Insulin is synthesized in the form of a precursor proteinthat requires cleavage of two different peptide segmentsbefore the mature protein is secreted from βcells in thepancreas. The first peptide is removed when the proteinenters the lumen of the ER. To find out when the second cleavage event takes place, investigators prepare a pairof antibodies: one recognizes the pro-insulin precursor,the other the mature insulin protein. They tag theantibody that binds to the precursor protein with a redfluorescent marker; the antibody that binds to matureinsulin is tagged with a green fluorescent marker. Theinvestigators then incubate an isolated βcell with bothantibodies at the same time and monitor the fluorescencein itsvarious membrane-bound compartments.Based on the data shown in the table, where is the second peptide removed from the pro-insulin precursor protein?

A. Immature secretory vesicles
B. Lysosomes
C. Mature secretory vesicles
D. The ER
E. The Golgi apparatus

Insulin is synthesized in pancreatic ß-cells. It is a small protein formed by two amino acid chains (A-chain, 21aa, and B-chain, 30aa) connected by two disulfuric chains.

The gene encoding is located in the short arm of chromosome 11. Its precursor, called preproinsulin, is synthesized in the endoplasmic reticulum. Right after its synthesis, this protein undergoes enzymatic cleavage to proinsulin (86 aa).

The prohormone is packed in small granules within the Golgi complex, that migrates to the cell surface. As the granules mature, 3 proteases split the proinsulin into equal amounts of insulin and C-peptide.

These secretory granules are stored within the intracellular space until their contents are released by exocytosis.

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Image from Guyton and Hall Textbook of Medical Physiology, unit XIV, chapter 79