Intracellular localization of a lipid transfer protein in Vigna unguiculata seeds

Lipid transfer proteins (LTP) facilitate transfer of lipids between membranes in vitro. Up to now, they have been found to be localized basically in the plant cell wall and in compartments linked to lipid metabolism, such as glyoxysomes. Accordingly, LTP are considered to be involved in the plant defence against pathogen microbes and lipid metabolism. We herein show, by immunoelectron microscopy, that besides the cell wall, LTP are localized in the lumen of organelles which we suggest to be the protein storage vacuoles, as well as in vesicles similar to the lipid-containing ones and in the extracellular space of Vigna unguiculata seeds. To further characterize these organelles, we performed subcellular fractionation of membranes isolated from imbibed seeds on a sucrose-density gradient. The analysis of these fractions revealed that the lightest membrane vesicles, derived probably from PSV, contain LTP, alpha-TIP and K+ independent PP(i)ase, but not gamma-TIP and K+ stimulated PP(i)ase. The presence of LTP and vicilins (typical storage protein) in the lumen of these vesicles was confirmed by immunoelectron microscopy. Taken together, the data suggest that the intracellular LTP in the V. unguiculata seeds are localized in protein storage vacuoles and in lipid-containing vesicles.