Acidosomes are organelles that in Paramecium are responsible for the acidification of phagosomes before phagosomes fuse with lysosomes. Using a combination

Acidosomes were first identified in living Paramecium mul timicronucleatum on the basis of their intense accumulation of weak bases such as acridine orange and neutral red (Allen and Fok, 1983c). Ultrastructurally, mature acidosomes have a distinctive membrane appearance, following freeze-fracture studies (Allen and Fok, 1983b) as well as a distinctive internal appearance, following the QF-DE technique (Schroeder et al., 1990). As digestive vacuoles or phagosomes grow in cells that are pulsed briefly with acridine orange, they acquire a layer of bright red-orange acidosomes next to their membranes. These acidosomes pulsate actively next to the growing vacuole membrane in these living cells. This pulsation is putatively powered by the oral cilia that facilitate food intake. As the digestive vacuoles are released from the cytopharynx (DV-I), these red-orange acidosomes accompany the vacuoles to the cell’s posterior region (Allen and Fok, 1983c). Induced by an as yet unknown stimulus these acidosomes fuse with the vacuole, fusion occurs synchronously often within 15 s of vacuole release (Allen and Fok, 1983c). At this time, the acidosome membrane is incorporated into the vacuole membrane (forming a DV-II) as the membrane of the original vacuole is being retrieved in the form of short tubules that are pinched from the vacuole surface (Allen and Fok, 1983a). Vacuoles do not become acid if acidosome-vacuole fusion is prevented as is the case when cytochalasin B is present (Allen and Fok, 1983c). Furthermore, lysosomes will not bind to unacidified phagosomes (Fok et al., 1987). Thus, it has been established that acidosome fusion is needed for the normal acidification of vacuoles in Paramecium and for the subsequent attachment of lysosomes to the vacuoles. Acidosomes have now been found in Paramecium bur saria (Meier and Wiessner, 1989) and published electron micrographs suggest that such electron-lucent organelles exist in Paramecium tetraurelia (Luthe and Plattner, 1986). Acidosomes are probably present in other ciliates as well, though a systematic search for such organelles has not been carried out. The isolated “acidosomes” reported to be present in Dictyostelium discoideum (Padh et al., 1991a; Nolta et al., 1991) do not appear to be homologous to those in Paramecium, as the “acidosomes” in Dictyostelium are not acidic (Padh et al., 1991b), although they possess a proton pump. An antibody raised against the Dictyostelium “acidosome” vacuolar ATPase strongly reacts with the dec411 Journal of Cell Science 106, 411-422 (1993) Printed in Great Britain © The Company of Biologists Limited 1993