Local conformations affect the histidine tag-Ni2+ binding affinity of BinA and BinB proteins

Binary toxin (Bin) is one of the bio-larvicidal toxin produced by Lysinibacillus sphaericus. Two component proteins, BinA and BinB toxins are required simultaneously to exert its larvicidal activity. The binary toxin has been proposed to act initially on the susceptible cell membrane. Here, the cell membrane binding of the binary toxin was imitated via specific histidine (His)-nickel ion (Ni2+) chelating. The N-terminal His-conjugated binary toxins (His-Bin) were attached onto the Ni2+-lipid bilayer surface besides its facilitating of purification process. The N-terminal conjugation of histidine tag did not interfere with the folding structure of both toxins. Subsequently, the attachment of binary toxins on the lipid membrane was successful with Ni2+-phosphatidylcholine (POPC)/phosphatidylethanolamine (POPE) bilayers (a model membrane that mimics the mosquito cell membrane) but not for Ni2+-phosphatidylcholine bilayer. However, His-BinA formed unstable attachment with Ni2+-POPC/POPE bilayers since it could be removed by buffer rinsing. In contrast, His-BinB required imidazole solution to detach from Ni2+-POPC/POPE. Particularly, His-BinB had higher binding affinity to Ni2+-ion than His-BinA. The lipid membrane attachment led to the initial finding that although BinA and BinB toxins share high homology structures, their capability for Ni2+ chelation was different. The local N-terminal structure of binary toxin seems to interfere the His-Ni2+ chelating of His-BinA.