The Modes of Action of MARTX Toxin Effector Domains

Abstract

Many Gram-negative bacterial pathogens directly deliver numerous effector proteins from the bacterium to the host cell, thereby altering the target cell physiology. The already well-characterized effector delivery systems are type III, type IV, and type VI secretion systems. Multifunctional autoprocessing repeats-in-toxin (MARTX) toxins are another effector delivery platform employed by some genera of Gram-negative bacteria. These single polypeptide exotoxins possess up to five effector domains in a modular fashion in their central regions. Upon binding to the host cell plasma membrane, MARTX toxins form a pore using amino- and carboxyl-terminal repeat-containing arms and translocate the effector domains into the cells. Consequently, MARTX toxins affect the integrity of the host cells and often induce cell death. Thus, they have been characterized as crucial virulence factors of certain human pathogens. This review covers how each of the MARTX toxin effector domains exhibits cytopathic and/or cytotoxic activities in cells, with their structural features revealed recently. In addition, future directions for the comprehensive understanding of MARTX toxin-mediated pathogenesis are discussed.

Keywords: MARTX toxin; bacterial protein toxin; effector domain; host–microbe interaction; host–pathogen interaction.

Figure 1

Multifunctional autoprocessing repeats-in-toxin (MARTX) toxin delivers various effector domains into the host cell cytosol. (a) Schematic diagrams of MARTX toxins from various pathogens. The sequences used in the analysis were downloaded from the NCBI website: Vibrio cholerae N16961 (WP_010895441.1); V. vulnificus MO6-24/O (WP_015728045.1); V. vulnificus CMCP6 (WP_011081430.1); V. vulnificus BAA87 (WP_039507922.1); V. splendidus 12F01 (WP_004732217.1); V. ordalii ATCC33509 (WP_010319615.1); Aeromonas hydrophila ATCC7966 (WP_011705266.1); and Xenorhabdus bovienii SS-2004 (WP_012987644.1). (b) The steps in MARTX toxin intoxication. After effector domain translocation, InsP₆ activates the cysteine protease domain (CPD) which then autoprocesses the inter-effector domain regions to release each effector domain from the holo-MARTX toxin.

Figure 2

The crystal structure of actin cross-linking domain (ACD) in V. cholerae VgrG1 (Protein Data Bank (PDB) code 4DTH). Overall structure (a) and close-up view for active site (b) are shown. Critical and important residues for the actin cross-linking reaction are represented with cyan and green sticks, respectively. An adenosine triphosphate (ATP) molecule is shown with a red stick. A sulfate ion mimicking the position of Glu270 in actin and two Mg²⁺ ions are shown in the sphere model. The residues are numbered according to the sequence of multifunctional autoprocessing repeats-in-toxin (MARTX) toxin from V. cholerae N16961. All effector domain structures in this review were visualized using PyMOL software ver. 1.5.0.4 (Schrödinger, LLC, New York, NY, USA).

Figure 3

The crystal structure of Rho GTPase-inactivation domain (RID) in the V. vulnificus MARTX toxin (PDB code 5XN7). Overall structure (a) and close-up view for active site (b) are shown. Catalytic residues are represented with cyan sticks. A membrane localization domain (MLD) and a four-helix pair are represented in light yellow and light orange, respectively. The residues are numbered according to the sequence of MARTX toxin from V. vulnificus MO6-24/O.

Figure 4

The modeled structure of alpha/beta hydrolase domain (ABH) generated by using the structure of P. haloplanktis FGH (PDB code 3LS2) as a template. Overall structure (a) and close-up view for active site (b) are shown. Catalytic residues are represented with cyan sticks, and eight beta-strands in the typical alpha/beta hydrolase fold are indicated. The residues are numbered according to the sequence of MARTX toxin from V. vulnificus MO6-24/O.

Figure 5

The modeled structure of makes caterpillars floppy-like domain (MCF) generated by using the structure of P. syringae AvrPphB (PDB code 1UKF) as a template. Overall structure (a) and close-up view for active site (b) are shown. Although MCF is predicted to have a typical catalytic triad (Cys3351, His3463, and D3482) for the C58 cysteine peptidase family, the previously proposed catalytic tripeptides are Arg3350, Cys3351, and D3352 represented with violet sticks [61]. The residues are numbered according to the sequence of MARTX toxin from V. vulnificus MO6-24/O.

Figure 6

The crystal structure of Ras/Rap1-specific endopeptidase (RRSP) in V. vulnificus MARTX toxin (PDB code 6A8J). Overall structure (a) and close-up view for active site (b) are shown. The 2His/2Glu active site residues are represented with magenta sticks. The membrane localization domain (MLD) and three additional helices in the N-lobe are represented in light blue and violet, respectively. The residues are numbered according to the sequence of MARTX toxin from V. vulnificus CMCP6.

Figure 7

The modeled structure of ExoY generated by using the structure of P. aeruginosa type III secretion effector ExoY (PaExoY) (PDB code 5XNW) as a template. Overall structure (a) and close-up view for active site (b) are shown. The structure was analyzed based on the analogy of the CyaA and Edema factor (EF) structures [76,77]. ATP-binding essential residues on the critical regions (CR1, CR2, and CR3) are represented with magenta, cyan, and yellow sticks, respectively. A sulfate ion mimicking the phosphate group of ATP is shown in the sphere model. The residues are numbered according to the sequence of MARTX toxin from V. vulnificus BAA87.

Figure 8

The modeled structure of DmX generated by using the structure of P. syringae AvrPphB (PDB code 1UKF) as a template. Overall structure (a) and close-up view for active site (b) are shown. The predicted catalytic triad is represented with magenta sticks. The residues are numbered according to the sequence of MARTX toxin from V. vulnificus BAA87.