The spectrum of lipid particles in Phaseolus vulgaris seeds

Abstract

Lipid particles with similar morphologies have been reported in cells and circulatory systems of many organisms. The best studied of these in plants are oil bodies found in the cytosol of cells from oil-bearing seeds. Oil bodies are spherical consisting of a triacylglycerol core surrounded by a monolayer of phospholipid which is coated with a protein shell composed of oleosins. Oleosin is the only protein reported to be associated with oil bodies at any point in their genesis.

In the present study, distinguishable populations of lipid particles were isolated from developing and germinating seeds of wax bean (Phaseolus vulgaris). These particles have been termed low-density particles (LDPs), intermediate-density particles (IDPs) and high-density particles (HDPs) to reflect their different densities. LDPs and IDPs are both enriched in triacylglycerol indicating that they are oil bodies, and they also have three major proteins none of which is oleosin. One of the three major proteins identified in these particles is a ~ 52 kDa protein, which, despite a protease protected fragment, is too large for an oleosin and does not appear in developing seeds, as do oleosins. A second major protein, 22.9 kDa in size, associated with LDPs and IDPs was identified, by amino terminal sequencing and enzyme activity, as an a-aylase inhibitor originating from protein bodies and artifactually associated with the lipid particles. The third major protein associated with LDPs and IDPs is 17.7 kDa in size and was shown to be too hydrophilic to be an oleosin. In addition, a protein of the same molecular weight was also detectable in protein bodies suggesting that the 17.7 kDa protein may also artifactually adhere to lipid particles during tissue homogenization of oil bodies. Thus LDPs, from Phaseolus vulgaris seeds, appear to be a unique class of oil bodies that do not contain oleosin. IDPs closely resemble LDPs and appear to be formed from LDPs by the shear forces generated during tissue homogenization.

The lipid composition of HDPs indicates that they are similar to lipid particles previously identified in carnation petals, called lipid-protein particles, which are thought to be involved in membrane turnover. HDPs are enriched in phospholipid metabolites including free fatty acids, diacylglycerol and steryl and wax esters and appear to have a different structure than that proposed for oil bodies.

The presence of phospholipid in LDPs and HDPs indicates that they both originate from membranes. This is also believed to be true for oil bodies and lipid particles from organisms other than plants. The mechanism employed by plants to accumulate triacylglycerol into domains within the membrane bilayer, prior to formation of an oil body, has not been described. The only proteins associated with oil bodies from developing seeds are oleosins, and it seems unlikely that oleosins are involved in this process, as naked triacylglyerol droplets have been described in different plants. Moreover, LDPs isolated from wax bean seeds do not appear to contain oleosins. A search of data bases using proteins involved in the generation of lipid particles in animals and bacteria resulted in the identification of several plant proteins with sequence similarities to these animal and bacterial proteins. Many of these plant proteins have been deduced from open reading frames identified by the Arabidopsis genome project and do not yet have assigned functions. This strategy appears to be promising in the search for proteins involved in the ontogeny of lipid particles in plants.