills strain A). Loss of the T6SS in both isolates allows non-identical swarms to merge and the lack of T6SS-dependent killing appears as “recognition” (white arrow) (reprinted with permission from Alteri CJ et al. 2013. PLoS Pathog 9: e1003608).

Figure 5. Immunity against T6SS is the basis for self-recognition

If two strains of Proteus mirabilis synthesize the same immunity protein then both strains are immune to killing by the T6SS and recognize as “self”. However, if two strains have divergent immunity proteins, then both strains are subject to killing by each of the other strain’s T6SS.