Food Science, Agricultural, Medicinal and Pharmaceutical Chemistry
Wiley InterScience Backfile Collection 1832-2000
The deposition and apolysis of insect cuticles have long been known to be regulated by ecdysone. Unsclerotized, chitin-containing procuticles contain evolutionarily conserved, hydrophobic proteins that are soluble in solutions of denaturing agents. The pupal procuticle of Drosophila is deposited by larval and imaginal epidermis starting 9 h after puparium formation when the ecdysone titer is low. Initially, a set of low molecular weight proteins (less than 25,000 daltons; low molecular weight pupal cuticle proteins = S-PCPs) is synthesized. However, about the time of pupation, synthesis of S-PCPs ceases, and high molecular weight proteins (greater than 50,000 daltons; H-PCPs) are synthesized. In vitro experiments indicate that the initial formation of the procuticle with synthesis of the S-PCPs requires a pulse of hormone followed by withdrawal (6 h with 20-hydroxyecdysone, 1 μg/ml). The switch from synthesis of S-PCPs to H-PCPs is facilitated by a second, short pulse of 20-hydroxyecdysone (0.1 μg/ml, 3 h). Ultrastructural localization demonstrates that the S-PCPs are located only in the external lamellae of the procuticle, while the H-PCPs are present only in internal lamellae. Developmental analyses with cloned genes indicate that cuticle protein genes are expressed during only one stage of Drosophila development. Some of the genes encoding S-PCPs are limited in their expression to larval (posterior) or imaginal (anterior) epidermis. Preliminary molecular analyses of the larval and pupal cuticle protein genes indicate that they are organized in different ways. For example, four larval genes exist in a cluster with divergent transcription, and one PCP gene, PCP-GART, is located within an intron of a “housekeeping” gene.
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