RAPID HULK REPLACEMENT OF ACCEPTOR MEMBRANE BY DONOR MEMBRANE DURING PHAGOSOME TO PHAGOACIDOSOME TRANSFORMATION IN PARAMECIUM

The extent to which a donor membrane will be retrieved, or if it is re trieved at all after it fuses with an acceptor membrane, is usually di fficult to determine. We have studied the dynamics of membrane retriev al in the phagosome system of Paramecium multimicronucleatum using six monoclonal antibody markers. Our previous freeze-fracture and transmi ssion electron microscopic studies have indicated that extensive chang es take place in the membrane of the young phagosome as it progresses through its cycle. Using immunofluorescence and immunoelectron microsc opy to determine the times of entry and exit of these individual antig ens into the digestive vacuole system, we showed that two hydrophilic antigens, one located on the cytosolic and one on the lumenal side of the discoidal membrane (phagosome membrane precursor), were completely retrieved from the phagosome by tubulation within the first three min utes. At the same time that this membrane was retrieved, membrane from a second population of vesicles, the acidosomes, fused with the phago some to produce the phagoacidosome. On the basis of immunogold localiz ation on cryosections of a total of six antigens, the two specific for phagosome/discoidal vesicle membrane as well as four specific for the acidosome/phagoacidosome membrane, this replacement is total. We also showed that in the presence of the actin-active drug cytochalasin B, this replacement was essentially prevented. However, when vacuole acid ification was neutralized by ammonium chloride, this replacement proce ss continued unaffected after a lag. Consequently, acidification, per se, is not required to trigger the replacement of the phagosome membra ne. We conclude, on the basis of these studies as well as our previous freeze-fracture studies that during phagoacidosome formation most of the acceptor membrane is retrieved and is replaced by the donor membra ne. This shows that at least one cell type possesses the mechanisms ne eded to substantially replace the membrane of a phagosomal compartment when radical and rapid changes are needed to modulate the digestive a nd absorptive processes.