Expression and crystallization of a Cry3Aa-Cry1Ac chimerical proteinof Bacillus thuringiensis

The insecticidal Cry1 proteins of Bacillus thuringiensis form a typical bipyramidal parasporal crystal and their protoxins contain a highly conserved C-terminal region. A chimerical gene was constructed with the coding regions of the Cry3Aa protein's toxic domain, and of the Cry1Ac protoxin's C-terminal fragment. This chimerical construction expressed a truncated (similar to 70 kDa) protein in the acrystalliferous strain 4Q7 of B. thuringiensis, assembled in spherical to amorphous parasporal crystals. This protein was recognized only by antibodies raised against the Cry3Aa protein. When the protease-deficient mutant BL21 of Escherichia coli was transformed with the same chimerical construction, a complete (similar to 140 kDa) chimerical protein was expressed. However, the formation of a crystalline inclusion was unclear. This protein was recognized by antibodies raised against the proteins Cry1Ac and Cry3Aa. Both chimerical proteins showed toxicity against larvae of Leptinotarsa texana, being much more active when expressed truncated in B. thuringiensis. These results suggest that the formation of bipyramidal crystals requires more than just the presence of the C-terminal region of Cryl protoxins. They also suggest that proteolysis plays an important role during the post-translational processing of Cry proteins.