Question

A cell that is synthesizing insulin is lacking Signal Recognition Particle (SRP) receptors at the level of the rough endoplasmic reticulum. In your own words, describe the impact, if any, on the synthesis and secretion (exocytosis) of insulin from the cell. Explain your answer..

Answer 1

Insulin is a hormone that regulates blood glucose levels in humans. It is synthesized and secreted by the beta cells of Islets of Langerhans of the pancreas. The mRNA is translated on ribosomes of rough endoplasmic reticulum (RER) as a single chain precursor. This precursor is called preproinsulin (110kDa). Preproinsulin has a signal peptide that causes its insertion into the endoplasmic reticulum. This insertion generates the proinsulin molecule.

The insertion of preproinsulin into ER to form proinsulin requires the N terminal signal peptide in preproinsulin. This N terminal signal peptide upon synthesis (along with some part of nascent chain) will interact with the signal recognition particle. SRP is a ribonucleoprotein present in cytoplasm. SRP along with Signal peptide and ribosome binds to the SRP receptors on ER. SRP also directs the ribosome binding to the ER membrane. Translation is halted at this stage. SRP then dissociates from the nascent preproinsulin along with SRP receptor. The signal peptide and the nascent polypeptide will then move through the internal peptide conducting channel into ER lumen. The N terminus of preproinsulin now faces the cytosol. The translation is resumed and the polypeptide chain elongates, In the ER lumen, the signal peptide is cleaved by signal peptidase enzyme to form the proinsulin. Protein translation is completed by ribosomes and the proinsulin will now be present in ER lumen.

This proinsulin is however unfolded. Folding of proinsulin involves formation of disulphide bonds. Folded proinsulin then moves from ER to Golgi apparatus. Here, it is packaged into secretor vesicles. Proinsulin is acted upon by several endopeptidases to form insulin in these immature secretory vesicles. These endopeptidases remove the C peptide of proinsulin to generate mature insulin. Mature insulin is then packaged along with free C peptide in the Golgi apparatus into secretory granules. These secretory vesicles will then be present in cytoplasm till they can be secreted. Secretion occurs when the beta cell is stimulated. Exocytosis is involved in secretion of insulin. The free C peptide also secreted has no known biological function.

SRP is therefore required for transport of the preproinsulin to the ER lumen. If the cell lacks SRP, then ribosome attached to ER will continue to synthesize the preproinsulin. Signal peptide will ensure that protein synthesis occurs on RER. However, this preproinsulin will not be able to bypass the ER membrane into the ER lumen. Although SRP binds to the signal peptide, it cannot bind to ER membrane due to lack of SRP receptors. As signal peptide is not cleaved, proinsulin and later mature insulin is not formed. Thus, the cell will have only preproinsulin present, possibly in the cytoplasm. The cell will not be able to form mature insulin. Thus, insulin cannot be secreted even upon stimulation by high glucose levels in the circulation.