Aim, Load, Fire: The Type VI Secretion System, a Bacterial Nanoweapon

Bacteria utilise specialised protein secretion systems to interact with host organisms, competitor bacteria, and the environment. The Type VI secretion system (T6SS) is a versatile weapon deployed by many bacterial species to target either host cells or rival bacteria. The widespread occurrence and significance of the T6SS is becoming increasingly appreciated, as is its intriguing mode of action. The T6SS delivers multiple, diverse effector proteins directly into target cells using a dynamic ‘firing’ mechanism related to the action of contractile bacteriophage tails. Here, we summarise the contribution of recent findings to our developing picture of how the T6SS assembles and fires, how it is loaded with different types of effectors, and how it can be aimed towards an incoming assault.

TrendsThe bacterial Type VI secretion system (T6SS) can be an important virulence factor used to target host cells or, perhaps more widely, an efficient weapon to target and compete against rival bacteria in polymicrobial environments.The T6SS translocates effector proteins into target cells in a one-step manner and represents a contractile, phage-related nanomachine composed of several subcomplexes.The T6SS features dynamic firing cycles, based on assembly, contraction, and disassembly of a sheath-like structure, and expulsion of a cell-puncturing device loaded with multiple toxins.Varied modes of post-translational regulation of the T6SS suggest adaptation to threats encountered in Nature.Effectors delivered by the T6SS are diverse, both in terms of distinct modes of recruitment (broadly categorised into ‘cargo’ or ‘specialised’ classes) and highly varied functions in target cells.