## 1 Introduction  
Pore-forming toxins (PFTs) are proteins produced by several pathogenic bacteria to disrupt the membrane integrity of their target cells, playing an important role in their virulence to invade or colonize their hosts. Most PFTs are produced as soluble proteins that must undergo substantial conformational changes to penetrate the hydrophobic barrier of the membrane bilayer. In different PFTs, oligomerization is a prerequisite for membrane insertion [1, 2]. Oligomer assembly of PFTsrequires structural rearrangements, allowing intermolecular interaction between monomers to form ring-shaped structures capable of making pores in the membrane [3; 4; 5]. The PFTs are classified in \(\beta\)- or \(\alpha\)-PFT according to the secondary structure that is inserted into the membrane. _Bacillus thuringiensis_ (Bt) bacteria produce different PFTs highly lethal for invertebrates, mainly insects. During vegetative growth, different Bt strains secrete Vip toxins, and in synchrony with the sporulation phase synthetize Cyt and Cry toxins, accumulating them as parasporal crystals [6; 7]. All of these toxins are PFTs, and some of them are used worldwide to control insect pests in agriculture or mosquitoes that transmit infectious diseases to humans. Among the Bt toxins described so far, Cry toxins are most abundant, and at least three different non-phylogenetically related Cry families have been found, such as the three-domain Cry toxins (named Cry), the Cry Bin-like (now named Tpp and Gpp) and the Cry Mtx-like toxins (now named Mpp and Mtx) [6; 7; 8]. The Cry family of toxins is considered as \(\alpha\)-PFT since domain I, composed of seven \(\alpha\)-helices, is involved in oligomerization and membrane insertion. These proteins are produced as prootoxins with two possible sizes of 130 or 70 kDa. After ingestion by the target insect pest, N- and C-terminal regions are cleaved out by midgut proteases, releasing a 65 kDa soluble globular protease-resistant core composed of three domains. Domains II and III are composed of \(\beta\)-strands and are involved in binding to receptors located in the apical membrane of midgut epithelial cells of the insect larvae [9]. The interaction of Cry toxins with those receptors triggers the formation of oligomers whose three-dimensional structure remains unknown. It is proposed that the hairpin conformed by helices \(\alpha\)4-\(\alpha\)5 of domain I is implicated in membrane insertion to form the pore [10; 11]. In the case of the Cry1A's toxins it was shown that, after receptor interaction and protease activation, toxin complexes forming oligomers with an apparent molecular weight of 150-250 kDa generate electrical conductivity in black lipid bilayers, suggesting that oligomeric structures may be composed by 3-4 monomeric subunits [9]. The formation of a trimeric complex has been proposed for Cry1 Ab and Cry4 Ba oligomer formation observed by SEM and AFM in synthetic liposomes [12; 13]. However, other reports support an arrangement of Cry1A toxins in tetrameric structures after liposome interaction [14]. For oligomer assembly, it was reported that after receptor binding additional proteolysis of helix \(\alpha\)-1 at the N-terminal end region of Cry1 Ab was required [15]. However, there are no reports describing the conformational changes in the N-terminal region of this protein leading to oligomer formation. For this reason, we decided to study the conformational changes at the N-terminal region of Cry1 Ab by different methodologies to determine the conformational changes that occur during oligomerization of this protein.