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. These tube and trans-envelope sub-assemblies are linked via TssK. This entry comprises family members such as the inner tube protein Hcp (TssD). Hcp proteins form hexamers that stack to form the inner tube/needle structure of the puncturing device. Other functions have also been described for Hcp proteins, for example, some Hcp proteins have been shown to have a chaperone function in that they bind to and stabilize effectors. In addition, there are 'evolved' Hcp proteins that have the Hcp domain at the N-terminal half of the protein and a toxic effector function present in the C-terminal portion of the protein [1].

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