Phagosome fusion vesicles of Paramecium. II. Freeze-fracture evidence for membrane replacement.

Phagosome fusion vesicles (PFVs), a new population of relatively large granules in Paramecium caudatum which fuse with the first stage of digestive vacuoles (DV-I) shortly after these vacuoles are released from the cytopharynx (their site of formation), have been studied by using the freeze-fracture technique. Identification of PFVs is possible in the resulting replicas at all sites where they are commonly found in thin sections, at the cytopharynx, bound but not fused with nascent digestive vacuoles and fused with released vacuoles in the cell's posterior end. These PFVs have membranes which do not resemble the membranes of the forming digestive vacuole membrane or the discoidal vesicle membranes from which vacuole membrane is derived. Their smooth E-fracture face with only 50 to 100 intramembrane particles (IMPs) per micrometers 2 and particulate P-face (approximately 2500 IMPs/micrometers) do resemble the second vacuole stage (DV-II) which is characterized by a smaller diameter and acid pH. Evidence is presented for PFV fusion with the DV-I and for membrane replacement, at least in part, as the DV-I becomes a DV-II. Membrane replacement entails first adding PFVs to the DV-I and then removing the original discoidal vesicle-derived membrane as tubules as the vacuole condenses. Implications of the possible role of PFVs in forming intravacuolar symbiotic relationships are also discussed.