Some Examples of Bacterial Toxins as Tools

Abstract

Pathogenic bacteria produce diverse protein toxins to disturb the host's defenses. This includes the opening of epithelial barriers to establish bacterial growth in deeper tissues of the host and to modulate immune cell functions. To achieve this, many toxins share the ability to enter mammalian cells, where they catalyze the modification of cellular proteins. The enzymatic activity is diverse and ranges from ribosyl- or glycosyl-transferase activity, the deamidation of proteins, and adenylate-cyclase activity to proteolytic cleavage. Protein toxins are highly active enzymes often with tight specificity for an intracellular protein or a protein family coupled with the intrinsic capability of entering mammalian cells. A broad understanding of their molecular mechanisms established bacterial toxins as powerful tools for cell biology. Both the enzymatic part and the pore-forming/protein transport capacity are currently used as tools engineered to study signaling pathways or to transport cargo like labeled compounds, nucleic acids, peptides, or proteins directly into the cytosol. Using several representative examples, this review is intended to provide a short overview of the state of the art in the use of bacterial toxins or parts thereof as tools.

Keywords: bacterial toxin; protein delivery; tool.

Figure 1

Toxins used as tools. SLO: The pore-forming toxin Streptolysin O binds to cholesterol-rich domains and inserts into the plasma membrane of mammalian cells. It oligomerizes to form water-filled pores through which ions and small molecules can enter cells by diffusion. CNF: Cytotoxic necrotizing factors are taken up by receptor-mediated endocytosis. Following proteolytic cleavage, the catalytic domain is released into the cytosol. This process requires acidification of the endosomes. In the cytosol, CNFs activate Rho proteins by deamidation. Cholera toxin: The toxin is composed of a cell-binding pentamer and an associated ADP-ribosyl-transferase. Toxin-containing endosomes are transported to the ER via the Golgi apparatus. From here, the transferase is released into the cytosol by the outwards transporter of misfolded proteins (endoplasmic-reticulum-associated protein degradation, ERAD). Cholera toxin was mainly used to study this transport route. Anthrax toxin: Anthrax protective antigen (PA) binds to the surface of mammalian cells. Cleavage by furin is required for oligomerization to a heptameric prepore. This allows the binding of lethal factor (LF), edema factor (EF), or an engineered cargo. The complex is then taken up by receptor-mediated endocytosis and released from the acidified endosome into the cytosol.

Figure 2

Photorhabdus luminescens toxin complex (PTC) is composed of three isolated proteins (TcA, TcB, and TcC), which are secreted by the bacteria. The A component (red) forms a pentamer with a needle-like structure surrounded by a shell (Adapted from [92]). TcB (blue) and TcC (yellow) monomers assemble into a cocoon-like structure enclosing the protein to be injected. The enzymatically active part of PTC (hypervariable region, hvr) is located within the C-terminus of TcC. It is proteolytically cleaved inside the cocoon by an intrinsic protease, as indicated. The engineered protein TcBC is a fusion of TcB and TcC. The catalytic hvr can be exchanged by diverse cargo proteins. x = times.