Degradation of aberrant proteins by larval tobacco hornworm,Manduca sexta L. (Sphingidae)

The tobacco hornworm Manduca sexta (Sphingidae) readily incorporates L-canavanine, the L-2-amino-4-(guanidinooxy)butyric acid structural analog of L-arginine, into newly synthesized proteins. As a result, the developing fifth-instar larva produces structurally aberrant canavanyl proteins that can exhibit severely impaired function. This situation is exacerbated by canavanine's ability to stimulate de novo protein synthesis. M. sexta larvae can respond to anomalous protein production by degrading canavanyl proteins nearly five times faster than normal proteins. The proteases of this insect can distinguish between normal and anomalous proteins and thereby avoid destruction of essential macromolecules. Aberrant protein degradative activity is not dependent upon de novo protein synthesis induced by canavanyl proteins. The fat body appears to be the source of proteases that degrade aberrant proteins; degradation is curtailed in the presence of sulfhydryl protease inhibitors as well as inhibitors of trypsin-like activity.