Limited functional redundancy and oscillation of cyclins in multinucleated Ashbya gossypii fungal cells

Cyclin protein behavior has not been systematically investigated in multinucleated cells with asynchronous mitoses. Cyclins are canonical oscillating cell cycle proteins, but it is unclear how fluctuating protein gradients can be established in multinucleated cells where nuclei in different stages of the division cycle share the cytoplasm. Previous work in A. gossypii, a filamentous fungus in which nuclei divide asynchronously in a common cytoplasm, demonstrated that one G1 and one B-type cyclin do not fluctuate in abundance across the division cycle. We have undertaken a comprehensive analysis of all G1 and B-type cyclins in A. gossypii to determine whether any of the cyclins show periodic abundance across the cell cycle and to examine whether cyclins exhibit functional redundancy in such a cellular environment. We localized all G1 and B-type cyclins and notably found that only AgClb5/6p varies in subcellular localization during the division cycle. AgClb5/6p is lost from nuclei at the meta-anaphase transition in a D-box-dependent manner. These data demonstrate that efficient nuclear autonomous protein degradation can occur within multinucleated cells residing in a common cytoplasm. We have shown that three of the five cyclins in A. gossypii are essential genes, indicating that there is minimal functional redundancy in this multinucleated system. In addition, we have identified a cyclin, AgClb3/4p, that is essential only for sporulation. We propose that the cohabitation of different cyclins in nuclei has led to enhanced substrate specificity and limited functional redundancy within classes of cyclins in multinucleated cells.