Figure 16.1.

Insulin biogenesis: Preproinsulin is synthesized in rough endoplasmic reticulum (RER) and is converted to proinsulin. Proinsulin is later transported to Trans-Golgi Network (TGN) and packed into immature granule via sorting for entry. The immature granule loses the clathrin coating and other unwanted components through sorting by exit. In parallel, proinsulin is converted to insulin and condensation/crystallization (sorting by retention) occurs. These processes account for the maturation of the immature insulin granules into a mature insulin secretory granule following TGN exit.