The new organelle is three times as large as a ribosome and may be just as important for the cell’s functioning.
Open any biology textbook and you’ll find a diagram of an animal cell, with its internal organelles--the nucleus that houses the genes, for instance, and the ribosomes that translate those genetic instructions into proteins--all neatly labeled. The picture looks complete. But the truth is that the average cell is loaded with particles that show up only dimly under microscopes, if at all, and have never been studied. They all look like little blobs, says biochemist Leonard Rome of the UCLA School of Medicine.
Rome has found, however, that one of those blobs deserves a label of its own. It is a newly identified organelle that dots most cells by the thousands, is three times as large as a ribosome (which biologists discovered decades ago), and may be just as important for the cell’s functioning.
The new organelle had never stood out because it is made almost entirely of proteins, and the stains that render things visible under an electron microscope don’t stick well to proteins. (The stains are good at highlighting DNA, RNA, and membranes.) Rome and his colleague Nancy Kedersha made a virtue of this flaw. They isolated a sample of the new organelle--which had been cluttering up another experiment of theirs--and stained it negatively, splashing enough stain onto the sample that the organelle was the only thing not stained.
The stain makes puddles around the organelle, Rome explains. You see it because parts of it are a little higher and they jut out. And some stain is trapped in little troughs in the middle of the particle. Under the electron microscope the troughs and prominences resolved into a consistent, regular shape: that of an octagonal barrel, about three- millionths of an inch long. In the multiple arches of the barrel’s skeleton, Rome and Kedersha saw some of the beauty of a cathedral ceiling, so they gave the organelles the architectural name vaults.
After discovering vaults in rat liver cells, the UCLA researchers have gone on to find them in everything from slime molds to humans. Here we have this new, very intricate looking particle that’s highly conserved throughout evolution, says Rome. And it’s in great abundance in all nucleated cells. It’s bound to have some important function.
A clue to what that function might be is provided by the octagonal structure. Something else in a cell is octagonal: the pores in the nuclear membrane, through which molecules pass between the nucleus and the cytoplasm. If vaults were corks, they would be just the right size and shape to plug these pores. Indeed, such plugs have been spotted on micrographs of the nucleus before, but researchers have never been able to isolate and identify them.
Vaults may be the plugs. Their job, says Rome, would not be to seal off the nucleus, but just the opposite. The vaults are hollow, and they are often split in half. Sometimes the halves unfold into eight- petaled flowers. Rome thinks vaults may be cellular mail trucks that dock at nuclear pores, open up to load molecules manufactured in the nucleus, and then deliver their cargo elsewhere in the cell. The researchers have observed vaults not just near the nucleus, but circulating throughout the cytoplasm--which makes sense. Mail trucks don’t dock at the post office and stay there, says Rome. Most of the time they’re out making deliveries.
What are the vaults delivering? The most likely cargo is messenger RNA, a molecule that is a transcript of the gene for a particular protein and that somehow travels out of the nucleus to the ribosomes, which assemble the protein. One place where vaults seem to clump together is where the cell is building long fibers of the protein actin that make up part of the cell’s internal skeleton. Rome proposes that vaults might be ferrying the blueprint for actin to the construction sites.
That scenario is speculative. To test it, the UCLA group plans to eliminate vaults from cultured cells by disabling the gene for the vault protein; if the cells then fail to make actin, it will be strong evidence for the mail truck theory. But even if we’re not right about the vaults’ function, it’s bound to be something fascinating, says Rome. I think they’re going to be as important as they are beautiful.