Cytoskeletal proteins aren't the only filament-forming molecules in cells, say Noree et al., who identify several metabolic enzymes and translation factors that also assemble into cytoplasmic threads.

Many cytosolic proteins assemble into large, supramolecular complexes such as the P bodies that regulate mRNAs. In an effort to identify novel components of these complexes, or even new structures entirely, Noree et al. screened a yeast library for GFP-tagged proteins that form cytoplasmic aggregates big enough to see by fluorescence microscopy.

The screen uncovered nine proteins that formed cytoplasmic filaments. Several translation factors co-assembled into one type of filament, while the metabolic enzymes glutamate synthase, GDP-mannose pyrophosphorylase, and CTP synthase each formed their own unique cytoplasmic strands. CTP synthase also formed filaments in Drosophila tissues and mammalian neurons. CTP synthase fibers probably comprise the inactive form of the enzyme, because a mutation blocking feedback inhibition by CTP also prevented filament assembly, whereas boosting CTP levels increased the enzyme's aggregation.

Self-assembly may therefore control enzymatic activity depending on the cell's metabolic state, though senior author James Wilhelm thinks these structures might also be used for other purposes. CTP synthase filaments are specifically found in the axons of rat nerve cells, suggesting they have a specialized function there, perhaps as a metabolically regulated scaffold for other proteins. Wilhelm now wants to investigate possible functions of these new cytoplasmic filaments, as well as study the mechanism and regulation of their assembly.

et al
J. Cell Biol.