The bacterial type VI secretion machine: yet another player for protein transport across membranes

Several secretion systems have evolved that are widespread among Gram-negative bacteria. Recently, a new secretion system was recognized, which is named the type VI secretion system (T6SS). The T6SS components are encoded within clusters of genes initially identified as IAHP for IcmF-associated homologous proteins, since they were all found to contain a gene encoding an IcmF-like component. IcmF was previously reported as a component of the type IV secretion system (T4SS). However, with the exception of DotU, other T4SS components are not encoded within T6SS loci. Thus, the T6SS is probably a novel kind of complex multi-component secretion machine, which is often involved in interaction with eukaryotic hosts, be it a pathogenic or a symbiotic relationship. The expression of T6SS genes has been reported to be mostly induced in vivo. Interestingly, expression and assembly of T6SSs are tightly controlled at both the transcriptional and the post-translational level. This may allow a timely control of T6SS assembly and function. Two types of proteins, generically named Hcp and VgrG, are secreted via these systems, but it is not entirely clear whether they are truly secreted effector proteins or are actually components of the T6SS. The precise role and mode of action of the T6SS is still unknown. This review describes current knowledge about the T6SS and summarizes its hallmarks and its differences from other secretion systems.