Brefeldin A: A Tool for Plant Cell Biologists?

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.

Transport of sterols to the plasma membrane of leek seedlings

To investigate the intracellular transport of sterols in etiolated leek (Allium porrum L.) seedlings, in vivo pulse-chase experiments with [1-14C]acetate were performed. Then, endoplasmic reticulum-, Golgi-, and plasma membrane (PM)-enriched fractions were prepared and analyzed for the radioactivity incorporated into free sterols. In leek seedlings sterols are present as a mixture in which (24R)-24-ethylcholest-5-en-3beta-ol is by far the major compound (around 60%). The other sterols are represented by cholest-5-en-3beta-ol, 24-methyl-cholest-5-en-3beta-ol, (24S)-24-ethylcholesta-5,22E-dien-3beta-ol, and stigmasta-5, 24(24(1))Z-dien-3beta-ol. These compounds are shown to reside mainly in the PM. Our results clearly indicate that free sterols are actively transported from the endoplasmic reticulum to the PM during the first 60 min of chase, with kinetics very similar to that of phosphatidylserine. Such a transport was found to be decreased at low temperature (12 degreesC) and following treatment with monensin and brefeldin A. These data are consistent with a membrane-mediated process for the intracellular transport of sterols to the PM, which likely involves the Golgi apparatus.