The SciZ protein anchors the enteroaggregative Escherichia coli Type VI secretion system to the cell wall

Type VI secretion systems (T6SS) are multi-component machines encoded within the genomes of most Gram-negative bacteria that associate with plant, animal and/or human cells, and therefore are considered as potential virulence factors. We recently launched a study on the Sci-1 T6SS of enteroaggregative Escherichia coli (EAEC). The Sci-1 T6SS is composed of all or a subset of the 21 gene products encoded within the cluster, 13 of which are shared by all T6SS identified so far. In the present work, we focussed our attention on the SciZ protein. We first showed that SciZ is required for the release of the Hcp protein in the culture supernatant and for efficient biofilm formation, demonstrating that SciZ is necessary for EAEC T6SS function. Indeed, SciZ forms a complex with SciP, SciS and SciN, three core components of the transport apparatus. Fractionation and topology studies showed that SciZ is a polytopic inner membrane protein with three trans-membrane segments. Computer analyses identified a motif shared by peptidoglycan binding proteins of the OmpA family in the SciZ periplasmic domain. Using in vivo and in vitro binding assays, we showed that this motif anchors the SciZ protein to the cell wall and is required for T6SS function.