Digestive system membranes: freeze-fracture evidence for differentiation and flow in Paramecium.

ABSTRACT

Freeze-fractured membranes of digestive vacuoles of randomly feeding Paramecium caudatum exhibit dramatic differences in intramembrane particle (IMP) number and distribution on both E- and P-fracture faces. By pulse-feeding latex spheres to cells we have demonstrated that these differences are related to the age of the digestive vacuoles, and that the membranes of such vacuoles undergo a specific sequence of changes during the digestive cycle. Young digestive vacuoles (DV-I; less than or equal to 6 min), nascent vacuoles still connected to the cytopharynx, and discoidal vesicles, from which vacuole membrane is derived, all have a highly particulate E face and a less particulate P face. As early as 3 min after feeding, a second category of digestive vacuoles (DV-II) can be recognized, which are both considerably smaller in diameter and lack particles on their E face. These findings suggest that the endocytic removal of DV-I membrane material associated with the formation of DV-II vacuoles involves a concomitant and selective removal of E-face particles, as essentially no changes are seen in the density of P-face particles on the two types of vacuoles. Beginning at 10 min the first DV-III vacuoles are encountered. These are both larger than the DV-II vacuoles and possess very prominent E-face particles, which resemble those on the E face of the numerous lysosomes bordering the digestive vacuoles. DV-III vacuoles also exhibit a substantial increase in P-face particles. These membrane changes closely parallel, and are probably correlated with, the physiological events occurring within the vacuole lumen concentration of food, killing of prey, and digestion. Calculations of the amount of membrane removed from DV-I to form DV-II and of the increase in membrane surface area during the transition from DV-II to DV-III indicate that as much as 90% of the initial phagosome (DV-I) membrane can be removed before digestion begins. The enlargment of DV-II must be caused by fusion with adjacent lysosomes which also contribute the new populations of IMPs to the DV-III membrane. The appearance of numerous endocytic structures on older DV-III vacuoles suggests that membrane is retrieved from DV-III before defecation.