A researcher is tracing the fate of C-peptide, a product of preproinsulin cleavage. Which of the following is a true statement regarding the fate of C-peptide?
1.C-peptide is immediately degraded by the proteasome
2.C-peptide is packaged with insulin in secretory vesicles
3.C-peptide exits the cells via a protein channel
4.C-peptide is further cleaved into insulin
5.C-peptide activates an intracellular signaling cascade
C-peptide is packaged with insulin in secretory vesicles and then jointly released from cells. Both proteins are released in equal molar amounts.
Preproinsulin is produced in the rough endoplasmic reticulums. From there, the protein is trafficked to the golgi apparatus where it is cleaved, resulting in insulin and C-peptide. The two molecules are then packaged into secretory vesicles. When glucose is present and the beta-cell becomes activated, there is release of the contents of these vesicles - equal amounts of insulin and C-peptide are released.
Illustration A depicts the production and packaging of preproinsulin and cleavage of the protein to form insulin and C-peptide.