Occurrence and transport of particle “tetrads” in the cell membranes of the unicellular red alga porphyridium visualized by freeze-fracture

Abstract

The supramolecular organization of biomembranes in the unicellular red algae Porphyridium cruentum and Porphyridium aerugineum is studied in replicas of rapidly frozen and fractured cells. Intramembrane particle complexes composed of four particles (“tetrads”, average diameter 9 nm) have been observed in the protoplasmic fracture (PF) face of the plasma membrane, Golgi cisternae, and Golgi-derived vesicles. The occurrence of particle tetrads in the Golgi cisternae and vesicles suggests that the Golgi complex is responsible for the selective transport and exocytosis of tetrads in the red algaPorphyridium, as has been previously indicated for rosettes of six particles in the algaMicrasterias (T. H. Giddings, Jr., D. L. Brower, and L. A. Staehelin (1980)J. Cell Biol.84, 327–339) and in higher plants (C. H. Haigler and R. M. Brown, Jr. (1986)Protoplasma134, 111–120). The plasma membrane exhibits two distinct populations of intramembrane particles (with diameters between 8.5 and 9.5 nm and between 10.5 and 11.5 nm, respectively). The tetrad frequency varies between 16 and 33 μm−2. The particle density of the PF face of the plasma membrane is 2400 μm−2, while that of the exoplasmic face is 260 μm−2. Considering that the Golgi complex is involved in the synthesis and export of cell surface polysaccharides in the red algaPorphyridium (J. Ramus (1972)J. Phycol.8, 97–111;J. Ramus and D. M. Robins (1975)J. Phycol.11, 70–74;L. V. Evans, M. E. Callow, E. Percival, and V. Farreed (1974)J. Cell Sci.16, 1–21;I. Tsekos (1985)Protoplasma129, 127–136) it could be assumed that the tetrads represent a membrane-bound multienzyme complex participating in the synthesis of extracellular polysaccharides.