Isoleucine at position 150 of Cyt2Aa toxin from Bacillus thuringiensis plays an important role during membrane binding and oligomerization

The key role of E418 carboxyl group in the formation of Nt.BspD6I nickase active site: Structural and functional properties of Nt.BspD6I E418A mutant

The mutated nickase Nt.BspD6I E418A has been obtained by site-directed mutagenesis. The purified protein has been crystallized, and its spatial structure has been determined at 2.45 Å resolution. An analysis of the crystal structures of the wild-type and mutated nickase have shown that the elimination of a carboxyl group due to the E418A mutation initiates marked conformational changes in both the N-terminal recognition domain and the C-terminal catalytic domain of nickase and insignificantly affects its linker domain. This is supported by changes in the functional properties of mutated nickase: an increase in the oligomerization capacity in the presence of a substrate, a reduction in the capacity to bind a substrate, and complete loss of catalytic activity.

Protein-Lipid Interaction of Cytolytic Toxin Cyt2Aa2 on Model Lipid Bilayers of Erythrocyte Cell Membrane

Cytolytic toxin (Cyt) is a toxin among Bacillus thuringiensis insecticidal proteins. Cyt toxin directly interacts with membrane lipids for cytolytic action. However, low hemolytic activity is desired to avoid non-specific effects in mammals. In this work, the interaction between Cyt2Aa2 toxin and model lipid bilayers mimicking the erythrocyte membrane was investigated for Cyt2Aa2 wild type (WT) and the T144A mutant, a variant with lower hemolytic activity. Quartz crystal microbalance with dissipation (QCM-D) results revealed a smaller lipid binding capacity for the T144A mutant than for the WT. In particular, the T144A mutant was unable to bind to the phosphatidylcholine lipid (POPC) bilayer. However, the addition of cholesterol (Chol) or sphingomyelin (SM) to the POPC bilayer promoted binding of the T144 mutant. Moreover, atomic force microscopy (AFM) images unveiled small aggregates of the T144A mutant on the 1:1 sphingomyelin/POPC bilayers. In contrast, the lipid binding trend for WT and T144A mutant was comparable for the 1:0.4 POPC/cholesterol and the 1:1:1 sphingomyelin/POPC/cholesterol bilayers. Furthermore, the binding of WT and T144A mutant onto erythrocyte cells was investigated. The experiments showed that the T144A mutant and the WT bind onto different areas of the erythrocyte membrane. Overall the results suggest that the T144 residue plays an important role for lipid binding.