SciN Is an Outer Membrane Lipoprotein Required for Type VI Secretion in EnteroaggregativeEscherichia coli

Abstract

Enteroaggregative Escherichia coli (EAEC) is a pathogen implicated in several infant diarrhea or diarrheal outbreaks in areas of endemicity. Although multiple genes involved in EAEC pathogenesis have been identified, the overall mechanism of virulence is not well understood. Recently, a novel secretion system, called type VI secretion (T6S) system (T6SS), has been identified in EAEC and most animal or plant gram-negative pathogens. T6SSs are multicomponent cell envelope machines responsible for the secretion of at least two putative substrates, Hcp and VgrG. In EAEC, two copies of T6S gene clusters, called sci-1 and sci-2, are present on the pheU pathogenicity island. In this study, we focused our work on the sci-1 gene cluster. The Sci-1 apparatus is probably composed of all, or a subset of, the 21 gene products encoded on the cluster. Among these subunits, some are shared by all T6SSs identified to date, including a ClpV-type AAA ؉ ATPase (SciG) and an IcmF (SciS) and an IcmH (SciP) homologue, as well as a putative lipoprotein (SciN). In this study, we demonstrate that sciN is a critical gene necessary for T6S-dependent secretion of the Hcp-like SciD protein and for biofilm formation. We further show that SciN is a lipoprotein, as shown by the inhibition of its processing by globomycin and in vivo labeling with [ 3 H]palmitic acid. SciN is tethered to the outer membrane and exposed in the periplasm. Sequestration of SciN at the inner membrane by targeting the ؉2 residue responsible for lipoprotein localization (Gly2Asp) fails to complement an sciN mutant for SciD secretion and biofilm formation. Together, these results support a model in which SciN is an outer membrane lipoprotein exposed in the periplasm and essential for the Sci-1 apparatus function.

Structural Characterization and Oligomerization of the TssL Protein, a Component Shared by Bacterial Type VI and Type IVb Secretion Systems

“…We recently reported coimmunoprecipitation data that identify four components, TssL, TssM, TssJ, and TagL, constituting the membrane-asso- ciated subcomplex (27). Although TssJ is an outer membrane lipoprotein (12), the three other components are anchored in the inner membrane. TssM interacts with TssJ, and this interaction has been characterized through different approaches (29,31).…”

“…More recently, the translocation of canonical protein effectors degrading the peptidoglycan layer into the periplasm of recipient bacterial cells has been demonstrated (10,11). The T6SS has also been shown to be involved in stress sensing or biofilm formation, although the mechanistic bases for these phenotypes have not been described yet (12)(13)(14). The T6SS is therefore a versatile machine that adapts to the specific needs of each bacterium.…”

“…At least three proteins, TssJ, TssL, and TssM, form a trans-envelope complex that may be augmented by TagL, an additional protein containing a peptidoglycan-binding domain (27,28). TssJ is an outer membrane lipoprotein (12) whose structure has been reported recently (29,30). TssJ interacts with the inner membrane TssM protein (29,31).…”

Identification of T6SS-dependent effector and immunity proteins by Tn-seq in Vibrio cholerae

“…Type T6SS proteins have been identified that correspond to orthologous components in phage tails that include the tail spike, tube, sheath, and base plate (4,6). Innerand outer-membrane-associated T6SS proteins likely anchor the T6SS organelle to the cell envelope (8)(9)(10). Because the functionality of the T6SS apparatus has been correlated with its ability to kill or inhibit both eukaryotic as well as prokaryotic cells, most models for T6SS function postulate that the contraction of the sheath minimally ejects a complex corresponding to the T6SS spike/tube complex out of the T6SS + cells and into target cells in close contact (4,6).…”

In vivo actin cross-linking induced by Vibrio cholerae type VI secretion system is associated with intestinal inflammation

“…The components constituting this multiprotein secretion complex are beginning to be defined. On the basis of work in several organisms, the T6SS IcmF ortholog seems to be a central scaffolding protein that interacts with itself, the T6SS DotU ortholog, and a T6SS-encoded outer membrane lipoprotein (6)(7)(8). In V. cholerae, T6SS proteins VipA and VipB form a tubular structure that is dissociated by ClpV protein in an ATP-dependent manner (9), which could be required for biogenesis of a functional secretion complex.…”