By David N. Leff
Any of the big overnight package-delivery services, which have been in the news lately, could learn a lot from the smallest and most sophisticated package delivery system in the world.
It operates nonstop inside every cell of every mammal and plant on earth. The products that this intracellular distribution enterprise ships are proteins.
As a newly fabricated protein moves off the ribosomal factory floor onto the loading dock of the endoplasmic reticulum (ER), it enters an elaborate sorting process, which first separates long-distance secretory proteins — bound for export beyond the cell — from local ones consigned to various users inside the cell.
From the ER, newly baked proteins move initially to the central sorting and finishing facility, the cell's Golgi complex.
Nobelist Camillo Golgi (1843-1926), an Italian physician, discovered the Golgi organelle just under a century ago, in 1898.
Since then, this ultra-minute mass of membranes and enzymes has remained, if not a black box, a gray one. Medical students learn from their textbooks, observed National Institutes of Health cell biologist Jennifer Lippincott-Schwartz, "that small, 50-nanometer vesicles bud off from the ER and carry proteins through the cytoplasm into the Golgi complex, then fuse."
That doctrine is challenged by an article in today's Nature, dated Sept. 4, 1997, of which Lippincott-Schwartz is the senior author. Its title: "ER-to-Golgi transport visualized in living cells." She directs the laboratory of intracellular organelle biology at the National Institute of Child Health and Human Development, in Bethesda, Md.
"The popular view," Lippincott-Schwartz told BioWorld Today, "has been that the little vesicles budded off from stable pre-Golgi structures adjoining the ER and tracked proteins into the Golgi complex.
"The alternative possibility," she went on, "was that these pre-Golgi structures themselves were not stable, but transient in nature, and that they, rather than vesicles, served as the transport vehicles for transferring membranes and proteins en masse between organelles. Our data in Nature show clearly that that's the case."
This interpretation, she pointed out, "has some important implications for the biogenesis of organelles. For instance, my personal view is that these pre-Golgi structures are the progenitors of the Golgi apparatus itself."
Academic-Research Offers Drug Designers Insights
Lippincott-Schwartz grants that these new insights into the inner workings of the cell are "on a more academic level than a lot of the drug-discovery people would be interested in. The only thing that would probably interest them is the clear role of microtubules in the ER-Golgi traffic."
But she added a flip side to that estimation: "If by refining our understanding of these intracellular pathways we can have a better grasp of how medicinal drugs work, that's where I think the importance is going to play, further down the road. Now we have to reevaluate how these various drugs interfere with this organelle system. We know the global phenotypic effects of drugs, but now that we see in more detail how these processes actually work, we can try to fine-tune our understanding of the drugs."
She compares the ER to a bowl of spaghetti, extending all over the cell's interior, and the Golgi apparatus to a meatball. "One of the things we found in this study," Lippincott-Schwartz pointed out, "is that proteins that leave the ER do so from the whole plate of spaghetti, at apparently random sites all over the place."
She continued: "That's why it's critical for them to travel over microtubules — which are there all the time, like railroad tracks — that track into the area where the Golgi is. Those distances can be 10 to 20 microns.
"One of the things we show in this paper," she went on, "is that when we inactivate a particular microtubule motor, these pre-Golgi containers don't move at all. That means they have a microtubule motor on their surface, which enables them to interact with these microtubule tracks.
"The motors," she explained, "are powered by energy from ATP [adenosine triphosphate]. That walking function is driven by a lever arm that undergoes a conformational change, which allows the protein to move in a direct fashion along the microtubule."
Two advanced technologies allowed her organelle laboratory to generate its new departure in membrane trafficking to the Golgi. One was a novel fluorescent cell marker; the other, creation of a motion picture, rather than static electron micrographs, to analyze the action.
Their secretory protein model was a mutant form of vesicular stomatitis virus glycoprotein. At high temperatures, this obliging molecule won't budge from the ER. Cooled, it departs very efficiently and heads for the Golgi.
Onto the tail of this working model, the team fused a fluorescent protein from a jellyfish named Aequrea victoria. This animal switches on its greenish glow when activated by light, as in this case, from a fluorescent microscope.
Live-Action Movie On A Web Site Near You
Armed with this construct, Lippincott-Schwartz and her team proceeded, in effect, to turn microscopy into a motion picture.
"What we did," she said, "was collect images at 3.6-second intervals by shining a light, and the protein fluoresces back, like a stroboscope. As our protein of interest moved to different places within the cell, we could follow its movements, watch the fluorescent pre-Golgi structures migrate into that region, and dock and fuse."
Lippincott-Schwartz has put a time-lapsed video on the world wide web for all to see: http://dir.nichd.nih.gov/CBMB/pb1labob.html.
Now topping her group's research agenda is "looking at paths by which this model glycoprotein that we have tagged with fluorescence moves on from the Golgi complex to the cell surface. That's a whole other area that's been really black box. Nobody has ever seen anything move in living cells from the Golgi to the plasma membrane," Lippincott-Schwartz concluded, "and so we're imaging that right now." *