Figure 4.

Model of the Bacteroidetes type VI secretion system

The type VI secretion system (T6SS) is a contractile nanomachine that injects antimicrobial proteins, termed effectors, into target cells. In the precontraction state of the system, the outer sheath of TSSB and TSSC multimers exists in a high energy state and covers the inner tube of Hcp multimers. When the outer sheath contracts to a lower energy state, the inner tube is forced out of the donor cell. At the end of the inner tube is the puncturing needle structure that contains a Valine Glycine Repeat G (VgrG) trimer and Proline-Alanine-Alanine-aRginine (PAAR) proteins that upon contact, are expelled into the target cell surface and cytoplasm. If the target cell lacks the cognate immunity proteins, death of the target organism may result. The T6SS assist Bacteroides spp. during interbacterial competition for nutrients with other commensals and pathogens and are predicted to be key players in intestinal homeostasis.