Endoplasmic Reticulum

ID #2269

In Palade's pulse-chase experiment, I'm interested in how they were able to show that the proteins made by the RER associated ribosomes were the ones that were secreted outside of the cell and not via the cytosolic ribosomes? Is it possible to "video-record" a live microscopic process?

The pulse-chase experiment was done in a time before you could watch a cellular process in a live cell using microscopy (yes, this can be done now; see note below). Palade's group would chemically freeze the cells (a process called fixing or crosslinking) at various time points after the pulse. They saw that there was a concentration of newly synthesized proteins in the ER which led them to the conclusion that proteins were being synthesized at the ER. Later data (like the identification of the signal sequence by Gunter Blobel in 1975) confirmed this hypothesis. The entire mechanism certainly wasn't fully understood until many years later. It also helped that the types of cells that Palade's group used were pancreatic cells. Most proteins in those cells are secreted causing an even higher concentration of proteins in the secretory pathway versus the cytoplasm.
Today, microscopy has come a long way since the 60s! For some info on live cell imaging: Here at U of I, Professor Taekjip Ha (along with other members of the physics department) has revolutionized the field of single molecule imaging in which one can even follow a singe, specific protein, RNA, DNA (whatever) in the cell using fluorescence labeling. If you want to check it out, here is the link to their webpage:

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