TRANSPORT OF STEROLS TO THE PLASMA-MEMBRANE OF LEEK SEEDLINGS

To investigate the intracellular transport of sterols in etiolated lee k (Allium porrum L.) seedlings, in vivo pulse-chase experiments with [ 1-C-14]acetate were performed. Then, endoplasmic reticulum-, Golgi-, a nd plasma membrane (PM)-enriched fractions were prepared and analyzed for the radioactivity incorporated into free sterols. In leek seedling s sterols are present as a mixture in which (24R)-24-ethylcholest-5-en -3 beta-ol is by far the major compound (around 60%). The other sterol s are represented by cholest-5-en-3 beta-ol, 24-methyl-cholest-5-en-3 beta-ol, (24S)-24-ethylcholesta-5,22E-dien-3 beta-ol, and stigmasta-5, 24(24(1))Z-dien-3 beta-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 phosphatidylse rine. Such a transport was found to be decreased at low temperature (1 2 degrees C) and following treatment with monensin and brefeldin A. Th ese data are consistent with a membrane-mediated profess for the intra cellular transport of sterols to the PM, which likely involves the Gol gi apparatus.