Membrane Permeabilization by Pore-Forming RTX Toxins: What Kind of Lesions Do These Toxins Form?

Abstract

Pore-forming toxins (PFTs) form nanoscale pores across target membranes causing cell death. The pore-forming cytolysins of the RTX (repeats in toxin) family belong to a steadily increasing family of proteins characterized by having in their primary sequences a number of glycine- and aspartate-rich nonapeptide repeats. They are secreted by a variety of Gram-negative bacteria and form ion-permeable pores in several cell types, such as immune cells, epithelial cells, or erythrocytes. Pore-formation by RTX-toxins leads to the dissipation of ionic gradients and membrane potential across the cytoplasmic membrane of target cells, which results in cell death. The pores formed in lipid bilayers by the RTX-toxins share some common properties such as cation selectivity and voltage-dependence. Hemolytic and cytolytic RTX-toxins are important virulence factors in the pathogenesis of the producing bacteria. And hence, understanding the function of these proteins at the molecular level is critical to elucidating their role in disease processes. In this review we summarize the current state of knowledge on pore-formation by RTX toxins, and include recent results from our own laboratory regarding the pore-forming activity of adenylate cyclase toxin (ACT or CyaA), a large protein toxin secreted by Bordetella pertussis, the bacterium causative of whooping cough.

Valid Presumption of Shiga Toxin-Mediated Damage of Developing Erythrocytes in EHEC-Associated Hemolytic Uremic Syndrome

“…Their common feature is the release via the type I secretion system and the typical glycine- and aspartate-rich nonapeptide repeats that can bind a large number of Ca 2+ ions [ 123 ]. The generation of pores by RTX toxins leads to the collapse of ion gradients and the membrane potential across the plasma membrane of target cells, which results in cell death [ 124 ]. A prototype member of the RTX pore-forming toxins is the α-hemolysin, which is often encoded by strains of uropathogenic E. coli (UPEC) [ 125 ].…”

RTX Toxins of Animal Pathogens and Their Role as Antigens in Vaccines and Diagnostics

“…The N-terminal domain that contains an amphipathic segment and several hydrophobic domains (Figure 1) is essential for the hemolytic and cytotoxic activity of RTX toxins. These hydrophobic domains can form α-helical structures that insert into lipid bilayers and form pores, which are finally responsible for membrane permeabilization [4,[32][33][34].…”

“…The repeats contain the common sequence structure G-G-X-G-(N/D) -D-X-(L/I/V/W/Y/F)-X (where X can be any amino acid) and are present in variable numbers ranging from six to more than 50 copies. These nonapeptide repeats form a parallel β-roll motif within a right-handed spiral, which binds Ca 2+ ions held by two sterically neighboring nonapeptide repeats [2][3][4]. Several RTX toxins have been shown to bind Ca 2+ in solution and to be dependent on Ca 2+ to exhibit their pore-forming and cytolytic activity.…”

Recent Developments in Cell Permeable Deubiquitinating Enzyme Activity-Based Probes

“…Ub-based ABPs possess a distinct advantage over peptide and small molecule based ABPs because of the specificity of their recognition element, and shortening the recognition element to allow greater cell permeability fails to preserve specificity or activity (Albrow et al, 2011;Safa et al, 2019). Each of the various approaches discussed above to force entry of Ub-based ABPs, including toxin pore formation, electroporation and so-called "cell-penetrating" peptides (CPPs), raise numerous concerns regarding host membrane repair responses that may be triggered as an unintended consequence (Ostolaza et al, 2019). For example, CPPs have been demonstrated to substantially disrupt membrane integrity, causing formation of non-physiological subcellular compartments (Gao et al, 2019).…”