Structural changes upon membrane insertion of the insecticidal pore-forming toxins produced by Bacillus thuringiensis

Abstract

Different Bacillus thuringiensis (Bt) strains produce a broad variety of pore-forming toxins (PFTs) that show toxicity against insects and other invertebrates. Some of these insecticidal PFT proteins have been used successfully worldwide to control diverse insect crop pests. There are several studies focused on describing the mechanism of action of these toxins that have helped to improve their performance and to cope with the resistance evolved by different insects against some of these proteins. However, crucial information that is still missing is the structure of pores formed by some of these PFTs, such as the three-domain crystal (Cry) proteins, which are the most commercially used Bt toxins in the biological control of insect pests. In recent years, progress has been made on the identification of the structural changes that certain Bt insecticidal PFT proteins undergo upon membrane insertion. In this review, we describe the models that have been proposed for the membrane insertion of Cry toxins. We also review the recently published structures of the vegetative insecticidal proteins (Vips; e.g. Vip3) and the insecticidal toxin complex (Tc) in the membrane-inserted state. Although different Bt PFTs show different primary sequences, there are some similarities in the three-dimensional structures of Vips and Cry proteins. In addition, all PFTs described here must undergo major structural rearrangements to pass from a soluble form to a membrane-inserted state. It is proposed that, despite their structural differences, all PFTs undergo major structural rearrangements producing an extended α-helix, which plays a fundamental role in perforating their target membrane, resulting in the formation of the membrane pore required for their insecticidal activity.

Bacillus thuringiensis Cry9Aa Insecticidal Protein Domain I Helices α3 and α4 Are Two Core Regions Involved in Oligomerization and Toxicity

“…In previous studies, Cry1A domain I helices α3 and α4 mutants altered pore formation and insecticidal activity. − , Also, Cry1Ab, Cry4Ba, or Cry5Ba α3 mutations affected oligomerization and insecticidal activity. , Cry9Aa helices α3 and α4 mutations showed phenotypes and structures similar to those of Cry1A (Figure S2), which further indicates that Cry9Aa and Cry1A have similar active structures.…”

“…In addition, Cry4Ba (PDB: 1w99) helix α3 is longer than that of Cry9Aa (rmsd: 1.418, Figure B). Cry proteins are pore-forming proteins where the interaction of α helices from domain I among different monomers is important for prepore oligomer formation …”

“…Although this salt bridge was found to be conserved in several Cry proteins, it is not present in the structure of Cry4Ba or Cry5Ba . However, different intramolecular salt bridges in Cry4Ba and Cry5Ba were identified that were shown to be needed for oligomerization and insecticidal activity. ,, Domains II and III are responsible for the binding of Cry proteins to their specific larval midgut receptors. Domain II is composed of antiparallel β-sheets connected by exposed loops involved in receptor recognition.…”

In Vivo and In Vitro Interactions between Exopolysaccharides from Bacillus thuringensis HD270 and Vip3Aa11 Protein

“…Furthermore, as the concentration of EPS elevates, the interaction between activated Vip3Aa11 or Cry1Ac protein [27] and BBMVs diminishes progressively, indicating that EPS may compete with activated proteins for binding sites on BBMVs. Currently, there is uncertainty regarding whether the activation of Vip3Aa11 protoxin precedes or follows receptor binding, along with the identification of the receptor protein [32]. It is…”

N-Terminal α-Helices in Domain I of Bacillus thuringiensis Vip3Aa Play Crucial Roles in Disruption of Liposomal Membrane

“…Domains II and III, rich in β-sheets, participate in receptor binding and target insect selection [ 21 , 22 ]. Similar to Vip3 toxins, Cry toxins were proposed to form 150- to 250-kDa oligomers after binding with receptor proteins located on the midgut epithelial membrane of target insects, although the precise mechanism of pore formation in this process remains elusive [ 23 ].…”