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]. VipB is a family of Gram-negative type VI secretion system components of the tail sheath. They have been known as COG3517. These sheath-components, of which there are many copies in the sheath, are also variously referred to as TssC. On contact with another bacterial cell the sheath contracts and pushes the puncturing device and tube through the cell envelope and punches the target bacterial cell [1]. 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 [2].

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: 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