Abstract

A Dictyostelium cell chooses its fate based on the time of its last division prior to the initiation of development. At the onset of development, freshly-divided cells tend to form the stalk of the fruiting body while late-G2 cells become the reproductive spores. How the phases of the cell cycle are linked to cell-type differentiation is unknown. To address this issue, we targeted for analysis two key cell-cycle regulators, Cdk1 and the Retinoblastoma orthologue RblA.
Using RNA-blot analysis and reporter-gene studies, we showed that cdk1 is expressed shortly before mitosis then again during mid-development. We also generated a series of doxycycline-inducible cdk1 mutants. Cells expressing cdk1Y15F were blocked in mitosis and displayed defects in spindle assembly, suggesting that Cdk1 dephosphorylation on tyrosine15 is a pivotal step in the G2/M transition of Dictyostelium. Those expressing cdk1T14A were smaller than the control strain, implying that Cdk1T14 phosphorylation is one mechanism by which Dictyostelium coordinates cell growth with cell division. When given the choice between becoming spores or stalk cells, Cdk1T14A cells opted for the stalk fate. Importantly the developmental phenotype was rescued when cdk1T14A was expressed in RblA-deficient cells. By comparing the transcriptional profiles of wild-type and RblA-deficient cells, we found that RblA-repressed genes associated with cell proliferation. These genes were expressed in late G2 then again in mid-development.
Collectively our findings support a model in which RblA and Cdk1 play opposing roles in the cell cycle. In this model RblA prevents cell-cycle progression by repressing mitosis and S-phase genes. Repression is relieved in late G2 and genes such as cdk1 are activated, thereby allowing cells to advance through the cell cycle. The antagonistic relationship between Retinoblastoma and Cdks is not new; it has long been established in other model systems. The novelty of our findings is that RblA and Cdk1 also influenced development. We found that cells with an active Cdk1 (or lacking a functional RblA) at the onset of development were ushered to the stalk pathway. This is important since it implies that these cell-cycle regulators are part of the intrinsic signalling pathway responsible for initial cell-type choice in Dictyostelium.