CHARACTERIZATION OF AN INTRACELLULAR CA2+ PUMP IN DICTYOSTELIUM

Calcium uptake by microsomal membranes from the cellular slime mould D ictyostelium discoideum was measured using Calcium Green-2 as a fluore scent probe of external free Ca2+ concentration. High-affinity Ca2+ up take was found to be completely inhibited by low concentrations of van adate, but not by thapsigargin, suggesting that the activity is mediat ed by a Ca2+-ATPase distinct from sarco(endo)plasmic reticulum type of higher animal cells. On sucrose density gradients, Ca2+ uptake distri butes with vacuolar proton pumpo activity and part of the observed Ca2 + uptake is dependent on the pH gradient generated by the vacuolar-typ e H+-ATPase, indicating that the Ca2+ pump is located on both acidic a nd non-acidic vesicles, possibly derived from the H+-ATPase-rich contr actile vacuole complex.