Annals of Botany 72: 213-222, 1993
© 1993 Annals of Botany Company
The Supramolecular Organization of Red Algal Vacuole Membrane Visualized by Freeze-fracture
Institute of Botany, University of Thessaloniki, 54006- Thessaloniki, Macedonia, Greece and Zellenlehre, Universität Heidelberg, Germany
The supramolecular organization of the vacuole membrane (or of the membranes of mucilage sacs) in 27 species of red algae is studied in replicas of rapidly frozen and fractured cells. Intramembranous particle complexes composed of four particles ('tetrads' with average diameters between 8·5 and 14·5 have been observed in the protoplasmic fracture (PF) face but most clearly and more frequently in the exoplasmic fracture (EF) face of the vacuole membrane of all red algae investigated. The tetrads lie individually within the vacuole membrane or form clusters in several species and are randomly distributed. In the species Ceramium diaphanum var. strictum and Laurencia obtusa the intramembranous particle complexes ('tetrads') have been observed both in the EF and PF faces of the vacuole membrane; the 'membrane tetrads' at least as regards these two species seem to span both the outer and inner leaflets of the vacuole membrane ('transmembrane particles'). The occurrence of particle tetrads in the plasma membrane is probably due to exocytosis either of the Golgi vesicles or of the mucilage sacs. Tetrad frequency in the EF face of the vacuole membranes of the investigated red algae varies between 2 and 87 µm-2, while that of single particles varies between 102 and 695 µm-2. The PF face of the vacuole membrane is characterized by a higher particle density than the EF face. The particle densities of the PF and EF faces of the plasma membrane for a given species are higher than those of the corresponding fracture faces of the vacuole membrane. Some members of Bangiophycidae bear smaller protein particles (diameter between 8·5 and 10·5 nm) in comparison with those of Florideophycidae (diameter between 10·5 and 14·5 nm). It is suggested, based upon the particle tetrads lying in depressions of the vacuole membrane and the origin of vacuoles (mucilage sacs) from ER, that the particle tetrads originate from the ER or the Golgi complex. Since vacuoles (mucilage sacs) in red algae, along with the Golgi complex, are involved in the synthesis and export of cell surface polysaccharides, it could be assumed that the 'membrane-tetrads' within the vacuole membrane represent a membrane-bound multienzyme complex, participating in the synthesis of amorphous extracellular matrix polysaccharides.Copyright 1993, 1999 Academic Press
Red algae, freeze-fracture, vacuole membrane, mucilage sacs, membrane tetrads, supramolecular organization