1.C.4.4.5
Mosquitocidal toxin, Mpp46Ab, natural product of Bacillus thuringensis (Uniprot Q6AW28) also called parasporin-2Ab andCry46Ab, and synthetic construct of 304 aas; 84% identical to 1.C.4.4.4.  Cry46Ab (Mpp46Ab) from Bacillus thuringiensis TK-E6 is a mosquitocidal toxin with an aerolysin-type architecture (Hayakawa et al. 2020). Cry46Ab mutants were constructed by targeting the putative transmembrane beta-hairpin region, showing that charged residues within the beta-hairpin control the flux of ions through channel pores and that channel-pore cation selectivity is correlated with insecticidal activity (Hayakawa et al. 2020). Pore cation selectivity is a major determinant of Bacillus thuringiensis Cry46Ab mosquitocidal activity (Hayakawa et al. 2020). The cryo-EM structure of the XaxAB pore complex from Xenorhabdus nematophila and the crystal structures of the soluble monomers of XaxA and XaxB have been determined (Schubert et al. 2018).. The structures revealed that XaxA and XaxB are built similarly and appear as heterodimers in the 12-15 subunits containing pore, classifying XaxAB as bi-component α-PFT. Major conformational changes in XaxB, including the swinging out of an amphipathic helix are responsible for membrane insertion. XaxA acts as an activator and stabilizer for XaxB that forms the actual transmembrane pore (Schubert et al. 2018). Two mutants, K155E and K155I, exhibited toxicity significantly higher than that of the wild-type toxin, and the cation selectivity was also increased (Miyazaki et al. 2023).  The charge of residue 155 may not directly affect the cation selectivity of Mpp46Ab channel pores. Replacement of K(155) with glutamic acid or isoleucine may induce a similar conformational change in the region associated with the ion selectivity of the Mpp46Ab channel pores. Mutagenesis targeting the transmembrane beta-hairpin seems to be an effective strategy for enhancing the ion permeability of the channel pores and the resulting mosquito- larvicidal activity of Mpp46Ab (Miyazaki et al. 2023).