Molecular basis for low temperature compartment formation by transitional endoplasmic reticulum of rat liver

Symposium review: Milk fat globule size: Practical implications and metabolic regulation

Milk fat globule (MFG) size ranges over 3 orders of magnitude, from less than 200 nm to over 15 µm. The significance of MFG size derives from its tight association with its lipidome and proteome. More specifically, small MFG have relatively higher content of membrane compared with large globules, and this membrane exerts diverse positive health effects, as reported in human and animal studies. In addition, MFG size has industrial significance, as it affects the physicochemical and sensory characteristics of dairy products. Studies on the size regulation of MFG are scarce, mainly because various confounders indirectly affect MFG size. Because MFG size is determined before and during its secretion from mammary epithelial cells, studies on the size regulation of its precursors, the intracellular lipid droplets (LD), have been used as a proxy for understanding the mechanisms controlling MFG size. In this review, we provide evidence for 2 distinct mechanisms regulating LD size in mammary epithelial cells: co-regulation of fat content and triglyceride-synthesis capacity of the cells, and fusion between LD. The latter is controlled by the membrane's polar lipid composition and involves mitochondrial enzymes. Accordingly, this review also discusses MFG size regulation in the in vivo metabolic context, as MFG morphometric features are often modulated under conditions that involve animals' altered energy status.

The puzzle of chloroplast vesicle transport - involvement of GTPases

In the cytosol of plant cells vesicle transport occurs via secretory pathways among the endoplasmic reticulum network, Golgi bodies, secretory granules, endosome, and plasma membrane. Three systems transfer lipids, proteins and other important molecules through aqueous spaces to membrane-enclosed compartments, via vesicles that bud from donor membranes, being coated and uncoated before tethered and fused with acceptor membranes. In addition, molecular, biochemical and ultrastructural evidence indicates presence of a vesicle transport system in chloroplasts. Little is known about the protein components of this system. However, as chloroplasts harbor the photosynthetic apparatus that ultimately supports most organisms on the planet, close attention to their pathways is warranted. This may also reveal novel diversification and/or distinct solutions to the problems posed by the targeted intra-cellular trafficking of important molecules. To date two homologs to well-known yeast cytosolic vesicle transport proteins, CPSAR1 and CPRabA5e (CP, chloroplast localized), have been shown to have roles in chloroplast vesicle transport, both being GTPases. Bioinformatic data indicate that several homologs of cytosolic vesicle transport system components are putatively chloroplast-localized and in addition other proteins have been implicated to participate in chloroplast vesicle transport, including vesicle-inducing protein in plastids 1, thylakoid formation 1, snowy cotyledon 2/cotyledon chloroplast biogenesis factor, curvature thylakoid 1 proteins, and a dynamin like GTPase FZO-like protein. Several putative potential cargo proteins have also been identified, including building blocks of the photosynthetic apparatus. Here we discuss details of the largely unknown putative chloroplast vesicle transport system, focusing on GTPase-related components.

Very-long-chain fatty acids are required for cell plate formation during cytokinesis in Arabidopsis thaliana

Acyl chain length is thought to be crucial for biophysical properties of the membrane, in particular during cell division, when active vesicular fusion is necessary. In higher plants, the process of cytokinesis is unique, because the separation of the two daughter cells is carried out by de novo vesicular fusion to generate a laterally expanding cell plate. In Arabidopsis thaliana, very-long-chain fatty acid (VLCFA) depletion caused by a mutation in the microsomal elongase gene PASTICCINO2 (PAS2) or by application of the selective elongase inhibitor flufenacet altered cytokinesis. Cell plate expansion was delayed and the formation of the endomembrane tubular network altered. These defects were associated with specific aggregation of the cell plate markers YFP-Rab-A2a and KNOLLE during cytokinesis. Changes in levels of VLCFA also resulted in modification of endocytosis and sensitivity to brefeldin A. Finally, the cytokinesis impairment in pas2 cells was associated with reduced levels of very long fatty acyl chains in phospholipids. Together, our findings demonstrate that VLCFA-containing lipids are essential for endomembrane dynamics during cytokinesis.

A chloroplast-localized vesicular transport system: a bio-informatics approach

BACKGROUND

The thylakoid membrane of higher plant chloroplasts is made of membrane lipids synthesized in the chloroplast envelope. As the inner envelope membrane and the thylakoid are separated by the aqueous stroma, a system for transporting newly synthesized lipids from the inner envelope membrane to the thylakoid is required. Ultrastructural as well as biochemical studies have indicated that lipid transport inside the chloroplast could be mediated by a system similar in characteristics to vesicular trafficking in the cytosol. If indeed the chloroplast system is related to cytosolic vesicular trafficking systems, a certain degree of sequence conservation between components of the chloroplast and the cytosolic systems could be expected. We used the Arabidopsis thaliana genome and web-based subcellular localization prediction tools to search for chloroplast-localized homologues of cytosolic vesicular trafficking components.

RESULTS

Out of the 28952 hypothetical proteins in the A. thaliana genome sequence, 1947 were predicted to be chloroplast-localized by two different subcellular localization predictors. In this chloroplast protein dataset, strong homologues for the main coat proteins of COPII coated cytosolic vesicles were found. Homologues of the small GTPases ARF1 and Sar1 were also found in the chloroplast protein dataset.

CONCLUSION

Our database search approach gives further support to that a system similar to cytosolic vesicular trafficking is operational inside the chloroplast. However, solid biochemical data is needed to support the chloroplast localization of the identified proteins as well as their involvement in intra-chloroplast lipid trafficking.

Phosphatidylserine increase in rat liver endomembranes during the acute phase response

The acute phase response (APR) is responsible for great changes in protein and lipid metabolism. For example, marked changes are observed in the metabolism of fatty acids, triglycerides, cholesterol and sphingolipids. Those lipids are partly recovered in the lipoproteins and subsequently in the plasma. Beside these lipid families, nothing is known about phospholipids and their synthesis in endomembranes during the APR. Our studies show that phosphatidylserine synthesis is stimulated during the APR and that this lipid is increased in the endoplasmic reticulum (ER) and the ER-derived vesicles.

Cell-free transfer of the vesicular stomatitis virus G protein from an endoplasmic reticulum compartment of baby hamster kidney cells to a rat liver Golgi apparatus compartment for Man8-9 to Man5 processing

We report the reconstitution of the transfer of a membrane glycoprotein (vesicular stomatitis virus glycoprotein, VSV-G protein) from endoplasmic reticulum to Golgi apparatus and its subsequent Man8-9GlcNAc2 to Man5GlcNAc2 processing in a completely cell-free system. The acceptor was Golgi apparatus from rat liver immobilized on nitrocellulose. The endoplasmic reticulum donor was from homogenates of VSV-G-infected BHK cells. Nucleoside triphosphate plus cytosol-dependent transfer and processing of radiolabeled VSV-G protein was observed with donor from BHK cells infected at 37 degrees C with wild-type VSV or at the permissive temperature of 34 degrees C with the ts045 mutant. With Golgi apparatus as acceptor, specific transfer at 37 degrees C in the presence of nucleoside triphosphate was eightfold that at 4 degrees C or in the absence of ATP. About 40% of the VSV-G protein transferred was processed to the Man5GlcNAc2 form. Processing was specific for cis Golgi apparatus fractions purified by preparative free-flow electrophoresis. Fractions derived from the trans Golgi apparatus were inactive in processing. With the ts045 temperature-sensitive mutant, transfer and processing were much reduced even in the complete system when microsomes were from cells infected with mutant virus and incubated at the restrictive temperature of 39.5 degrees C but were able to proceed at the permissive temperature of 34 degrees C. Thus, Man8-9GlcNAc2 to Man5GlcNAc2 processing of VSV-G protein occurs following transfer in a completely cell-free system using immobilized intact Golgi apparatus or cis Golgi apparatus cisternae as the acceptor and shows temperature sensitivity, donor specificity, requirement for ATP, and response to inhibitors similar to those exhibited by transfer and processing of VSV-G protein in vivo.

ATP-Dependent formation of phosphatidylserine-rich vesicles from the endoplasmic reticulum of leek cells

Leek (Allium porrum) plasma membrane is enriched in phosphatidylserine (PS) by the vesicular pathway, in a way similar to that already observed in animal cells (B. Sturbois-Balcerzak, D.J. Morre, O. Loreau, J.P. Noel, P. Moreau, C. Cassagne [1995] Plant Physiol Biochem 33: 625-637). In this paper we document the formation of PS-rich small vesicles from leek endoplasmic reticulum (ER) membranes upon addition of ATP and other factors. The omission of ATP or its replacement by ATPgamma-S prevents vesicle formation. These vesicles correspond to small structures (70-80 nm) and their phospholipid composition, characterized by a PS enrichment, is compatible with a role in PS transport. Moreover, the PS enrichment over phosphatidylinositol in the ER-derived vesicles is the first example, to our knowledge, of phospholipid sorting from the ER to ER-derived vesicles in plant cells.

Lipid headgroup spacing and peptide penetration, but not peptide oligomerization, modulate peptide-induced fusion

In this study, the mechanism by which an amphipathic negatively charged peptide consisting of 11 amino acids (WAE) induces fusion of liposomal phosphatidylcholine membranes is investigated. WAE-induced fusion, which only occurs when the peptide is covalently attached to the bilayer, shows a highly remarkable dependence on naturally occurring phosphatidylcholine species. The initial rate of fusion increased in the order 1-palmitoyl 2-arachidonoyl PC (PAPC) > 1-palmitoyl 2-oleoyl PC (POPC) > 1-stearoyl 2-oleoyl PC (SOPC) > dioleoyl PC (DOPC) > egg yolk PC. Interestingly, the susceptibility of the various PC species toward WAE-induced fusion matched a similar order of increase in intrinsic lipid headgroup spacing of the target membrane. The degree of spacing, in turn, was found to be related to the extent by which the fluorescence quantum yield of the Trp residue increased, which occurred upon the interaction of WAE with target membranes. Therefore, these results demonstrate an enhanced ability for WAE to engage in hydrophobic interactions when headgroup spacing increases. Thus, this latter parameter most likely regulates the degree of penetration of WAE into the target membrane. Apart from penetrating, WAE oligomerizes at the site of fusion as revealed by monitoring the self-quenching of the fluorescently derivatized lipid anchor to which WAE is attached. Clustering appears specifically related to the process of membrane fusion and not membrane aggregation. This is indicated by the fact that fusion and clustering, but not aggregation, display the same strict temperature dependence. However, evidence is presented indicating that clustering is an accompanying event rather than a prerequisite for fusion. The notion that various biologically relevant fusion phenomena are accompanied by protein clustering and the specific PC-species-dependent regulation of membrane fusion emphasize the biological significance of the peptide in serving as a model for investigating mechanisms of protein-induced fusion.

Non-bilayer lipids and biological fusion intermediates

Disparate biological fusion reactions and fusion of purely lipid bilayers are similarly influenced by 'non-bilayer' lipids (lipids which do not form lipid bilayers in water by themselves). Lipid composition of membranes affects biological fusion at a stage downstream of activation of fusion proteins and prior to fusion pore formation. These data suggest that actual merger of membrane lipid bilayers in different fusion reactions proceeds via the same pathway. The effects of non-bilayer lipids specifically correlate with their ability to bend lipid monolayers in different directions, and appear to be consistent with the specific hypothesis of membrane fusion suggesting that fusion proceeds through highly bent intermediates--stalks, local connections between contacting monolayers of fusing membranes.

Temperature-sensitive steps in the transport of Semliki Forest virus envelope proteins in mosquito C6/36 cells

We have analysed the temperature dependence of the transport of Semliki Forest virus (SFV) envelope proteins in mosquito cells, the natural host cells of alphaviruses. These cells are cultivated at a lower temperature (28 degrees C) and have a different lipid composition as compared to mammalian cells. When the incubation temperature was reduced at early times after infection, the onset of virus shedding was delayed and the maximal titers decreased correspondingly to the temperature. No virus was shed at 12 degrees C. No evidence was observed for a block of virus release due to a shift of the sites of virus maturation. When the incubation temperature was reduced at later times after infection a critical temperature of 12 degrees C was again observed. At this temperature no transport of viral proteins took place, p62 remained uncleaved, the glycan processing of E1 did not occur and the envelope proteins accumulated in a pre-Golgi compartment. We suggest a mathematical formula which allows the extrapolation of transport data to the temperature at which intracellular protein transport becomes blocked.

Cell-free transfer of phospholipids between the endoplasmic reticulum and the Golgi apparatus of leek seedlings

The transfer of lipids between the endoplasmic reticulum and the Golgi apparatus was investigated in vitro using a cell-free system from leek seedlings. Lipids of the donor membranes (endoplasmic reticulum) were radiolabeled either by incubating leek seedlings with [1-14C]acetate or [3H]acetate. Acceptor membranes (Golgi apparatus) were unlabeled and immobilized on nitrocellulose strips. The assay measured the lipid transfer resulting from both an ATP-independent process and an ATP- and cytosol-dependent process. A significant ATP- and cytosol-dependent lipid transfer was observed only in the case of the endoplasmic reticulum as donor and the Golgi apparatus as acceptor. Lipids transferred in an ATP-dependent manner were chiefly phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine. The stimulation of lipid transfer by ATP as compared to the ATP-independent process was +79% (PC), +123% (PS) and +69% (PE). On the other hand, PI was not transferred in an ATP-dependent manner (the stimulation by ATP was only 20%). This supports the theory that a sorting of phospholipids takes place in the donor membrane. Moreover, a formation of lysoPC was observed only in the presence of ATP (+330%). The ATP-dependent lipid transfer was inhibited by N-ethylmaleimide, indicating the involvement of cytosolic (but no phospholipid transfer proteins) or membrane proteins in the transfer process. The ATP-dependent transfer of lipids was also diminished at 12 degrees C showing the sensitivity to low temperatures of the transfer of lipids between the endoplasmic reticulum and the Golgi apparatus.