CALCIUM-SEQUESTERING ORGANELLES OF DICTYOSTELIUM-DISCOIDEUM - CHANGESIN ELEMENT CONTENT DURING EARLY DEVELOPMENT AS MEASURED BY ELECTRON-PROBE X-RAY-MICROANALYSIS

Starving Dictyostelium discoideum amoebae aggregate within a few hours by chemotaxis towards the attractant cAMP to form a multicellular org anism. The differentiation cells possess rapid and efficient calcium b uffering and sequestration systems which enable them to restrict chang es in the cytosolic free calcium concentration temporally and spatiall y during their chemotactic reaction and allow the continous accumulati on of Ca2+ during development. In order to identify and to characteriz e calcium storage compartments, we analyzed the element content of amo ebae at three consecutive stages of differentiation. Determination of the element distribution was done using energy-dispersive X-ray microa nalysis of freeze-dried cryosections of rapid frozen cells. Amoebae we re frozen in the vegetative and aggregation-competent state and after formation of aggregates. Aggregation-competent as well as aggregated c ells contained mass dense granules with large amounts of calcium toget her with phosphorous and either potassium or magnesium: in aggregation -competent cells calcium was colocalized with potassium, whereas in ag gregated cells the mass dense granules contained calcium and magnesium . Although mass dense granules were also present in undifferentiated, vegetative cells, they contained only low amounts of phosphorous and p otassium together with little Ca and Mg. We conclude that during their differentiation D. discoideum cells use an intracellular storage comp artment to sequester Ca and other cations constantly throughout develo pment.