Characterization of inhibitor A, a protease from Bacillus thuringiensis which degrades attacins and cecropins, two classes of antibacterial proteins in insects.

The insect pathogen Bacillus thuringiensis produces an exoprotease, inhibitor A, at the beginning of the stationary growth phase. In vitro, the enzyme selectively destroys cecropins and attacins, two antibacterial proteins found in immune hemolymph from Hyalophora cecropia. The specificity of this enzyme was investigated using cecropin A(1-33) and HPLC for separation and characterization of the fragments obtained. A maximum of 12 different peptides were produced and their positions in the known sequence of cecropin A(1-33) were deduced from their amino acid compositions. The enzyme did not show a stringent requirement for a specific amino acid sequence at the cleavage site but prefers a hydrophobic residue on the C-terminal side. The specificity of the enzyme is explained in terms of the open structure of the cecropins and a pronounced inability of inhibitor A to attack globular proteins.