Paramecium Phagosome Membrane: From Oral Region to Cytoproct and Back Again1

Abstract

Ten years of research on digestive vacuoles (phagosomes) of Paramecium caudatum have revealed sequential changes both within the vacuole lumen as well as within the surrounding membrane. Four vacuole stages can be recognized by a combination of thin section and freeze-fracture ultrastructural features. Three sets of vesicles (discoidal vesicles, acidosomes, and lysosomes) fuse with the vacuole, each at a predetermined stage, to bring about these membrane and physiological changes. At various times membrane is removed as vesicles from the vacuole surface, which has the effect of regulating vacuole size. Membrane recycling, membrane replacement, and specific membrane to membrane recognition all appear to be operating during the digestive cycle. Details of these events are summarized in this address and a number of unanswered questions suggest areas for future research.HE digestive system of Paramecium has captured the in-

Distinct Subcellular Localization of a Group of Synaptobrevin-Like SNAREs in Paramecium tetraurelia and Effects of Silencing SNARE-Specific Chaperone NSF

“…In agreement with this the SNARE-specific chaperone NSF was also found enriched at the cytostome and the cytoproct after treatment of carefully permeabilized cells with ATP-␥-S and NEM, which inhibits NSF function (36). In fact, these sites are places of intense vesicle trafficking (2,3,5). Our present work identifies a previously unexpected high number of different synaptobrevin-like SNAREs associated with the cytostome and cytopharynx.…”

“…Their shape varies from round to slightly elongate and they are smaller (ϳ0.1 to 0.2 m) ( Fig. 7AЈ and BЈ) than acidosomes (ϳ0.8 m) (2,3,20). It is well possible that the observed differences in localization, i.e., along the peniculus, at the cytopharynx and on nascent food vacuoles, on early endosomes, and on trails of vesicles along microtubules ema- The cell surface staining pattern of PtSyb11 (Fig.…”

“…Vesicles of different sizes and origins travel close to the oral cavity and are frequently associated with the structures just mentioned. Once the food vacuole reaches a certain size, the nascent food vacuole is pinched off the cytopharynx and takes a defined route through the cytoplasm of the cell, termed cytoplasmic streaming or cyclosis (2), which is supported by specialized microtubule structures (54). Vesicles of an ϳ0.8-m size (acidosomes) situated at the site of food vacuole formation at the cytopharynx fuse with the nascent food vacuole after it has detached from the cytopharynx, and they drastically lower the pH of the phagosome lumen (48).…”

“…The whole cycle of digestion is completed after ϳ20 min. Membranes and digestive enzymes are recycled from the digestive vacuole, and undigested waste products are excreted by fusion of the digestive vacuole at a specialized site on the cell surface, the cytoproct (2,3). The membrane of the defecated vacuole is retrieved as ϳ100-nm discoidal vesicles and transported back along microtubular ribbons to the cytostome (2).…”

Food plaquette digestion in the ciliated protozoan Hyalophysa chattoni

“…Using apostomes, more may be learned about digestion and metamorphosis in symbiotic ciliates. Currently, much of what is known of the ciliate digestive cycle has been gathered from the free-living genera Paramecium and Tetrahymena (Elliott & Clemmons 1966;Nilsson 1976;Allen 1984;Radek & Hausmann 1996); less is known about digestion in symbiotic ciliates, whose endocytic pathways may shed light on basic cellular processes. Future experiments with H. chattoni will address the role of lysosomes in the digestive cycle.…”