Jan. 29, 1999 CHAPEL HILL - In a study of life's beginnings, scientists at the University of North Carolina at Chapel Hill have moved a step closer to unraveling the biochemical mystery of embryogenesis, the process by which an egg cell transforms into an embryo.
At the heart of this new research is a pair of cellular proteins - SLBP1 and SLBP2 - which were identified in frog oocytes (immature egg cells) by researchers headed by Dr. William F. Marzluff, professor of biochemistry and biophysics at UNC-CH School of Medicine. In a report of the findings published in January's Molecular and Cellular Biology, Marzluff and his study colleagues suggest that the proteins act as separate "on" or "off" biochemical switches that trigger synthesis of histone proteins crucial to normal cell functioning during embryogenesis and throughout the organism's life. About half of the nucleoprotein complex known as DNA are histone proteins.
Although the new study looked at frog oocytes - largely because of their accessibility in large quantities - proteins structurally similar to SLBP1 have been identified in other organisms, including worms, flies, and mice, according to Dr. Thomas C. Ingledue, a postdoctoral fellow in Marzluff's laboratory. Ingledue is a lead co-author of the report along with Dr. Zeng-Feng Wang, formerly of UNC-CH and now with the Carnegie Institute of Embryology. "The new findings are already focusing our studies on other model systems, including mammalian," Ingledue says.
Studies have established that all genes are inactive - turned off - during early embryogenesis. In frog cells, they remain off until the embryo has gone through 12 rounds of division. "And if you don't activate genes during development but you have a need for a large amount of proteins to be used during development, then you better have those proteins stored. Or you better be ready to translate, or activate, stored RNA to make those proteins," Ingledue explains. "That's what the crux of our work became: to figure out how the oocyte activates stored RNA in its cytoplasm so it can generate enough protein to get it through the early stages of embryogenesis."
Part of the answer for the UNC-CH team was glimpsed in 1996 when Marzluff's researchers discovered SLBP1, stem-loop binding protein-1. This novel protein is located within the cell nucleus, where it "binds," or sticks, to histone messenger-RNA (mRNA) -- DNA's blueprint for histone proteins. The UNC-CH researchers had a hunch that the binding of SLBP1 to histone mRNA might play a role in processing this molecular blueprint - perhaps allowing it to leave the nucleus and enter the cytoplasm, the cell's factory floor. The new findings suggest this is so.
"What really opened the door to our understanding the entire process was identifying another SLBP protein, SLBP2," Ingledue says. "If you compare the proteins' respective amino acid chains, you notice the only regions of similarity are in the middle, roughly a third of each protein. But their ends are completely different. That suggests the proteins have some function in common, but those differences also suggest that there could be a great potential for the proteins to have different functions, and that was interesting to us."
But the biggest clue of all was finding differences in their patterns of localization inside the cell and in their temporal expression, Ingledue says.
SLBP1 was found in both the nucleus and cytoplasm. SLBP2 occurred only in the cytoplasm. "And if a protein is found only in the cytoplasm, that suggests that it does not have any nuclear functions," Ingledue points out. Subsequent tests revealed that to be true. SLBP2 did not assist in the processing of histone mRNA, he adds. Therefore, SLBP2 might suppress histone protein synthesis from amino acids. Indeed, as the oocyte becomes an egg, SLBP2 drops dramatically, virtually disappearing. Thus, suppression of histone protein synthesis is released.
As to SLBP1, Ingledue is impressed with this protein's molecular duties.
"It has a biochemical signal that takes it to the nucleus," he says. "It has another signal that localizes it to a specific region of the nucleus. Then it interacts with other proteins to make sure histone mRNA is properly processed. Then we also believe it's involved in getting the mRNA out of the nucleus. And if that's not enough, now it's making sure that histone mRNA instructions get translated -- all this from one very compact protein."
Ingledue's UNC-CH co-authors include Dr. Marzluff, Dr. Zbigniew Dominski, research assistant professor of biochemistry, and graduate student Ricardo Sanchez.
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