From enzyme to zymogen: engineering Vip2, an ADP-ribosyltransferase from Bacillus cereus, for conditional toxicity

The adenosine diphosphate (ADP)-ribosyltransferase, Vip2 (vegetative insecticidal protein), from Bacillus cereus in combination with another protein from the same organism, Vip1, has insecticidal activity against western corn rootworm larvae. The Vip2 protein exerts its intracellular poisoning effect by modifying actin and preventing actin polymerization. Due to the nature of this toxin, expression of Vip2 in planta is lethal. In this work, we attempted to build an enzyme precursor (proenzyme, zymogen) that would silently reside in one biological system (e.g. plants or yeast) and be activated in the other (insect larvae). Our approach involved engineering a random propeptide library at the C-terminal end of Vip2 and selecting for malfunctional enzyme variants in yeast. A selected proenzyme (proVip2) possesses reduced enzymatic activity as compared with the wild-type Vip2 protein, but remains a potent toxin toward rootworm larvae. In addition, upon analysis of the digestive fate of the engineered enzyme precursor in rootworm larvae, we demonstrated that 'zymogenized' Vip2 can be proteolytically activated by rootworm digestive enzyme machinery. This report represents an example of applying a protein engineering strategy for the creation of a plant-tolerated, zymogen-like form of an otherwise toxic protein. This approach may outline a novel path to address challenges associated with utilizing toxic proteins in certain biotechnological applications.