Diverse enzymatic specificities of digestive proteases, 'intestains', enable Colorado potato beetle larvae to counteract the potato defence mechanism

In response to insect attack, high levels of proteinase inhibitors are synthesised in potato leaves. This can cause inefficient protein digestion in insects, leading to reduced growth, delayed development and lower fecundity. It has been suggested that Colorado potato beetle overcomes this defence mechanism by inducing the production of a set of cysteine proteases that are resistant to potato proteinase inhibitors. Experiments with gut extracts showed that these proteases have unusual inhibition profiles as they are not inhibited by most of the cystatins but are strongly inhibited by thyropins. In this study we have isolated three cysteine proteases from adapted guts of Colorado potato beetle larvae, named intestains 1, 2 and 3, the first cysteine proteases known to be involved in extracellular protein digestion. The N-terminal sequences suggest their classification into the papain family. Intestains differ in substrate specificities and inhibitory profiles. Their substrate specificities suggest that intestains 1 and 2 are general digestive enzymes, while intestain 3 has a more specific function. The inhibitory profile of intestain 1 is similar to that of proteases of the papain family. However, the Ki values for the interaction of intestain 2 with the same set of inhibitors are several hundred fold higher, which would enable the enzyme to circumvent the potato defence mechanism characterised by high concentrations of protease inhibitors in attacked potato leaves. A further, different strategy of the Colorado potato beetle to avoid potato defence is exhibited by intestain 3, which is able to cleave off the N-terminus of model cystatin and thus inactivate the inhibitor. These results suggest that the Colorado potato beetle combines different strategies to counteract plant defence mechanisms.