PUX10 Is a CDC48A Adaptor Protein That Regulates the Extraction of Ubiquitinated Oleosins from Seed Lipid Droplets in Arabidopsis.

Postgerminative mobilization of neutral lipids stored in seed lipid droplets (LDs) is preceded by the degradation of oleosins, the major structural LD proteins that stabilize LDs in dry seeds. We previously showed that Arabidopsis thaliana oleosins are marked for degradation by ubiquitination and are extracted from LDs before proteolysis. However, the mechanisms underlying the dislocation of these LD-anchored proteins from the LD monolayer are yet unknown. Here, we report that PUX10, a member of the plant UBX-domain containing (PUX) protein family, is an integral LD protein that associates with a subpopulation of LDs during seed germination. In pux10 mutant seedlings, PUX10 deficiency impaired the degradation of ubiquitinated oleosins and prevented the extraction of ubiquitinated oleosins from LDs. We also showed that PUX10 interacts with ubiquitin and CDC48A, the AAA ATPase Cell Division Cycle 48, through its UBA and UBX domains, respectively. Collectively, these results strongly suggest that PUX10 is an adaptor recruiting CDC48A to ubiquitinated oleosins, thus facilitating the dislocation of oleosins from LDs by the segregase activity of CDC48A. We propose that PUX10 and CDC48A are core components of a LD-associated degradation machinery, which we named the LD-associated degradation system. Importantly, PUX10 is also the first determinant of a LD subpopulation described in plants, suggesting functional differentiation of LDs in Arabidopsis seedlings.

N-terminus of seed caleosins is essential for lipid droplet sorting but not for lipid accumulation.

Caleosin, a calcium-binding protein associated with plant lipid droplets, stimulates lipid accumulation when heterologously expressed in Saccharomyces cerevisiae. Accumulated lipids are stored in cytoplasmic lipid droplets that are stabilised by incorporated caleosin. We designed a set of mutants affecting putative crucial sites for caleosin function and association with lipid droplets, i.e. the N-terminus, the EF-hand motif and the proline-knot motif. We investigated the effect of introduced mutations on caleosin capacity to initiate lipid accumulation and on caleosin sorting within cell as well as on its association with lipid droplets. Our results strongly suggest that the N-terminal domain is essential for proper protein sorting and targeting to lipid droplets but not for enhancing lipid accumulation.

Combining chymotrypsin/trypsin digestion to identify hydrophobic proteins from oil bodies.

Oil bodies, lipid-storage organelles, are stabilized by a number of specific proteins. These proteins are very hydrophobic, which complicates their identification by "classical" proteomic protocols using trypsin digestion. Due to the lack of trypsin cleavage sites, the achievable protein coverage is limited or even insufficient for reliable protein identification. To identify such proteins and to enhance their coverage, we introduced a modified method comprising standard three-step procedure (SDS-PAGE, in-gel digestion, and LC-MS/MS analysis). In this method, chymotrypsin, single or in combination with trypsin, was used, which enabled to obtain proteolytic peptides from the hydrophobic regions and to identify new oil bodies' proteins. Our method can be easily applied to identification of other hydrophobic proteins.

New protein isoforms identified within Arabidopsis thaliana seed oil bodies combining chymotrypsin/trypsin digestion and peptide fragmentation analysis.

Plant seed oil bodies, subcellular lipoprotein inclusions providing storage reserves, are composed of a neutral lipid core surrounded by a phospholipid monolayer with several integrated proteins that play a significant role in stabilization of the particles and probably also in lipid mobilization. Oil bodies' proteins are generally very hydrophobic, due to the long uncharged sequences anchoring them into the lipid core, which makes them extremely difficult to handle and to digest successfully. Although oil bodies have been intensively studied during last decades, not all their proteins have been identified yet. To overcome the problems connected with their identification, a method based on SDS-PAGE, in-gel digestion and LC-MS/MS analysis was used. Digestion was carried out with trypsin and chymotrypsin, single or in combination, which increased significantly the number of identified peptides, namely the hydrophobic ones. Thanks to this methodology it was possible to achieve an extensive coverage of proteins studied, to analyze their N-terminal modifications and moreover, to detect four new oil bodies' protein isoforms, which demonstrates the complexity of oil bodies' protein composition.

Caleosin of Arabidopsis thaliana : Effect of Calcium on Functional and Structural Properties.

A non-radioactive blot binding assay has proved the capacity of a purified recombinant form of Arabidopsis thaliana caleosin (AtClo1), a key protein of this plant oil body, to bind calcium. Calcium affected recombinant caleosin aggregation state, solubility, and electrophoretic mobility on SDS-PAGE. The effect of calcium on interfacial behavior of recombinant caleosin was studied at three interfaces: air/water (A/W), purified oil/water (O/W), and air/phosholipid/water (A/PLs/W). Recombinant caleosin was able to decrease interfacial tension (IFT) at A/W and O/W interfaces as a function of concentration and calcium, whereas no interaction was detected at the A/PLs/W interface. Effect of calcium was time dependent, and its amplitude strongly varied with the interface considered. Reconstituted oil bodies were used to prove the involvement of recombinant caleosin in their calcium-driven aggregation and coalescence. Calcium ions at concentration as low as 100 nM were able to strongly modify the shape and aggregation state of purified oil bodies, as well as their behavior within a monolayer, reflecting potentially profound changes in their structure and dynamic.

Structure and function of seed lipid-body-associated proteins.

Many organisms among the different kingdoms store reserve lipids in discrete subcellular organelles called lipid bodies. In plants, lipid bodies can be found in seeds but also in fruits (olives, ...), and in leaves (plastoglobules). These organelles protect plant lipid reserves against oxidation and hydrolysis until seed germination and seedling establishment. They can be stabilized by specific structural proteins, namely the oleosins and caleosins, which act as natural emulsifiers. Considering the putative role of some of them in controlling the size of lipid bodies, these proteins may constitute important targets for seed improvement both in term of oil seed yield and optimization of technological processes for extraction of oil and storage proteins. We present here an overview of the data on the structure of these proteins, which are scarce, and sometimes contradictory and on their functional roles.

Structural properties of caleosin: a MS and CD study.

We have investigated the covalent and secondary solution structure of caleosin, a 27-kDa protein also called ATS1 or AtClo1 (At4g26740) found within Arabidopsis thaliana seed lipid bodies. The native protein was partly phosphorylated at S225. Purified bacterially expressed caleosin (recClo) was not phosphorylated; cysteine residues C221 and C230 were connected by a disulfide bridge. In solution it exists as a mixture of predominant monomers and covalent dimers. We have used recClo as a model for the study of AtClo1 secondary structure. recClo is folded in aqueous solution (16% alpha-helix, 29% beta-sheet), its secondary structure being dramatically influenced by the polarity of media, as deduced from CD spectra measured in the presence of increasing concentrations of various aliphatic alcohols.