Characterization of Atg8 in lepidopteran insect cells.

Yeast Atg8 and mammalian microtubule-associated protein light chain 3 (LC3) are landmark proteins essential for autophagy. Here the lepidopteran Atg8, a homolog of LC3, is characterized. Sequence analysis reveals that Atg8 proteins are highly conserved in lepidopteran species. The abundance of endogeous Atg8 and the ratios of Atg8 conjugation to phosphatidylethanolamine (Atg8-PE)/Atg8 are different among several lepidopteran cell lines and different tissues of Helicoverpa armigera larvae. Both the density of fluorescent pre-autophagosomal structures with GFP-Ha Atg8 and the abundance of Atg6 are positively correlated with levels of Atg8-PE in different cell lines. The mutant GFP-Atg8(G116A) has lost the function in punctual formation, suggesting that G116 is important for autophagy. Exogenous factors have significant influences on the conversion of Atg8 in lepidopteran cells. Bacillus thuringiensis enhances the degradation of Atg8 in Spodoptera litura Sl-HP cells. Atg8-PE degrades gradually with extension of amino acid starvation, and bafilomycin A1 can block the decrease through the inhibition of autophagosome fusion with lysosome. Interestingly, high pH is more effective than amino acid starvation in Bombyx mori Bme cells to induce the conversion of BmAtg8 to BmAgt8-PE. Change of the quality of fetal bovine serum in the culture medium results in alteration of the ratio of Atg8-PE/Atg8 in some lepidopteran cell lines.

Expression of recombinant and mosaic Cry1Ac receptors from Helicoverpa armigera and their influences on the cytotoxicity of activated Cry1Ac to Spodoptera litura Sl-HP cells.

Bacillus thuringiensis (Bt) toxin receptors play important roles in the killing of pests, and investigation on characterization of the receptors is essential for utilization of Bt and management of insect resistance. Here, recombinant and mosaic receptors of Bt Cry1Ac toxin from Helicoverpa armigera were expressed in Spodoptera litura Sl-HP cells and their influences on cytotoxicity of activated Cry1Ac toxin were investigated. When H. armigera aminopeptidase N1 (APN1), alkaline phosphatase 2 (ALP2) and cadherin fused with or without GFP tag were, respectively, expressed in Sl-HP cells, live cell-immunofluorescence staining detection revealed that the quantity of the toxin binding to cadherin or cadherin-GFP was much more than that binding to ALP2 and APN1 or their fusion proteins with GFP, and only the cadherin- or cadherin-GFP-expressing cells showed aberrant cell morphology after the treatment of the toxin at low concentrations. ALP2 and APN1 fused with or without GFP tag did not significantly enhance the cadherin-mediated cytotoxicity of the toxin. The mosaic ALP-TBR-GFP-GPI was located on cell membrane, but did not bind to the toxin. The mosaic truncated cadherin-GFP-GPI was not located on cell membrane even if the signal peptide was sustained. The concentrations of the toxin resulting in swelling of 50 % cells for noncadherin-expressing Sl-HP cells and cadherin-expressing Hi5 cells were 5.08 and 9.50 µg/ml within 1 h, respectively. Taken together, our data have indicated that the binding affinity of ALP2 and APN1 to activated Cry1Ac toxin is much weaker than that of cadherin and both ALP2 and APN1 do not enhance the cytotoxicity of the toxin even though cadherin is co-expressed, and the mosaic receptor of ALP2 inserted with cadherin toxin binding domain does not mediate cytotoxicity of the toxin. In addition, the noncadherin-expressing Sl-HP cells are more susceptible to activated Cry1Ac than the cadherin-expressing Hi5 cells.