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In studying secreted proteins, you find that Substance X inhibits the secretion of a labeled protein. However, you do find a fully synthesized, folded, and glycosylated proteins in the cell. Where is the most likely site in the synthesis and secretion of proteins for Substance X to act?a. translation on the ribosomeb. budding of the secretory granulec. enzyme modification in the Golgid. All of the abovee. nucleus during translation

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Answer:

The correct answer is: b. Budding of the secretory granule.

Explanation:

  • In eukaryotic cells, the pathway by which secretory proteins are produced and are secreted can be explained below:
  1. The gene {DNA (Deoxyribonucleic Acid) sequence}, located on the genome, encoding the secretory protein is transcribed into an mRNA (messenger Ribonucleic Acid) by the RNA polymerase. This process happens in the Nucleus.
  2. The mature mRNA is translocated from the nucleus into the cytoplasm.
  3. In the cytoplasm it associates with the ribosome but translation process does not begin now.
  4. This is because, as the mRNA sequence has the tag of forming a secretory protein, it can be translated only when the ribosome bound mRNA sequence attaches itself to the ER (Endoplasmic Reticulum).
  5. On attachment to the ER, the process of translation begins and the polypeptide formed is released into the lumen of the ER.
  6. The process of glycosylation starts in the ER.
  7. The glycosylated protein is then transported from the ER in membrane bound vesicles to the cis, medial and trans Golgi, where the process of glycosylation gets completed and the protein gets completely folded.
  8. After this, secretory vesicles buds off from the Golgi and migrate towards the plasma membrane.
  9. At the plasma membrane, the secretory vesicles fuse with the plasma membrane and releases the protein out of the cell by the process of Exocytosis.
  • In the given question completely folded glycosylated protein is formed but it is not secreted. This means the protein is able to reach to the Golgi bodies. But secretory vesicle formation does not take place.
  • Hence the Substance X might inhibit the formation or budding of Secretory Vesicles.