DIGESTION OF DELTA-ENDOTOXIN BY GUT PROTEASES MAY EXPLAIN REDUCED SENSITIVITY OF ADVANCED INSTAR LARVAE OF SPODOPTERA-LITTORALIS TO CRYIC

The present study describes the correlation between gut protease activ ity of lepidopteran larvae of different instars, the inactivation of B acillus thuringinesis delta-endotoxins in crystalline and noncrystalli ne forms, and the reduced susceptibility of advanced larval instars of Spodoptera littoralis to the toxin, The original assembly of delta-en dotoxins in a crystal structure is essential for causing efficient lar val mortality, Denaturation and renaturation (D/R) of delta-ednotoxin crystals increased the vulnerability of the toxin molecules to proteol ysis, reduced their capability to kill neonate larvae of S. littoralis , but sustained most of their larval growth-inhibition activity, E. ca ll-produced CryIC delta-endotoxin applied as a fraction of inclusion b odies exerted a growth inhibition effect, similar to the molecules rel eased from the crystals by denaturation and subsequent renaturation. I ncubation of CryIC with gut juice of Ist or 2nd instar larvae, left pa rt of the CryIC toxin intact, while the toxin was completely degraded when incubated with gut juice of 5th instar larvae. The degradation ra te was consistent with the increase of protease specific activity of t he gut juice during larval development, This increase in toxin degrada tion may account for the loss of sensitivity of 5th instar larvae to C ryIC, Specific protease inhibitors such as PMSF and Leupeptin were sho wn to inhibit gut proteases activity in all instar larvae, while, 1,10 phenanthroline, TLCK and TPCK were effective only in young instar lar vae, The differential effect of protease inhibitors on proteases obtai ned from different larval instars indicated that gut juice protease pr ofiles change with larval age, The observed quantitative and qualitati ve differences in degradation of S-endotoxin by larval gut proteases t hat occur during larval maturation may account for the difference in s usceptibility to the delta-endotoxin, This finding should be taken int o consideration when designing strategies for the development of trans genic crops expressing delta-endotoxins as potent insecticidal protein s, Copyright (C) 1996 Elsevier Science Ltd.