Ultrastructure of the Golgi Apparatus and Contractile Vacuole in Chlamydomonas reinhardi

Brefeldin A action and recovery in Chlamydomonas are rapid and involve fusion and fission of Golgi cisternae

CHLAMYDOMONAS NOCTIGAMA has a non-motile Golgi apparatus consisting of several Golgi stacks adjacent to transitional ER. These domains are characterized by vesicle-budding profiles and the lack of ribosomes on the side of the ER proximal to the Golgi stacks. Immunogold labelling confirms the presence of COPI-proteins at the periphery of the Golgi stacks, and COPII-proteins at the ER-Golgi interface. After addition of BFA (10 microg/ml) a marked increase in the number of vesicular profiles lying between the ER and the Golgi stacks is seen. Serial sections of cells do not provide any evidence for the existence of tubular connections between the ER and the Golgi stacks, supporting the notion that COPI- but not COPII-vesicle production is affected by BFA. The fusion of COPII-vesicles at the CIS-Golgi apparatus apparently requires the presence of retrograde COPI-vesicles. After 15 min the cisternae of neighbouring Golgi stacks begin to fuse forming "mega-Golgis", which gradually curl before fragmenting into clusters of vesicles and tubules. These are surrounded by the transitional ER on which vesicle-budding profiles are still occasionally visible. Golgi remnants continue to survive for several hours and do not completely disappear. Washing out BFA leads to a very rapid reassembly of Golgi cisternae. At first, clusters of vesicles are seen adjacent to transitional ER, then "mini Golgis" are seen whose cisternae grow in length and number to produce "mega Golgis". These structures then divide by vertical fission to produce Golgi stacks of normal size and morphology roughly 60 min after drug wash-out.

Inhibition of Contractile Vacuole Function by Brefeldin A

Brefeldin A (BFA) causes a block in the secretory system of eukaryotic cells. In the scaly green flagellate Scherffelia dubia, BFA also interfered with the function of the contractile vacuoles (CVs). The CV is an osmoregulatory organelle which periodically expels fluid from the cell in many freshwater protists. Fusion of the CV membrane with the plasma membrane is apparently blocked by BFA in S. dubia. The two CVs of S. dubia swell and finally form large central vacuoles (LCVs). BFA-induced formation of LCVs depends on V-ATPase activity, and can be reversed by hypertonic media, suggesting that water accumulation in the LCVs is driven by osmosis. We suggest that the BFA-induced formation of LCVs represents a prolonged diastole phase. A normal diastole phase takes about 20 s and is difficult to investigate. Therefore, BFA-induced formation of LCVs in S. dubia represents a unique model system to investigate the diastole phase of the CV cycle.

Small G proteins of two green algae are localized to exocytic compartments and to flagella

The Ypt/Rab proteins are small GTPases, which belong to the Ras superfamily and have been shown to be involved in endo- and exocytosis in mammalian cells and yeast. Using affinity-purified antibodies specific for four Ypt proteins, namely Ypt1p, Ypt4p, Ypt5p and Ypt6p, of the multicellular green alga Volvox carteri (YptVp) and its close unicellular relative Chlamydomonas reinhardtii (YptCp), we examined the abundance of the corresponding antigens during the asexual life cycle of Volvox, and their intracellular localization. The YptV proteins were found in all stages throughout the asexual life cycle and are tightly associated with intracellular membranes. Indirect immunofluorescence revealed that YptV4p, YptV5p and YptV6p are present in perinuclear regions of the cell, indicating an association with the Golgi region. Golgi localization of YptV4p and YptV6p in Volvox was confirmed by immunogold electron microscopy. In contrast, we found Ypt1p associated with the contractile vacuole in both V. carteri and C. reinhardtii. Furthermore, the YptV proteins were also detected along the entire length of the flagella of somatic Volvox cells. This flagellar location was substantiated by western blot analysis of extracts prepared from isolated flagella of both algae. While localization to exocytic compartments is in agreement with the established Ypt/Rab function in intracellular vesicle transport of eukaryotic cells, presence in the algal flagellum is the first hint of a possible role for small G proteins also in motility organelles.