Endoplasmic Reticulum

ID #2268

In a pulse-chase experiment, if the radioactively-labeled agent was an amino acid (AA), how was the researcher able to distinguish the proteins created by the cytosolic ribosomes from those created by membrane-bound ribosomes? Methionine is a universal AA to all proteins so how would there be anyway to specifically tell that the proteins were coming from the ER and not just a cytosolic cluster of ribosomes?

Good question. During the pulse step of the experiment, all proteins that are being made (both at the ER and in the cytoplasm) are radioactively labeled. Because the proteins inside the ER and all other organelles of the secretory pathway are spatially concentrated, they are detectable via autoradiography. All the proteins that are made in the cytoplasm are very spread out and not nearly as concentrated as those that are inside of the organelles. The radioactivity in the cytolasm is much less intense than what is found in localized compartments like the ER and it is fairly easy to discern between the two during the chase part of the experiment. 

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