Insulin is a Secretory Protein Made by the β Cells

Question

Insulin is a secretory protein made by the β cells in the pancreas. This protein …
A) is released by the β cells in response to increased intracellular Ca²⁺ concentration.
B) is first synthesized as preproinsulin.
C) is stored as aggregates inside secretory vesicles in the β cells.
D) undergoes proteolytic cleavage before secretion.
E) All of the above.

Accepted Answer

Starting from a pre-pro-protein (e.g. preproinsulin), the pre-peptide is cleaved off in the ER, and the pro-peptide is cleaved later normally during maturation of secretory vesicles. The formation and maturation of secretory vesicles at the TGN involves selective segregation and concentration of secretory proteins such as insulin.