The Evolution of Hexamerins and the Phylogeny of Insects
In response to a rise in ecdysteroid titre, fat body cells of insect larvae take up storage proteins from the haemolymph by receptor-mediated endocytosis. Here we show that the receptor responsible for incorporation of the major haemolymph protein arylphorin of the blowfly, Calliphora vicina, is subject to an unusual posttranslational processing that involves three distinct cleavage steps. After the removal of a 17-amino-acid signal peptide, a receptor precursor of 141 kDa is released. Before reaching the cell surface, the precursor is cleaved a second time, giving rise to the active 92-kDa arylphorin receptor, plus a 48-kDa peptide. The function of this 48-kDa peptide may be the prevention of premature ligand-receptor interaction in the endoplasmic reticulum. 20-Hydroxyecdysone initiates a third cleavage step of the arylphorin receptor, which results in a 62-kDa arylphorin binding protein and a 30-kDa peptide. Contrary to the standard model of steroid hormone action, the process which give rise to receptor cleavage can be induced by 20-hydroxyecdysone in vivo and in vitro even in absence of protein biosynthesis.