CmCatD, a cathepsin D-like protease has a potential role in insect defense against a phytocystatin

When fed on a diet containing a proteinaceous cysteine protease inhibitor from soybean (scN), cowpea bruchid larvae enhance their overall digestive capacity to counter the inhibitory effect. Elevated proteolytic activity is attributed not only to the major digestive cysteine proteases (CmCPs), but also to aspartic proteases, a minor midgut protease component. In this study, we isolated a CmCatD cDNA from cowpea bruchid midgut that shares substantial sequence similarity with cathepsin D-like aspartic proteases of other organisms. Its transcript profile was developmentally regulated and subject to alteration by dietary scN. CmCatD transcripts were more abundant in scN-fed 3rd and 4th instar midguts than in control. The bacterially expressed recombinant CmCatD proprotein was capable of autoprocessing under acidic conditions, and mature CmCatD also exhibited pH-dependent proteolytic activity which was inhibited specifically by pepstatin A, indicative of its aspartic protease nature. CmCatD trans-activated CmCPs and vice versa, suggesting a cooperation between the minor midgut CmCatD and major digestive CmCPs. Further, CmCatD was able to degrade scN after extensive incubation. This activity partially restored CmCP proteolytic activity otherwise inhibited by scN. Thus CmCatD could facilitate insects' coping with the challenge of dietary scN by exerting its scN-insensitive and scN-degrading activity, freeing cysteine proteases for food degradation. Taken together, cowpea bruchids coordinate the functionality of the two classes of digestive proteases to fend off the negative effect of scN, and fulfill their nutrient requirements.