Structure of the mosquitocidal toxin fromBacillus sphaericus

Abstract

Bacillus spfzaericus produces insecticidal proteins during sporulation that are highly toxic to larvae of certain mosquitoes. including vectors of certain encephalitis and mala1ia, but a1•e not toxic to any other insects or mammals (Porter et al. (1994) MicrobiaL Rev. 57:833). Two proteins, with molecular weights of 51 and 42l<Da. bind tightly together as a complex and cause toxicity by a mechanism that is not well understood. It has been suggested that one protein acts as a specific protease on the other or that one protein serves as a chaperone to bring the other into the celL Altematively, each protein may exert a different effect on the host that is insufficient to cause host death when acting independently. To understand these interactions at a molecular level we have begun a study of the three dimensional structure of the individual proteins and the complex using X-ray diffraction. The proteins are sepa.I•ately purified using an E. Coli expression system. Tetragonal crystals (P43212) of the 51 kDa protein have been obtained that a1•e up to 0.2 mm in size with cell constants of a=b=l33.6 A and c= 69.7 A. These crystals diffract to a resolution of2.6 A. Phasing with five derivatives followed by solvent flattening yielded an interpretable electron density map. Tracing of the polypeptide backbone is nea1•ly complete. The structure of the toxin will be presented in its cunent state.