Figure 3.Insulin biosynthesis and secretion. Model of the intracellular synthesis, processing, and transport of preproinsulin, proinsulin, and insulin. Preproinsulin is synthesized in the RER and is converted to proinsulin within 1-2 min. Proinsulin is folded in the RER and transported to the Golgi apparatus. Clathrin-coated granules bud from the trans-Golgi contain proinsulin and the converting enzymes to cleave the prohormone to insulin. For further explanations see text.