Insect cuticle, an in vivo model of protein trafficking

In the course of this study more than 20 proteins have been isolated from t he larval cuticle of Manduca sexta. Synthesis, secretion, transport and acc umulation of four particular proteins, representative members of four chara cteristic groups, were followed during metamorphosis by immunoblot and immu ncytochemical methods and are described in detail in this paper. We establi shed that only some of the proteins of the soft cuticle of Lepidopteran lar vae are synthesized in epidermal cells at the beginning of the larval stage s and are digested during the moulting period (MsCP29), Other proteins (MsC P30/11) are secreted into the cuticle by the epidermal cells in different f orms during various developmental stages. Some proteins are secreted apical ly during the feeding period, but before ecdysis they are then taken up by epidermal cells and transported in a basolateral direction back into the he molymph and saved in an immunologically intact form by the fat body cells ( MsCP12.3), Some cuticle proteins have a non-epidermal origin. They are tran sported from the hemolymph into the cuticle. Before and during ecdysis thes e molecules reappear in the hemolymph and are detectable again in the pupal cuticle (MsCP78). Our data prove that the cuticle is not a non-living part of the insect body: it is not only an inert, protective armor, but maintai ns a continuous and dynamic metabolic connection with the other organs of t he organism.