Expression of Insulin in Yeast: The Importance of Molecular Adaptation for Secretion and Conversion

The globular, two...chain and 51 amino acid residue peptide-hormone insulin is produced and secreted by the ~-cellsof the pancreatic islets of Langerhans. Insulin is synthesized as preproinsnlin (110 amino acids). The pre-peptide (signal peptide) is removed upon entrance into the endoplasmic reticulum. Proinsulin folds in the endoplasmic reticulum, is transported to the Goigi apparatus and subsequently processed into the mature insulin molecule that is stored in well-defined storage vesicles (Figure 5.1) (Steiner etaI., 1967, 1986; Dodsonand Steiner, 1998). Proinsulin and insulin have self-assembling properties that play an important role in processing and storage in the J3-cell's secretory pathway and both associate to dimers and in the presence of zinc these further assemble into hexamers (Dodson and Steiner, 1998). In the late Golgi apparatus proinsulin is targeted to acidifying secretory granules and conversion ofproinsulin to insulin occurs by removal of the C-peptide by cleavage at dibasic processing sites by the endoproteases PC3 (or PCl) and pe2 (mammalian