C-Terminal Regions of Insecticidal Cry Toxins Form Amyloid Fibrils Affecting Metabolism of Insect Cells.

Crystal (Cry) toxins produced by Bacillus thuringiensis are highly specific insecticidal proteins that are widely used as biological insecticides and function through proteolytic activation in insect midguts, which generates a three-domain toxic core responsible for membrane disruption as well as cleaved N- and C-terminal domains. While the structure and function of this core have been extensively characterized, the fate of the cleaved C-terminal region remains poorly understood. Amyloids are protein fibrils with a characteristic cross-β structure that participate in a wide range of functions in bacteria. Here, using bioinformatic approaches, we demonstrated that the C-terminal domains of Cry are rich in potentially amyloidogenic regions. We investigated the aggregation behavior of C-terminal domains from Cry1Ea11 and Cry1Ab12 proteins using a combination of biochemical and structural approaches. We found that the C-terminal domains of Cry1Ea11 and Cry1Ab12 undergo self-assembly in vitro, forming aggregates with fibrillar morphology. These assemblies bind amyloid-associated dyes, exhibit partial resistance to ionic detergents and proteolytic digestion, and display X-ray diffraction patterns confirming their amyloid properties. Functional assessment in insect cells showed that although these amyloids do not induce detectable cytotoxicity, amyloids of the C-terminal domains of Cry1Ea11 alter the readouts of the MTT assay in a manner consistent with previously described amyloid-associated effects on cellular formazan handling. Together, our findings demonstrate that the C-terminal domains of Cry toxins, which are released in the insect midgut as a result of proteolytic cleavage, are able to form bona fide amyloids that affect the metabolism of insect cells.