Oil bodies isolated from the mature seeds of rape (Brassica napus L.), mustard (Brassica juncea l.), cotton (Gossypium hirsutum L.), flax (Linus usifafis simum), maize (Zea mays L.), peanut (Arachis hypogaea L.), and sesame (Sesamum indicum L.) had average diameters that were different but within a narrow range (0.6-2.0 fim), as measured from electron micrographs of seria1 sections. Their contents of triacylglycerols (TAC), phospholipids, and proteins (oleosins) were correlated with their sizes. The correlation fits a formula that describes a spherical particle surrounded by a shell of a monolayer of phospholipids embedded with oleosins. Oil bodies from the various species contained substantial amounts of the uncommon negatively charged phosphatidylserine and phosphatidylinositol, as well as small amounts of free fatty acids. These acidic lipids are assumed to interact with the basic amino acid residues of the oleosins on the surface of the phospholipid layer. lsoelectrofocusing revealed that the oil bodies from the various species had an isoelectric point of 5.7 to 6.6 and thus possessed a negatively charged surface at neutra1 pH. We conclude that seed oil bodies from diverse species are very similar in structure. In rapeseed during maturation, TAC and oleosins accumulated concomitantly. TAC-synthesizing acyltransferase activities appeared at an earlier stage and peaked during the active period of TAC accumulation. l h e concomitant accumulation of TAC and oleosins is similar to that reported earlier for maize and soybean, and the finding has an implication for the mode of oil body synthesis during seed maturation.Seeds store TAG as food reserves for germination and postgerminative growth of the seedlings. The TAG are present in small, discrete intracellular organelles called oil bodies (Yatsu and Jacks, 1972; Appelquist, 1975; Stymne and Stobart, 1987; Huang, 1992). Isolated oil bodies have a spherical shape and possess diameters ranging from about 0.5 to 2.0 pm. They contain mostly TAG and small amounts of PL and proteins called oleosins. It is generally agreed that the oil body has a matrix of TAG surrounded by a layer of PL embedded with oleosins. The PL form a monolayer such that the acyl moieties of the molecules face inward to interact with the hydrophobic TAG in the matrix, and the hydrophilic PL head groups are exposed to the cytosol. The embedded oleosin molecule is composed of three structural domains: an N-terminal amphipathic domain, a central hydrophobic domain, and a C-terminal amphipathic a-helical domain (Vance and Huang, 1987; Qu and Huang, 1990; Murphy et al., 1991; Tzen et al., 1992). It is predicted that the hydrophobic portion Supported by U.S. Department of Agriculture grant 91-01430 (A.H.C.H.).* Corresponding author; fax 1-714-787-4437. 267 of the oleosin molecule penetrates the PL layer into the TAG matrix, and its amphipathic portion resides on the PL layer or protrudes to the exterior. The structure of an oil body as described in the preceding paragraph implies that the relative...

Abstract. Storage triacylglycerols (TAG) in plant seeds are present in small discrete intracellular organ-

Oil bodies are lipid storage organelles which have been analyzed biochemically due to the economic importance of oil seeds. Although oil bodies are structurally simple, the mechanisms involved in their formation and degradation remain controversial. At present, only two proteins associated with oil bodies have been described, oleosin and caleosin. Oleosin is thought to be important for oil body stabilization in the cytosol, although neither the structure nor the function of oleosin has been fully elucidated. Even less is known about caleosin, which has only recently been described [Chen et al. (1999) Plant Cell Physiol 40: 1079-1086; Naested et al. (2000) Plant Mol Biol 44: 463-476]. Caleosin and caleosin-like proteins are not unique to oil bodies and are associated with an endoplasmatic reticulum subdomain in some cell types. Here we review the synthesis and degradation of oil bodies as they relate to structural and functional aspects of oleosin and caleosin.

Plant seed oil bodies comprise a matrix of triacylglycerols surrounded by a monolayer of phospholipids embedded with abundant oleosins and some minor proteins. Three minor proteins, temporarily termed Sops 1-3, have been identified in sesame oil bodies. A cDNA sequence of Sop1 was obtained by PCR cloning using degenerate primers derived from two partial amino acid sequences, and subsequently confirmed via immunological recognition of its over-expressed protein in Escherichia coli. Alignment with four published homologous sequences suggests Sop1 as a putative calcium-binding protein. Immunological cross-recognition implies that this protein, tentatively named caleosin, exists in diverse seed oil bodies. Caleosin migrated faster in SDS-PAGE when incubated with Ca2+. A single copy of caleosin gene was found in sesame genome based on Southern hybridization. Northern hybridization revealed that both caleosin and oleosin genes were concurrently transcribed in maturing seeds where oil bodies are actively assembled. Hydropathy plot and secondary structure analysis suggest that caleosin comprises three structural domains, i.e., an N-terminal hydrophilic calcium-binding domain, a central hydrophobic anchoring domain, and a C-terminal hydrophilic phosphorylation domain. Compared with oleosin, a conserved proline knot-like motif is located in the central hydrophobic domain of caleosin and assumed to involve in protein assembly onto oil bodies.

Oleosins are unique and major proteins localized on the surface of oil bodies in diverse seed species. We purified five different KD 20), and raised chicken antibodies against them. These antibodies were used to test for immunological cross-reactivity among oleosins from diverse seed species. Within the same seed species, antibodies raised against one oleosin isoform did not cross-react with the other oleosin isoform (i.e. between maize oleosins KD 16 and KD 18, and between soybean oleosins KD 18 and KD 24). However, the respective antibodies were able to recognize oleosins from other seed species. Where interspecies cross-reactivity occurred, the results suggest that there are at least two immunologically distinct isoforms of oleosins present in diverse seed species, one of lower Mr, and another one of higher Mr. This suggestion is also supported by the relative similarities between the amino acid sequence of a small portion of rapeseed oleosin KD 20 and those of maize oleosins KD 16 and KD 18. In maize kemel, there was a tissue-specific differential presentation of the three oleosins, KD 16, KD 18, and KD 19, in the oil-storing scutellum, embryonic axis, and aleurone layer. The phylogenetic relationship between the high and low Mr isoforms within the same, and among diverse, seed species is discussed.