Hanover, NH -- For some proteins made deep within a cell, those that work elsewhere in the body have a complex journey aboard carrier vessels with many possible stops before they exit the cell and continue through the blood to reach their destination. Now Dartmouth Medical School biochemists have discovered a key receptor for sorting which proteins stay and which leave, documenting for the first time what many scientists theorized but had not confirmed about how proteins travel through cells for export.
Their results, published in the November 16 Science, help resolve a major piece of the cell traffic puzzle, paving the way for further understanding of how hormones such as insulin as well as other important factors are conveyed through cells for secretion into the blood stream. The authors are Charles Barlowe, PhD, associate professor of biochemistry and William J. Belden, a graduate student.
Transporting proteins that need to get out of the cell to function is essential. After they are manufactured on membranes in the cell interior, the proteins are captured in little vesicles that take them to the cell boundary to be discharged. Packaging the proteins to board the vesicles takes a series of steps not yet well delineated.
"Internal membranes have thousands of proteins in them and some of those proteins depend on vesicles to travel to another location. But the first step is getting it into a vesicle. That's called sorting," says Barlowe.
He makes the analogy of a bus. "If there is a bus to carry passengers and the passengers need seats, then if we look at the bus carefully we should be able to find some seats." Barlowe, using baker's yeast as a model system, has found a seat on the cellular bus.
A popular cell transport model holds that a receptor will bind to a protein that needs to travel and capture it into the vesicle. However, no receptors that actually operate on this theory of selected transport had been found. Now Barlowe has discovered a "transport receptor," the first of its kind to select a soluble protein and help ferry it out of its compartment.
Yeast, like many animals, give off certain pheromones when mating. The investigators found that one little pheromone in yeast binds to a protein, Erv29p, for its journey through the cell, and that without that receptor protein, the pheromone is not secreted.
"If you knock out Erv29p, you still make the bus and it still goes to the right place. But it's missing the seat for this passenger to get on, so the passenger stays off." That is, the vehicle moves along, but it canΥt carry passengers.
The concept that proteins to be secreted rely on a receptor may be more general and widespread than appreciated, according to Barlowe. The Erv29p receptor may have parallel functions in higher cells or there may be other receptors as well. DMS postdoctoral fellow Stefan Otte in Barlowe's laboratory was among those who identified Erv29p and other protein components of this cell vehicle in yeast.
Hundreds of proteins are secreted in different cell types, and diseases related to faulty secretion could be linked to flawed receptors. In humans, for instance, there is evidence that a protein similar to Erv29p is required to get a blood clotting factor out of cells. So, a genetic mutation that causes a form of hemophilia, a bleeding disorder, is linked to a protein that is in the same place as the protein whose function Barlowe documented in yeast, reinforcing the possibility that a similar receptor mechanism is required for some proteins in humans as well.
The yeast cells in the Dartmouth experiments survive, even when their vesicles do not carry the passengers. But in some cells, the receptor process might be so essential that any defects in these receptors would be lethal, Barlowe suggests. Now, he and his team are exploring how the receptor binds and lets go -- how the passenger gets on and off the seat, as well as how other protein parts of the cellular vehicle function for safe transit.
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