Identification and characterization of cysteine protease inhibiting activity in the midgut fluid of Trichoplusia ni larvae

Plant cysteine proteases play a role in plant defense against herbivorous lepidopterans by targeting the insect midgut peritrophic membranes (PMs). In this study, the action of plant cysteine proteases on the midgut PM of Trichoplusia ni larvae was investigated and cysteine protease inhibiting activity was identified in the midgut fluid of T. ni larvae. Biochemical analysis showed that the cysteine protease inhibiting activity in the midgut fluid of T. ni larvae protects the PM proteins from degradation by plant cysteine proteases. The cysteine protease inhibiting activity is thermostable, resistant to degradation by digestive proteases, and appears to be conferred by molecules with a mass less than 3kDa. The cysteine protease inhibiting activity is active over a pH range 6-10, but partially susceptible to reducing agents.