DEVELOPMENTALLY CONTROLLED CLEAVAGE OF THE CALLIPHORA ARYLPHORIN RECEPTOR AND POSTTRANSLATIONAL ACTION OF THE STEROID-HORMONE 20-HYDROXYECYSONE

In response to a rise in ecdysteroid titre, fat body cells of insect l arvae take up storage proteins from the haemolymph by receptor-mediate d endocytosis. Here we show that the receptor responsible for incorpor ation of the major haemolymph protein arylphorin of the blowfly, Calli phora vicina, is subject to an unusual posttranslational processing th at involves three distinct cleavage steps. After the removal of a 17-a mino-acid signal peptide, a receptor precursor of 141 kDa is released. Before reaching the cell surface, the precursor is cleaved a second t ime, giving rise to the active 92-kDa arylphorin receptor, plus a 48-k Da 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 3 0-kDa peptide. Contrary to the standard model of steroid hormone actio n, the process which give rise to receptor cleavage can be induced by 20-hydroxyecdysone in vivo and in vitro even in absence of protein bio synthesis.