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

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.

Crystal Structure of the Human Cytomegalovirus pUL50-pUL53 Core Nuclear Egress Complex Provides Insight into a Unique Assembly Scaffold for Virus-Host Protein Interactions

“…Cases in which this space group is observed appear not to be fortuitous but rather reflect a preferred functional assembly of biological relevance. For example, space group P6 has been detected in viral capsid proteins such as the hexameric assembly of the human immunodeficiency virus type 1 (HIV-1) capsid protein (39) and in proteins that are part of highly symmetric complexes of bacterial secretion systems (40). For these reasons, the possibility arises that such a hexameric arrangement of the relevant full-length proteins might correspond to a preferred oligomeric state of pUL50-pUL53 readily assembled under specific conditions of viral replication.…”

The Self-Identity Protein IdsD Is Communicated between Cells in Swarming Proteus mirabilis Colonies

“…Many T6SS-exported effectors are under 50 kDa, and the inner Hcp tube constituting the channel of many T6SSs has a width of 40 Å in multiple bacteria (8,(44)(45)(46)(47). In fact, a variety of T6SS effectors bind to the inside of the Hcp tube, which allows them to be exported (43).…”

The structure of VgrG1 fromPseudomonas aeruginosa, the needle tip of the bacterial type VI secretion system

“…Only a few X-ray structures related to the T6SS puncturing device have been solved. So far, there are several crystal structures of Hcp homologues from P. aeruginosa (Mougous et al, 2006), Acinetobacter baumannii (Ruiz et al, 2015), Edwarsiella tarda (Jobichen et al, 2010), Burkholderia pseudomallei (Lim et al, 2015) and Yersinia pestis (PDB entry 3v4h; Center for Structural Genomics of Infectious Diseases, unpublished work) and a double mutant from enteroaggregative Escherichia coli (Douzi et al, 2014). While this work was being performed, the coordinates of a low-resolution structure of full-length P. aeruginosa PAO1 VgrG1 were deposited in the Protein Data Bank (PDB entry 4mtk; L. V. Sycheva, M. M. Shneider, M. Basler, B. T. Ho, J. J. Mekalanos & P. G. Leiman, unpublished work).…”