Cyclin B and Cdc2 Expression and Cd2 Kinase Activity During Dictyostelium Differentiation

Although Dictyostelium differentiation occurs in the absence of external nutrients, two periods of mitosis occur, one during early development and one during the formation of the migrating pseudoplasmodium. We showed previously that cyclin B mRNA levels vary in a cell cycle dependent manner during vegetative cell growth. In the present study, we report that cyclin B mRNA levels change dramatically during development, reaching a maximum at the tipped aggregate stage. However, amounts of cyclin B protein vary only slightly, peaking during early development and decreasing during late aggregation and pseudoplasmodial formation. Cdc2 protein levels also remain relatively constant during development. Cdc2-histone H1 kinase activity was considerably higher in vegetative cell extracts of transformants that expressed large amounts of truncated cyclin B protein in comparison to extracts of the parental Ax-2 cells. These results suggest that Cdc2 kinase activity is dependent upon the level of cyclin B in vegetative cells. This result is consistent with the idea that variations in the level of cyclin B during growth regulate the cell cycle. When Cdc2 histone H1 kinase activity was determined during development, it was also found that activity correlated reasonably well with the amount of cyclin B protein. Thus, there was an increase in Cdc2 histone H1 kinase activity early in development, and then levels decreased as development progressed. The increase in Cdc2 histone H1 kinase activity that occurs early in development following starvation may be important in accelerating G₂-phase cells through into mitosis. There was no increase in Cdc2 histone H1 kinase that accompanied the previously reported late developmental mitosis. When cells were grown into stationary phase, both cyclin B and Cdc2 protein levels decreased to negligible levels. When these stationary phase cells were allowed to differentiate, cyclin B protein was synthesized within the first 4 hr of development and levels then remained relatively constant for the remainder of the developmental period. In contrast, Cdc2 protein was synthesized later with accumulation peaking between 14 and 19 hr. These results indicate that both proteins are synthesized during development. The levels of Dictyostelium cdcA and cyclin B (cycB) mRNAs are coordinately regulated during growth and early development, but as development progresses cdcA mRNA accumulates earlier than cycB mRNA. Both mRNA species disappear during the terminal stages of differentiation.