ACIDOSOMES - RECIPIENTS OF MULTIPLE SOURCES OF MEMBRANE AND CARGO DURING DEVELOPMENT AND MATURATION

Acidosomes are organelles that in Paramecium are responsible for the a cidification of phagosomes before phagosomes fuse with lysosomes. Usin g a combination of (a) the quick-freeze deep-etch (QF-DE) technique, ( b) monoclonal antibodies (mAbs) that label specific membrane pools inc luding those of the acidosomes, and (c) horseradish peroxidase (HRP)-u ptake studies, we followed the development of acidosomes from the Golg i complex as well as the rapid transfer of HRP into the acidosomes. We also studied some of the characteristics of the involved membrane poo ls. Morphologically, acidosomes were first detected in the cytosol nea r the ER and Golgi stacks as clumps of tubules and vesicles, which app arently coalesced to form larger spherical or elongated preacidosomes. These clumped vesicles and preacidosomes had a QF-DE morphology resem bling that of the mature acidosomes and were specifically labeled with mAbs that also labeled mature acidosomes. Within 10 s HRP cargo could be internalized by acidosomes while they were docked at the nascent v acuole membrane. This rapid uptake of HRP along with membrane occurs b y vesicle fusion, a conclusion supported by QF-DE images. Thus the aci dosome obtains its membrane from at least two sources, from the trans- Golgi network, and from the small HRP-containing vesicles. Cargo can a lso be acquired from two sources, the Golgi apparatus and the transpor t vesicles. Since it acquires non-particulate exogenous marker we conc lude that the acidosome is linked to the endocytic pathway.