New Cellular 'Bones' Revealed

Filament-making proteins offer hints to cell structure

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By Tina Hesman Saey, Science News

PHILADELPHIA—Scientists may have uncovered a new type of skeleton in cells’ closets.

Cells harbor several newly discovered types of filaments, Jim Wilhelm of the University of California, San Diego reported December 12 at the annual meeting of the American Society for Cell Biology. These filaments, formed from strings of metabolic proteins, could give researchers clues about how the cell’s internal skeleton evolved.

In experiments with yeast, Wilhelm and his colleagues discovered that an enzyme called CTP synthase can make filaments. The enzyme produces a molecule that is similar to ATP, a cell’s main energy currency. CTP is necessary for many chemical reactions, which it participates in and also fuels.

The team found that when CTP levels in the cell rise in yeast, the enzyme forms filaments.

Fruit flies also harbor CTP synthase filaments in their egg cells, the researchers demonstrated. At about the same time, other research groups discovered filaments of the enzyme in bacteria and in human brain cells. Wilhelm says the researchers don’t yet know if the filaments help form the cellular skeleton in the human, fruit fly and yeast cells in which they are found. But another group showed that the filaments do affect the shape of some bacterial cells.

The discovery of the filaments in organisms as diverse as bacteria and humans suggests that the structures may have an important function, says Dyche Mullins, a cell biologist at the University of California, San Francisco.

Cell biologists already knew that actin, one of the most important proteins for forming a cell’s skeletal structure, is closely related to another metabolically important enzyme called hexokinase. The assumption has been that actin started out as an enzyme but that its ability to build structures within the cell eventually became its primary function. The newly discovered filaments may be a snapshot of an enzyme in the process of taking on a new role as a structural protein, Mullins says. But because CTP synthase hasn’t fully made the transition to structural protein in billions of years of evolution, these filaments are probably as far as it will go. “It could be that CTP synthase is not as well suited to do multiple things as actin is,” Mullins says.

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