Question

26. The mRNA for a well-known secretory protein is isolated and placed in a test tube in the presence of all the substances (or reagents) required for in vitro protein synthesis. The amino acid sequence of the protein produced in vitro (in a test tube) is then compared to the sequence of the purified secretory protein (purified from a eukaryotic cell). The sequences of the two proteins are not the same. Please give a possible explanation for this observation. Think about the model you learned about how secretory proteins are synthesized in a cell.

Answer 1

In eukaryotes, a secretory protein is produced by the endomembrane system.
The endomembrane system includes the ER and Golgi complex.

The nascent mRNA is transcribed by the ribosomes located on the RER. The nascent polypeptide is released into the ER lumen where it undergoes folding and post-translational modifications. The protein is then sent to the Golgi complex for further modifications. These modifications include proteolytic cleavage, disulfide bond formation, and phosphorylation, etc..
The mature protein is then packed into vesicles and fused with the membrane for secretion.

The endomembrane system is absent in the in vitro conditions. The protein modifications cannot be performed. Hence the protein sequenced synthesized in vivo and in vitro are different.

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