Pancreatic beta cells

Insulin mRNA codes for preproinsulin (with hydrophobic N-terminal signal peptide)

N-terminal signal peptide is cleaved to form proinsulin in the RER

conformational changes = addition of 3 disulfide bonds in the RER -> transport to Golgi apparatus, where it is packed into secretory granules

Proinsulin is cleaved by endopeptidases in the secretory granules to form insulin and C-peptide, which are then stored until they are secreted in equimolar maounts.