ARTICLE

MEMBRANE FUSION IN A MODEL SYSTEM : Mucocyst Secretion in Tetrahymena

Birgit Satir ¹, Caroline Schooley ¹, and Peter Satir ¹

¹ From the Department of Physiology-Anatomy and the Electron Microscope Laboratory, University of California, Berkeley, California 94720

The freeze-fracture, freeze-etch technique can be employed to reveal new details of the process of fusion of two unit membranes For this study, mucocyst discharge in Tetrahymena pyriformis provides a model system with certain general implications The undischarged mature mucocyst is a saclike, membrane-bound, secretory vesicle containing crystalline material The organelle tip finds its way toward a special site, a rosette of 150 Å diameter particles within the plasma membrane. To match this site, the mucocyst membrane forms an annulus of 110 Å diameter particles, above whose inner edge the rosette particles sit. Discharge of some mucocysts is triggered by fixation. As discharge proceeds, the organelle becomes spherical and its content changes from crystalline to amorphous. The cytoplasm between the two matching membrane sites is squeezed away and the membranes fuse Steps in membrane reorganization can be reconstructed from changes in rosette appearance in the fracture faces. First, a depression in the rosette—the fusion pocket—forms. The rosette particles spread at the lip as the pocket deepens and enlarges from 60 to 200 nm. The annulus particles then become visible at the lip, indicating completed fusion of the A fracture faces of mucocyst and plasma membranes The remaining B faces of the two membranes have opposite polarities When the content of the mucocyst is released, the edges of these faces join so that the unit membrane runs uninterruptedly around the lip and into the pocket.

CiteULike    Complore    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this?

This article has been cited by other articles:

• Nass, R., Rao, R. (1998). Novel Localization of a Na+/H+ Exchanger in a Late Endosomal Compartment of Yeast. IMPLICATIONS FOR VACUOLE BIOGENESIS. J. Biol. Chem. 273: 21054-21060 [Abstract] [Full Text]  
• White, J. (1992). Membrane fusion. Science 258: 917-924 [Abstract]  
• Cohen, F., Akabas, M., Finkelstein, A (1982). Osmotic swelling of phospholipid vesicles causes them to fuse with a planar phospholipid bilayer membrane. Science 217: 458-460 [Abstract]  
• Satir, B., Oberg, S. (1978). Paramecium fusion rosettes: possible function as Ca2+ gates. Science 199: 536-538 [Abstract]  
• Schober, R, Nitsch, C, Rinne, U, Morris, S. (1977). Calcium-induced displacement of membrane-associated particles upon aggregation of chromaffin granules. Science 195: 495-497 [Abstract]  
• Orci, L., Perrelet, A. (1973). Membrane-Associated Particles: Increase at Sites of Pinocytosis Demonstrated by Freeze-Etching. Science 181: 868-869 [Abstract]  

Home | Help | Feedback | Subscriptions | Archive | Search | Table of Contents