Intra- and extracellular domains of the Helicoverpa armigera cadherin mediate Cry1Ac cytotoxicity

Authors: ZHANG, HAONAN - Nanjing Agricultural University; YU, SHAN - Nanjing Agricultural University; SHI, YU - Nanjing Agricultural University; YANG, YIHUA - Nanjing Agricultural University; Fabrick, Jeffrey; WU, YIDONG - Nanjing Agricultural University

Submitted to: Journal of Insect Biochemistry and Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/28/2017
Publication Date: 5/31/2017
Citation: Zhang, H., Yu, S., Shi, Y., Yang, Y., Fabrick, J.A., Wu, Y. 2017. Intra- and extracellular domains of the Helicoverpa armigera cadherin mediate Cry1Ac cytotoxicity. Journal of Insect Biochemistry and Molecular Biology. 86:41-49.

Interpretive Summary: The cotton bollworm (Helicoverpa armigera) is one of the world's most important pests of cotton and transgenic cotton producing toxins from the bacterium Bacillus thuringiensis (Bt) is currently used to manage this pest. Such Bt transgenic crops are beneficial as they only target pest insects that ingest plant material and have the necessary receptor proteins to render the Bt compound toxic. Unfortunately, natural mutations that alter such receptor proteins have been identified in several insect pests that allow for survival on Bt crops (e.g. resistance). To better understand how changes to a specific receptor protein for this toxin may lead to resistance, ARS scientists at Maricopa, AZ and collaborators used an insect cell expression system to produce variant forms of a Bt receptor and show that a specific region found on the cell surface is needed for both toxin binding and cell toxicity. Results also show that while a mutation that alters a region of the receptor inside the cell receptor does not affect toxin binding, it does reduce cell toxicity. These data clarify how Bt toxins target important insect pests and how insects can adapt to current management practices. Such information is vital for delaying the onset of resistance and for extending the life of essential Bt technologies.

Technical Abstract: Diverse midgut cadherin mutations confer resistance to Cry1A toxins in at least three lepidopteran pests, including the cotton bollworm, Helicoverpa armigera. Most of these cadherin mutations are inherited as recessive alleles and result in changes within the cadherin repeat (CR) regions of the extracellular protein domain. However, the H. armigera r15 cadherin mutation results in a deletion of 55 amino acid residues within the cytoplasmic domain, and Cry1A resistance is inherited as a non-recessive trait. Here, eight recombinant H. armigera cadherin (HaCad) proteins, including seven variants containing different combinations of CRs and the cytoplasmic domain, were expressed in cultured insect cells using a baculovirus expression system and were analyzed for Cry1Ac binding and toxicity. Cells expressing either the wild-type HaCad or a mutant lacking only the region corresponding to the first nine CRs bound Cry1Ac and were equally susceptible to Cry1Ac. Cells expressing mutant HaCad proteins without the Cry1A toxin binding region (TBR) located in the CR nearest the plasma membrane did not bind Cry1Ac and were not killed by the toxin. Among the mutant proteins, loss of toxicity was observed in all cells producing HaCad variants lacking the amino acids 1422-1440, indicating that this TBR motif is important for both toxin binding and to confer susceptibility to Cry1Ac. Cells expressing the HaCad variant, which lacks the entire cytoplasmic domain, retained Cry1Ac binding, but were significantly less susceptible to Cry1Ac than were cells producing either wild-type HaCad or HaCad lacking the first nine CRs. These results suggest that both the extracellular and the cytoplasmic domains of HaCad participate in Cry1Ac intoxication.