Intracellular Bacteria Encode Inhibitory SNARE-Like Proteins

Abstract

Pathogens use diverse molecular machines to penetrate host cells and manipulate intracellular vesicular trafficking. Viruses employ glycoproteins, functionally and structurally similar to the SNARE proteins, to induce eukaryotic membrane fusion. Intracellular pathogens, on the other hand, need to block fusion of their infectious phagosomes with various endocytic compartments to escape from the degradative pathway. The molecular details concerning the mechanisms underlying this process are lacking. Using both an in vitro liposome fusion assay and a cellular assay, we showed that SNARE-like bacterial proteins block membrane fusion in eukaryotic cells by directly inhibiting SNARE-mediated membrane fusion. More specifically, we showed that IncA and IcmG/DotF, two SNARE-like proteins respectively expressed by Chlamydia and Legionella, inhibit the endocytic SNARE machinery. Furthermore, we identified that the SNARE-like motif present in these bacterial proteins encodes the inhibitory function. This finding suggests that SNARE-like motifs are capable of specifically manipulating membrane fusion in a wide variety of biological environments. Ultimately, this motif may have been selected during evolution because it is an efficient structural motif for modifying eukaryotic membrane fusion and thus contribute to pathogen survival.

Chlamydial Intracellular Survival Strategies

“…In support of this notion, at least two Golgi-specific SNARES, including Syntaxin 6 and GS15 (Moore et al 2011;Pokrovskaya et al 2012), as well as vesicles containing the endocytic SNARES, Vamp3, Vamp7, and Vamp8, are recruited to the inclusion (Delevoye et al 2008;Paumet et al 2009). Interestingly, a subset of Incs contains motifs that display similarities to eukaryotic SNAREs, including IncA, CT813, and CT223 (Delevoye et al 2008;Paumet et al 2009). It is speculated that these SNARE-like domains pair with host-SNARE proteins on target vesicles to assemble a fourhelix bundle that provides the energy to promote fusion of opposing membranes.…”

“…It is speculated that these SNARE-like domains pair with host-SNARE proteins on target vesicles to assemble a fourhelix bundle that provides the energy to promote fusion of opposing membranes. Indeed, IncA, along with CT813, binds to host SNAREs, Vamp3, 7, and 8, suggesting that host SNAREs may work in concert with IncA to regulate membrane fusion (Delevoye et al 2004(Delevoye et al , 2008Paumet et al 2009). However, in vitro liposome fusion assays indicate that instead of promoting fusion with endocytic compartments, IncA acts as an inhibitory SNARE to limit fusion with these compartments (Paumet et al 2009).…”

“…Indeed, IncA, along with CT813, binds to host SNAREs, Vamp3, 7, and 8, suggesting that host SNAREs may work in concert with IncA to regulate membrane fusion (Delevoye et al 2004(Delevoye et al , 2008Paumet et al 2009). However, in vitro liposome fusion assays indicate that instead of promoting fusion with endocytic compartments, IncA acts as an inhibitory SNARE to limit fusion with these compartments (Paumet et al 2009). IncA has also been shown to participate in homotypic fusion of individual inclusions, as naturally occurring IncA-deficient variants or infected cells microinjected with anti-IncA antibodies display an aberrant multilobed inclusion structure (Hackstadt et al 1999;Delevoye et al 2004;Xia et al 2005).…”

Manipulation of Host Vesicular Trafficking and Membrane Fusion During Chlamydia Infection

“…C. trachomatis IncA is not known to be phosphorylated and no kinase for C. psittaci IncA has been identified . Furthermore, C. trachomatis IncA-mediated inhibition of endosomal fusion (discussed in detail below) was not reported to depend on phosphorylation [Paumet et al, 2009]. In contrast, C. caviae IncA was shown to be phosphorylated at serine 17, and this post www.intechopen.com translational modification was required for the inhibition of C. caviae inclusion development in cell lines [Alzhanov et al, 2004].…”

Cloning, Expression, and Biophysical Investigations of Truncated Inclusion Protein A (IncA) from Chlamydia trachomatis

“…24 Second, bioinformatics and modeling suggest that the polar residue located in the center of the motif may organize the coiled coils in the hydrophilic domain into a single amphipathic structure. 27,37 SNAREs have a helix formed with either a glutamine or arginine residue located in the center. 28,36 The structure of IncA is postulated to form a helix with a threonine residue located in the center.…”