Structural proteins of sarcophagid larval exoskeleton. Composition and distribution of radioactivity derived from [7-14C]dopamine.

In the cyclorrhaphid flies, exoskeletal proteins from the last larval instar cross-link by arylation and glycosylation to form the sclerotized puparial case. Cuticular proteins from maggots killed just prior to tanning were resolved into 21 soluble components by isoelectric focusing and sodium dodecyl sulfate-polyacrylamide electrophoresis. Isoelectric points ranged from pH 4.5 to 6.0, molecular weights were distributed between Mr = 16,000 and 24,000. Aspartic and glutamic acids, glycine, serine, valine, and lysine were abundant in all the proteins while sulfur-containing residues were uniformly absent. Heterogeneity was manifest among NH2 termini of the soluble fractions, while the insoluble chitin-linked protein showed only aspartic acid in this position. The sclerotized matrix was assembled by a concerted bridging of protomers without accumulation of di-, tri-, or higher n-mers in the urea-soluble fraction. This mechanism was also favored by uniform distribution of the bridge precursor, [7-14C]dopamine, among the individual larval protomers including the polypeptide bound to chitin. Following administration of isotopic catecholamine 2 to 10 h prior to sclerotization, unbridged larval cuticle retained 3% of the radioactivity, puparial and adult in integument 7% and 18%, respectively. Proteolytic digestion afforded labeled peptides with molecular weights in register with the degree of cross-linking. Nonradioactive larval proteins did not incorporate labeled dopamine and exchange incubation of labeled proteins with nonisotopic precursor failed to diminish recoveries of 14C. Since protein synthesis was low as assessed by minimal incorporation of [3H]leucine, metabolites derived from dopamine may have been added after translation in the course of presclerotal activation of the polypeptides destined for cross-linking.