T6SSs are toxin delivery systems. It is a multiprotein complex requiring numerous core proteins (Tss proteins) including cytoplasmic, transmembrane, and outer membrane components. The needle or tube apparatus is comprised of a phage-like complex, similar to the T4 contractile bacteriophage tail, which is thought to be anchored to the membrane by a trans-envelope complex [1]. VipA is a family of Gram-negative bacterial proteins that form part of the type VI pathogenic secretion system. Members have been variously defined as VC_A0107 family, Hcp2, TssB and VipA, for ClpV-interacting proteins. VipB and VipA proteins interact very closely to form the shaft of the pathogenic penetrating needle system [2,3,4]. VipA and VipB (TssB and TssC) proteins were shown to form a cog-wheel like tubular structure in V. cholerae that was noticed to resemble T4 phage gp18 polysheath. Two beta-strands of VipA and four beta-strands of VipB intertwine forming the middle layer of the sheath. The sheath assembles around an inner Hcp tube and is attached to a structure called a baseplate that spans the bacterial membranes. Importantly, VipA/VipB sheath was shown to form a long contractile organelle in V. cholerae and in E. coli, suggesting that sheath contraction powers the secretion [5].

1: Type VI secretion systems of human gut Bacteroidales segregate into three genetic architectures, two of which are contained on mobile genetic elements. Coyne MJ, Roelofs KG, Comstock LE; BMC Genomics. 2016;17:58. PMID:26768901

2: [Study on BCG vaccination and incidence of children's tuberculous meningitis in Liaoning province]. Wu QR; Zhonghua Jie He He Hu Xi Za Zhi. 1991;14:173-175. PMID:1913969

3: Type VI secretion: a beginner's guide. Bingle LE, Bailey CM, Pallen MJ; Curr Opin Microbiol. 2008;11:3-8. PMID:18289922

4: ClpV recycles VipA/VipB tubules and prevents non-productive tubule formation to ensure efficient type VI protein secretion. Kapitein N, Bonemann G, Pietrosiuk A, Seyffer F, Hausser I, Locker JK, Mogk A; Mol Microbiol. 2013;87:1013-1028. PMID:23289512

5: Structure of the type VI secretion system contractile sheath. Kudryashev M, Wang RY, Brackmann M, Scherer S, Maier T, Baker D, DiMaio F, Stahlberg H, Egelman EH, Basler M; Cell. 2015;160:952-962. PMID:25723169