Species-specific gene duplication in Clostridia produces variations of cholesterol-dependent cytolysin with different cytotoxicity

Cholesterol-dependent cytolysin (CDC) is a bacterial toxin that binds to eukaryotic cholesterol-containing membranes, forms oligomeric complexes, and is inserted into the bilayer to create large aqueous pores. Recently, we reported a species-specific duplication of the hemolysin gene in group III Clostridium botulinum. The duplicated genes (bly1 and bly2) encoded two separate CDC proteins (botulinolysins; BLY1 and BLY2). Here, we aimed to investigate whether BLY1 and BLY2 exert differential cytotoxicity. We isolated two bly genes from C. botulinum and evaluated the cytotoxicity of two recombinant BLY proteins (rBLY1 and rBLY2) in HeLa, IEC-6, and NRK cells. rBLYs were cytotoxic to equine erythrocytes. rBLY1 showed higher hemolytic activity than rBLY2. rBLY2 showed no or very weak cytotoxicity to the HeLa, IEC-6, and NRK cells, whereas rBLY1 showed high cytotoxicity to these cells. The comparison of the amino acid sequence of BLYs with those of other CDCs revealed that the already-known amino acid residues involved in cholesterol-containing membrane recognition, oligomerization, and insertion into membranes are well conserved in both BLYs. However, several amino acid substitutions were observed in the conserved regions, particularly in L2 and L3 regions involved in cell binding. These findings suggest that gene duplication in group III C. botulinum evolved distinct functional specializations, and differential cytotoxicity of BLY1 and BLY2 could be due to the amino acid substitution in the conserved regions. However, the structural and functional comparisons of the two BLYs are essential to gain insights into the function of the CDCs.