347. Gladfelter et al. suggest that the Ashbya gossypii filamentous fungus evolved a new means to control cyclin activity that might make asynchronous nuclear division possible.
Cyclin activity drives the mitotic cycle in eukaryotes. To prevent untimely mitosis, most cells degrade mitotic cyclins at telophase and show an oscillating pattern of cyclin protein levels. As cyclins can enter and exit nuclei and diffuse throughout the cytoplasm, multinucleated cells normally have synchronous nuclear divisions.
But the new report shows that A. gossypii nuclei divide independently of their neighbors. Nuclei at various stages of the cell cycle were found within single cells. Artificially synchronized nuclei did not remain in synch for long, suggesting that each nucleus divides on its own time frame. Such autonomy might allow the cell to grow in some spots (perhaps where nutrients are richest) without burdening the other nuclei.
This unusual nuclear independence might be possible, according to the authors, because A. gossypii does not rely on oscillations in cyclin levels to control mitosis. G1 and mitotic cyclins were present at every nucleus independent of its division status. And the loss of proteosome-mediated cyclin degradation did not interfere with cell cycle progression, as it does in other cells.
The group has not proven the mechanistic basis of asynchrony, but they propose that cyclin activity, rather than abundance, drives the A. gossypii cell cycle. Cyclin activity might be controlled by Sic1, which in budding yeast is a CDK–cyclin inhibitor. Budding yeast cells normally degrade Sic1 to escape G1, but the filamentous fungus instead seems to regulate its localization. Before division, Sic1 was found throughout the nucleoplasm. At times when cyclin activity should be highest, Sic1 was concentrated at spindle pole bodies, perhaps thereby turning it off.
Most cells have intricate and redundant mechanisms that assure precise cell cycle progression. If indeed cyclin oscillation has been lost as one such means, A. gossypii might have traded nuclear accuracy for autonomy. Its abundant nuclei, however, may protect the fungus from any resulting sloppiness.