Host Tropism and Structural Biology of ABC Toxin Complexes

Abstract

ABC toxin complexes are a class of protein toxin translocases comprised of a multimeric assembly of protein subunits. Each subunit displays a unique composition, contributing to the formation of a syringe-like nano-machine with natural cargo carrying, targeting, and translocation capabilities. Many of these toxins are insecticidal, drawing increasing interest in agriculture for use as biological pesticides. The A subunit (TcA) is the largest subunit of the complex and contains domains associated with membrane permeation and targeting. The B and C subunits, TcB and TcC, respectively, package into a cocoon-like structure that contains a toxic peptide and are coupled to TcA to form a continuous channel upon final assembly. In this review, we outline the current understanding and gaps in the knowledge pertaining to ABC toxins, highlighting seven published structures of TcAs and how these structures have led to a better understanding of the mechanism of host tropism and toxin translocation. We also highlight similarities and differences between homologues that contribute to variations in host specificity and conformational change. Lastly, we review the biotechnological potential of ABC toxins as both pesticides and cargo-carrying shuttles that enable the transport of peptides into cells.

Keywords: ABC toxins; X-ray crystallography; biotechnology; cryo-EM; host tropism; insecticide; structural biology; translocase.

Figure 1

Life cycle of entomopathogenic nematodes and simplified view of ABC Tc intoxication. (A) Entomopathogenic nematode reproductive cycle starting from a juvenile nematode all the way through reproduction and release of offspring. (B) Zoomed in view of the larvae midgut epithelial cell being exposed to various virulence factors released by the bacterial symbiont of nematodes. (C) Simplified mechanism of ABC Tc intoxication of cells. Note: Direct injection mechanism is depicted for simplicity, although an alternative endosomal mechanism has been proposed. This figure was produced in Biorender.

Figure 2

Schematic of toxin complex formation and pre-pore to pore state transition of TcAs. (A) Structures of the Pl-TcdA1 and Xn-XptA2 in the pre-pore state showing that the full complex with Pl-TcdB2-TccC3 can be formed between both TcAs and Pl-TcdA1 in the pore state after exposure to basic conditions. (B) Structures of Pl-TcdA1-TcdB2-TccC3 complex in the pre-pore state (neutral pH) and pore state (basic pH) and Ye-YenTcA, which has been shown to form a complex with Ye-YenB-YenC.

Figure 3

Phylogenetic layout, sequence identity, and domain composition of TcAs. (A) Domain layout of the 7 TcAs that have been structurally solved to date. (B) Phylogenetic tree grouping a subset of TcAs by specificity and pathogenicity. (C) Graphical and pie chart representation of the sequence identity and amino acid composition comparisons between the 7 TcAs that have been structurally solved to date.

Figure 4

Structural comparisons of TcAs. (A) The 7 TcAs that have been structurally solved are depicted in their respective multimeric forms and grouped according to the species of bacteria they are produced in. The depiction shows side views of each TcA as well as bottom views in the second row. (B) A single monomer from each protein color coded by domain. The bottom pane displays a representative TcA monomer form Xenorhabdus nematophila Xn-XptA2 and serves as a key depicting the color code layout of domains that is represented in all TcAs shown.

Figure 5

Analyses of translocation channels, pore-forming loops, and pore-closing loops between TcAs. (A) Comparison of all TcA translocation channels measuring the channel density and highlighting the most constricted region of the channel apart from the pore forming loops. (B) Pore-forming loop amino acid sequence conservation between TcAs. (C) Pore-closing loop amino acid sequence conservation between TcAs.

Figure 6

TcC mechanisms of cytotoxicity. (A) TccC3 ADP ribosylates G-actin, inhibiting the actin thymosin–β4 interaction, resulting in G-actin sequestration and actin polymerization. (B) ADP ribosylation of RhoA inhibits GTP hydrolysis, rendering the GTP-binding protein obstinately active.

Figure 7

Comparison of ABC toxin mechanism to Bt toxin mechanism. (A) Schematic representation of the two proposed mechanisms for ABC toxin binding and cytotoxicity. (B) Schematic representation of Bt Cry toxin assembly and membrane permeation.