In the bustling economy of the cell, little bubbles called vesicles serve as container ships, ferrying cargo to and from the port - the cell membrane. Some of these vesicles, called post-Golgi vesicles, export cargo made by the cell's protein factory. Scientists have long believed that other, similar vesicles handle the reverse function, importing life-supporting nutrients and proteins through an independent process. By using a finely honed type of microscopy to more precisely examine these transactions, new research shows the processes are not as independent as assumed: certain molecules handle cargo moving in both directions. Like stevedores, they're involved in both loading and unloading the cell's container ships.
Jyoti Jaiswal, a research assistant professor and Sanford Simon, head of the Laboratory of Cellular Biophysics at Rockefeller University, examined the most common form of cellular export process called constitutive exocytosis, a continual ferrying of goods involved in the regular life and maintenance of all eukaryotic cells. This sort of shipping was assumed to end with the vesicles fusing completely to the membrane and delivering their whole load of proteins and lipids, in contrast to the more discriminating process by which similar container ships import proteins from outside the cell, called endocytosis. But, in a paper to be published June 26 in Cell, Jaiswal and Simon show that some of the key molecules regulating endocytosis, such as clathrin, dynamin and actin, are also at work in exocytosis.
"In retrospect, it makes perfect sense," Jaiswal says. "But at first we thought we had to be wrong because they had been defined as endocytic molecules." Adds Simon: "Once we took a step back from the dogma, we saw that cells employ these molecules for import and export. Then everything fell into place. We should stop stereotyping molecules as dedicated for this or that purpose. It puts on the blinders. There's an advantag
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