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A pathway for secretion

The Rockefeller University Press
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DOI: 10.1083/jcb1684fta3
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1684fta FROM THE ARCHIVE • THE JOURNAL OF CELL BIOLOGY 525 Text by William A. Wells [email protected] A pathway for secretion ocation, location, location. It has been, and remains, one of the best initial clues to understand the function of a protein or process. The same was true for those trying to localize protein synthesis in the 1950s and 1960s, but their experiments came with a twist. This early in the history of cell biology, the localization effort was not far behind, or even inspired, the definition and naming of the locations. The cells of choice were pancreatic or liver cells that were furiously churning out secreted proteins. Littlefield et al. (1955) made a promising start by showing that radioactive amino acids accumulated first in the detergent-resistant particles of the microsomes (i.e., the ribosomes) before moving into the membranous portion. The authors felt confident to state that “the cytoplasmic ribonucleo- protein particles are the site of initial incorporation of free amino acids into protein.” Siekevitz and Palade (1958) came to a similar conclusion, and extended the work to show that the label continued on from the microsome fraction to reach the pancreatic zymogen granules. The same sequence held true for individual purified secretory proteins (Siekevitz and Palade, 1960). Localization moved from biochemistry to cytology with Caro and Palade (1964), who used electron micrographs to trace radioactive leucine as it moved through the endoplasmic reticulum, then the Golgi, and finally to zymogen granules. The Golgi had been destroyed by the earlier biochemical fractionations, L Radioactive leucine is incorporated first into “attached particles” (AP; ribosomes) before entering the general microsomal content (MC) and then zymogen granules (Z). SI EK EV IT Z so this was its first appearance in the pathway. All these in vivo studies were hampered by overly long pulse and chase times. Thus it remained formally possibl

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