Sci. STKE, 7 February 2006
CELL CYCLE Dividing on a Different Schedule
Some multinucleated cells progress through the cell cycle synchronously, and the linear progression occurs by regulated degradation of cyclins, just as in mononucleated cells. However, there are other examples where the nuclei undergo asynchronous divisions. Gladfelter et al. show that the nuclei of the multinucleated filamentous fungi Ashbya gossypii divide asynchronously without requiring degradation of cyclins. A. gossypii have genes that are homologous to those in Saccharomyces cerevisiae for cell cycle regulatory components, and the A. gossypii G1 cyclins have degradation motifs. Yet, two A. gossypii G1 cyclins (AgCln1/2 and AgClb1/2) were present in nuclei at all stages of mitosis as well as present (although less abundantly) in the cytoplasm, and the abundance of AgClb1/2 was unchanged as the nuclei exited mitosis. Indeed, strains containing nondegradable forms of AgClb1/2 (with deleted D-box motifs, which are degradation sequences) showed no change in rate of mitotic exit or nuclear asynchrony. (In other systems, these mutations would produce cell cycle arrest due to the inability to exit mitosis.) Instead, the localization of the cyclin-dependent kinase inhibitor AgSic1 appeared to be the mitotic clock. AgSic1 abundance in the nucleus, where it appeared to associate with the spindle apparatus, correlated with different stages of the cell cycle. Strains deleted for Agsic1 exhibited growth arrest, further implicating this as a key regulator. Thus, this represents a new mechanism for regulating the cell cycle. It will be interesting to see if this regulatory mechanism applies in multinucleated cells in higher organisms.
Citation: Dividing on a Different Schedule. Sci. STKE 2006, tw50 (2006).
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