Kv. Nolta et Tl. Steck, ISOLATION AND INITIAL CHARACTERIZATION OF THE BIPARTITE CONTRACTILE VACUOLE COMPLEX FROM DICTYOSTELIUM-DISCOIDEUM, The Journal of biological chemistry, 269(3), 1994, pp. 2225-2233
The contractile vacuole complex serves to excrete excess cytosolic wat
er from protists. In the amoeba, Dictyostelium discoideum, the organel
le had a bipartite morphology: a large main vacuole (bladder) marked b
y lumenal alkaline phosphatase was surrounded by numerous satellite va
cuoles (spongiomes). Bladders and spongiomes have now been purified fo
r the first time. The spongiome membranes had a high density of surfac
e projections identified as catalytically-active vacuolar proton pumps
(V-H+-ATPase). Spongiomes were resolved from the pump-poor bladders b
y immunogold buoyant density shift with antibodies to the V-H+-ATPase;
they contained little protein other than this pump. It appears that,
following homogenization, most of the spongiome dissociated from bladd
ers and populated the proton pump-rich membrane fraction called acidos
omes. Isolated bladders were enriched >40-fold in alkaline phosphatase
and phosphodiesterase, the activities of which were >85% latent. Blad
ders depleted of spongiomes bore several distinctive polypeptides; the
y also had an excess of the basepieces of the proton pump over the cat
alytic heads. Bladder membranes were also lipid-rich and had a distinc
tive lipid composition. We conclude that the contractile vacuole syste
m in Dictyostelium is a complex of discrete, separable bladder and spo
ngiome membranes. The V-H+-ATPase in the spongiome may catalyze the pr
imary energy transduction step for pumping water out of the cytoplasm.