Glucose-induced pathways for actin tyrosine dephosphorylation during Dictyostelium spore germination

In the presence of germination signals, dormant spores of Dictyostelium discoideum rapidly germinate to start a new life cycle. Previously we have shown that half of the actin molecules in spores are maintained in a tyrosine-phosphorylated state, and a decline of the actin phosphorylation levels is a prerequisite for spore swelling. In this study, we have established d-glucose as a trigger molecule for the actin dephosphorylation. Present in a nutrient germination medium, d-glucose both may act as a trigger molecule and/or may serve as a substrate within a pathway for actin dephosphorylation depending upon spore age. However, the glucose-induced actin dephosphorylation was insufficient for spores to swell. Other factors in the nutrient medium were required for complete germination of young spores aged 1 to 5 days. In contrast, dispersion in nonnutrient buffer was necessary and sufficient for a decline of actin phosphorylation levels and even the emergence of amoebae in older spores (6 days and beyond). Moreover, the dephosphorylation pathway in the older spores was independent of energy production. We propose that the diversification of the actin dephosphorylation pathway may enable spores to increase their probability of germination upon spore aging.