Dec. 16, 1998 ATHENS, Ga. -- Like commuters pushing onto a train, certain proteins in cells always have a ticket to ride. They move through small cavities called vesicles from one place to another so that certain crucial biochemical tasks can take place. These vesicles bud off specialized and organ-like cell parts called organelles, carrying their protein passengers to another site in the cell.
This elegant catch and release keeps a proper protein level in organelles so they can keep working in the cell. Scientists at the University of Georgia have, for the first time, described the shape of two important yeast proteins that make such transport possible in eukaryotic cells -- those with well-defined nuclei.
"The goal of our laboratory is to understand the structural basis of docking and fusion at the molecular level," said Dr. Leigh Ann Lipscomb, an assistant professor of biochemistry. Lipscomb presented her findings at the Protein Society meeting in San Diego earlier this year.
Here's how it works in yeast, the organism Lipscomb is studying. The cell releases the vesicles with their protein "passengers" in a process called budding. After budding, an interaction between proteins on the surface of the vesicle and the target organelle leads the "train" to its station where it fuses with the organelle and delivers its proteins. (Two proteins are involved, v-SNARE, which is in the vesicle, and t-SNARE, which is on the target organelle. SNARE stands for soluble N-ethylmaleimide sensitive fusion attachment protein receptor.)
The question scientists had not unraveled was exactly what structure these two proteins have in eukaryotic cells -- a crucial factor in understanding how they work. Only recently have the structures of two mammalian SNARE proteins been described, but presently no x-ray or nuclear magnetic resonance structures are available for these structures in yeast, so Lipscomb and her colleagues used several techniques, including infrared spectroscopy, to study the proteins.
"Our results suggest that v-SNAREs and t-SNAREs from yeast are highly alpha helical," said Lipscomb. "Mechanisms of protein transport and structures of v- and t-SNAREs are believed to be similar among eukaroytic organelles, so our results may reveal general features of these important proteins."
In other words, these proteins are coiled, and each has a region with a high propensity to wrap around its target protein like a snake wrapping around a limb.
Uncovering these structures is an important step forward for Lipscomb in understanding just how these transport proteins work. Before her work, researchers had speculated that these proteins folded in a rather unstructured way and only "knotted up" after they had interacted. Instead, Lipscomb found that both the v-SNAREs and t-SNAREs are already helical. Because this winding structure was heretofore unknown, scientists may have been off base is describing how they worked.
While the new information on these proteins is important, a number of questions about them remain unresolved. Researchers have found that the elimination of the gene that controls the v-SNARE proteins kills the cells, and so the proteins may have secondary and thus far unknown functions.
Lipscomb's laboratory designed an expression system so the scientists would have enough of the proteins to study. They used a common expression system using a bacterium called E. coli to serve as a host to the proteins, which are expressed as the bacterium grows.
Unfortunately, making v-SNARE crystals for study has proved elusive. Lipscomb has doubts now that v-SNAREs can be crystallized, though she holds out some hope for t-SNAREs.
"The good thing is that we can learn a lot about v-SNAREs from t-SNARES," she said. "There doesn't seem to be a lot of homology in the genetic sequences between them, but we strongly suspect that there are structural similarities."
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