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Crystal Structure of Hcp from Acinetobacter baumannii: A Component of the Type VI Secretion System.

Ruiz FM, Santillana E, Spínola-Amilibia M, Torreira E, Culebras E, Romero A - PLoS ONE (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Physical Biology, Centro de Investigaciones Biológicas-CSIC, Madrid, Spain.

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.

No MeSH data available.


Related in: MedlinePlus

Key residues in the surface of the hexameric ring.(A) A hydrogen bond network stabilizes the position of Arg37, pointing to the central axis of the Hcp ring. Note the continuous β-sheet surface formed between neighboring chains (colored grey and cyan). (B) Electrostatic charge at the surface of the Hcp ring showing, in one of the edges, cavities with strong negative charge.
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pone.0129691.g003: Key residues in the surface of the hexameric ring.(A) A hydrogen bond network stabilizes the position of Arg37, pointing to the central axis of the Hcp ring. Note the continuous β-sheet surface formed between neighboring chains (colored grey and cyan). (B) Electrostatic charge at the surface of the Hcp ring showing, in one of the edges, cavities with strong negative charge.

Mentions: Using the 6-fold crystallographic symmetry, one hexameric ring can be generated from each Hcp chain (Fig 2C) interacting between them in both, head-to-head and head-to tail behavior (Fig 2D). The average contact area between intra-ring molecules is 1325 Å2, with 19 inter-chain hydrogen bonds. Among all the interfaces analyzed by the PISA web server in this structure, this is the only one that appears to play an essential role in complex formation. The inner diameter of this donut-shaped hexamer is 40 Å and the outer one is 80 Å. The 10-residue α-helix of each monomer is almost parallel to the crystallographic axis and it serves as a contact surface for the neighboring molecule. This helix interacts with residues Ser112 to Thr117 of β6 from the same chain, and with residues Val152 to Trp156 of a β-sheet (β8) from the symmetry related molecule. Pro71 faces Val152 and the aromatic ring of Tyr139, whereas Glu75 forms a hydrogen bond with the N atom of the Lys155 main chain. The side chain of Trp74 projects into the hydrophobic cavity formed by Leu66, Pro71 and Pro116 from the same chain, and by Trp32, Ile 36, Leu102 and Trp137 from the flanking chain. In addition, Trp74 interacts through a hydrogen bond with Glu126 from the same chain. The extended loop from the neighboring chain acts as a cap for the helix, interacting with their first residues. Moreover, hydrogen bonds are established between the O atom of the main chain of Gly80 and Cys77 with Lys40 and Gln38, respectively. Furthermore, Ser78 forms a hydrogen bond with the Nδ atom of His57and the main chain of Ser78 stacks against the side chain of Val55. The region from Ser112 to Glu120 (β6) interacts with residues Asn30 to Gln38 (β2) of the neighboring molecule. Accordingly, the six molecules form a continuous 24 β-strands surface and this β-barrel represents the inner surface of the ring. The Arg37 side chain points to the central axis of the hexagon, projecting from the barrel flat surface. A net of hydrogen bonds, involving Asn35, Ser58 and Thr115 from the next chain, maintains the particular position of Arg37 (Fig 3A).


Crystal Structure of Hcp from Acinetobacter baumannii: A Component of the Type VI Secretion System.

Ruiz FM, Santillana E, Spínola-Amilibia M, Torreira E, Culebras E, Romero A - PLoS ONE (2015)

Key residues in the surface of the hexameric ring.(A) A hydrogen bond network stabilizes the position of Arg37, pointing to the central axis of the Hcp ring. Note the continuous β-sheet surface formed between neighboring chains (colored grey and cyan). (B) Electrostatic charge at the surface of the Hcp ring showing, in one of the edges, cavities with strong negative charge.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4469607&req=5

pone.0129691.g003: Key residues in the surface of the hexameric ring.(A) A hydrogen bond network stabilizes the position of Arg37, pointing to the central axis of the Hcp ring. Note the continuous β-sheet surface formed between neighboring chains (colored grey and cyan). (B) Electrostatic charge at the surface of the Hcp ring showing, in one of the edges, cavities with strong negative charge.
Mentions: Using the 6-fold crystallographic symmetry, one hexameric ring can be generated from each Hcp chain (Fig 2C) interacting between them in both, head-to-head and head-to tail behavior (Fig 2D). The average contact area between intra-ring molecules is 1325 Å2, with 19 inter-chain hydrogen bonds. Among all the interfaces analyzed by the PISA web server in this structure, this is the only one that appears to play an essential role in complex formation. The inner diameter of this donut-shaped hexamer is 40 Å and the outer one is 80 Å. The 10-residue α-helix of each monomer is almost parallel to the crystallographic axis and it serves as a contact surface for the neighboring molecule. This helix interacts with residues Ser112 to Thr117 of β6 from the same chain, and with residues Val152 to Trp156 of a β-sheet (β8) from the symmetry related molecule. Pro71 faces Val152 and the aromatic ring of Tyr139, whereas Glu75 forms a hydrogen bond with the N atom of the Lys155 main chain. The side chain of Trp74 projects into the hydrophobic cavity formed by Leu66, Pro71 and Pro116 from the same chain, and by Trp32, Ile 36, Leu102 and Trp137 from the flanking chain. In addition, Trp74 interacts through a hydrogen bond with Glu126 from the same chain. The extended loop from the neighboring chain acts as a cap for the helix, interacting with their first residues. Moreover, hydrogen bonds are established between the O atom of the main chain of Gly80 and Cys77 with Lys40 and Gln38, respectively. Furthermore, Ser78 forms a hydrogen bond with the Nδ atom of His57and the main chain of Ser78 stacks against the side chain of Val55. The region from Ser112 to Glu120 (β6) interacts with residues Asn30 to Gln38 (β2) of the neighboring molecule. Accordingly, the six molecules form a continuous 24 β-strands surface and this β-barrel represents the inner surface of the ring. The Arg37 side chain points to the central axis of the hexagon, projecting from the barrel flat surface. A net of hydrogen bonds, involving Asn35, Ser58 and Thr115 from the next chain, maintains the particular position of Arg37 (Fig 3A).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Physical Biology, Centro de Investigaciones Biológicas-CSIC, Madrid, Spain.

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.

No MeSH data available.


Related in: MedlinePlus