Abstract
The type VI secretion system (T6SS) is a bacterial macromolecular machine widely distributed in Gram-negative bacteria, which transports effector proteins into eukaryotic host cells or other bacteria. Membrane complexes and a central tubular structure, which resembles the tail of contractile bacteriophages, compose the T6SS. One of the proteins forming this tube is the hemolysin co-regulated protein (Hcp), which acts as virulence factor, as transporter of effectors and as a chaperone. In this study, we present the structure of Hcp from Acinetobacter baumannii, together with functional and oligomerization studies. The structure of this protein exhibits a tight β barrel formed by two β sheets and flanked at one side by a short α-helix. Six Hcp molecules associate to form a donut-shaped hexamer, as observed in both the crystal structure and solution. These results emphasize the importance of this oligomerization state in this family of proteins, despite the low similarity of sequence among them. The structure presented in this study is the first one for a protein forming part of a functional T6SS from A. baumannii. These results will help us to understand the mechanism and function of this secretion system in this opportunistic nosocomial pathogen.
Highlights
Pathogenic bacteria use macromolecular protein complexes (1 to 20 molecules), known as secretion systems, in order to transport toxic proteins into the environment or directly into neighboring target cells
Single bacterial colony was added to 2 ml Muller-Hinton broth (MHB) and the cultures have grown for 15–18 h in 37°C rocking water bath (160 rpm)
To determine whether the T6SS is active in vitro, we assessed the presence of hemolysin co-regulated protein (Hcp) in culture supernatants using a polyclonal antiserum generated against a recombinant, purified and crystallized tag-free Hcp protein from A. baumannii AB0057
Summary
Pathogenic bacteria use macromolecular protein complexes (1 to 20 molecules), known as secretion systems, in order to transport toxic proteins into the environment or directly into neighboring target cells. The most recently discovered is the Type VI Secretion System (T6SS), initially described in Vibrio cholerae in 2006 [3]. This protein translocation pathway was originally shown to involve a cluster of around 15 genes. A T6SS was characterized in Pseudomonas aeruginosa, together with the first structure of a protein from this complex [4]. In P. aeruginosa the T6SS delivers toxins to other bacteria, showing that this secretion system is used to attack host cells and, to compete with other
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