Rd. Allen et al., RAPID HULK REPLACEMENT OF ACCEPTOR MEMBRANE BY DONOR MEMBRANE DURING PHAGOSOME TO PHAGOACIDOSOME TRANSFORMATION IN PARAMECIUM, Journal of Cell Science, 108, 1995, pp. 1263-1274
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.