Lipids of plant membrane rafts

Lipids tend to organize in mono or bilayer phases in a hydrophilic environment. While they have long been thought to be incapable of coherent lateral segregation, it is now clear that spontaneous assembly of these compounds can confer microdomain organization beyond spontaneous fluidity. Membrane raft microdomains have the ability to influence spatiotemporal organization of protein complexes, thereby allowing regulation of cellular processes. In this review, we aim at summarizing briefly: (i) the history of raft discovery in animals and plants, (ii) the main findings about structural and signalling plant lipids involved in raft segregation, (iii) imaging of plant membrane domains, and their biochemical purification through detergent-insoluble membranes, as well as the existing debate on the topic. We also discuss the potential involvement of rafts in the regulation of plant physiological processes, and further discuss the prospects of future research into plant membrane rafts.     

Calcium-dependent association of membrane rafts

The plasma membrane of eucariotic cells is composed from spatially distinct regions enriched in sphingomyelin and cholesterol (so called membrane rafts), which are surrounded by glycerophospholipid milieu. The ability of these regions (membrane microdomains) to recruit specific enzymes and structural proteins results in spatial segregation of elements of different signaling systems. To the great extent, such segregation is a result of membrane raft association. Our results demonstrate that association of individual rafts is a controlled process, which is regulated, as many other biological processes, by Ca2+. Annexin II, a member of phospholipid-binding protein family, interacts with preparations of membrane rafts in a Ca(2+)-dependent manner thus resulting in reversible raft-association. Occurring in the presence of physiologically relevant calcium concentrations these interactions may play a role in regulation of smooth muscle contraction.     

Lipid rafts and membrane traffic

Membrane rafts are regions of increased lipid acyl chain order that differ in their lipid and protein composition from the surrounding membrane. By providing an additional level of compartmentalization they have been proposed to serve many functions in cellular signal transduction and trafficking. We will review their potential involvement in different forms of membrane traffic, explicitly excluding signalling, and discuss select aspects of the raft hypothesis in its current form.     

Characteristics of the vacuolar membrane ofNitella

Summary Taking advantage of vacuolar perfusion, concentrations of K⁺, Cl^–, and H⁺ in the vacuole ofNitella pulchella were changed in a wide range. Both the potential difference (E _vo ) and specific resistance (R _vo ) between the vacuole and the external medium were scarcely affected by K⁺ in the vacuole, while they responded sensitively to K⁺ in the external medium. E _vo also responded to Cl^– in both internal (vacuolar) and external medium. However, the sign of the response was opposite to that expected from the constant field assumption.R _vo was almost independent of Cl^–-concentrations of both internal and external medium.The response ofE _vo to internal pH was similar to that of external pH. Between pH's 4 and 8,E _vo changed by about 10 mV for one unit change of both external and internal pH.E _vo responded very sensitively to internal pH in the strongly acid region (30–60 mV at pH 3–4) irrespective of the concentration of KCl in the vacuole. In the alkaline region, however,E _vo responded to vacuolar pH only when the KCl concentration in the vacuole was low (0.1 mM).R _vo increased significantly when the vacuolar pH was lowered to 4 or 3.Increase in tonicity of the vacuolar medium to twice normal caused no significant change in bothE _vo andR _vo , while it raised the threshold for excitation.Even when the chemical potential gradient between the internal and external medium was made zero by replacing the cell sap for the same solution used for the external medium, a significant amount ofE _vo was observed. The short-circuit current which was first outward decreased to zero or changed its direction with time. Light did not affect the current. These facts show that the possibility for the contribution of an ion pump toE _vo can be excluded.The results were discussed under the assumption that responses ofE _vo andR _vo to either internal or external ions reflect the passive property of either tonoplast or plasmalemma.     

Stoichiometry of lipid-protein interaction and integral membrane protein structure

The stoichiometries of lipid-protein interaction obtained from spin label electron spin resonance experiments with integral membrane proteins are compared with simple geometric models for the intramembranous perimeter that are based on the predicted numbers of transmembrane helices. Deviations from the predicted values provide evidence for oligomerization of the protein in the membrane and/or more complex arrangements of the transmembrane segments. Received: 16 January 1997 / Accepted: 5 March 1997     

Barley anther culture using membrane rafts

When compared to agarose solidified media in small petri dishes, membrane rafts used in conjunction with liquid induction media significantly improved anther culture response in the Australian, malting-quality, spring barley cultivar Clipper. In contrast, the German cultivar Gimpel did not show an increased response on rafts.Abbreviations BA 6-benzylaminopurine - IAA indoleacetic acid - DH doubled haploid     

An update on plant membrane rafts

The dynamic segregation of membrane components within microdomains, such as the sterol-enriched and sphingolipid-enriched membrane rafts, emerges as a central regulatory mechanism governing physiological responses in various organisms. Over the past five years, plasma membrane located raft-like domains have been described in several plant species. The protein and lipid compositions of detergent-insoluble membranes, supposed to contain these domains, have been extensively characterised. Imaging methods have shown that lateral segregation of lipids and proteins exists at the nanoscale level at the plant plasma membrane, correlating detergent insolubility and membrane-domain localisation of presumptive raft proteins. Finally, the dynamic association of specific proteins with detergent-insoluble membranes upon environmental stress has been reported, confirming a possible role for plant rafts as signal transduction platforms, particularly during biotic interactions.     

Roles of lipid rafts in membrane transport.

Cholesterol-sphingolipid microdomains (lipid rafts) are part of the machinery ensuring correct intracellular trafficking of proteins and lipids. The most apparent roles of rafts are in sorting and vesicle formation, although their roles in vesicle movement and cytoskeletal connections as well as in vesicle docking and fusion are coming into focus. New evidence suggests that compositionally distinct lipid microdomains are assembled and may coexist within a given membrane. Important clues have also been uncovered about the mechanisms coupling raft-dependent signaling and endocytic uptake.     

Measles virus assembly within membrane rafts

During measles virus (MV) replication, approximately half of the internal M and N proteins, together with envelope H and F glycoproteins, are selectively enriched in microdomains rich in cholesterol and sphingolipids called membrane rafts. Rafts isolated from MV-infected cells after cold Triton X-100 solubilization and flotation in a sucrose gradient contain all MV components and are infectious. Furthermore, the H and F glycoproteins from released virus are also partly in membrane rafts (S. N. Manié et al., J. Virol. 74:305-311, 2000). When expressed alone, the M but not N protein shows a low partitioning (around 10%) into rafts; this distribution is unchanged when all of the internal proteins, M, N, P, and L, are coexpressed. After infection with MGV, a chimeric MV where both H and F proteins have been replaced by vesicular stomatitis virus G protein, both the M and N proteins were found enriched in membrane rafts, whereas the G protein was not. These data suggest that assembly of internal MV proteins into rafts requires the presence of the MV genome. The F but not H glycoprotein has the intrinsic ability to be localized in rafts. When coexpressed with F, the H glycoprotein is dragged into the rafts. This is not observed following coexpression of either the M or N protein. We propose a model for MV assembly into membrane rafts where the virus envelope and the ribonucleoparticle colocalize and associate.     

脂筏与精子膜信号传导

脂筏是细胞膜内由特殊脂质与蛋白质构成的微域。小窝是脂筏的一种形式,小窝标记蛋白有小窝蛋白和小窝舟蛋白。脂筏或小窝与生物信号传导、细胞蛋白转运和胆固醇平衡有关。最近实验证实哺乳动物精子膜具有脂筏结构,脂筏与膜胆固醇外逸对于启动受精的信号传导具有重要作用。     

Fatty acids of plant vacuolar membrane lipids

Fatty acid (FA) composition of vacuolar membrane lipids from storage tissues of umbelliferous plants, viz., parsnip (Pastinaca sativa L.), parsley (Petroselinium crispum L.), and carrot (Daucus carota L.) is studied by gas-liquid chromatography and the FA biosynthetic pathways are considered. Vacuolar membrane lipids are characterized by high (78% of the total FA pool) content of unsaturated FA among which linoleic acid is predominant. Its content in vacuolar lipids of parsnip, parsley and carrot is 53.5, 55.1, and 54.9%, respectively. Parsnip and parsley vacuolar lipids contain large amounts of hexadecadienoic C16:2ω6 acid (8.0 and 4.6%, respectively). The content of α-linolenic acid in vacuolar lipids of tested plants varies from 4.8 to 7.3%. Palmitic acid (18.0–20.7%) predominates among saturated FA. High content of linoleic and hexadecadienoic acid in parsnip and parsley vacuolar lipids is suggestive of a crucial role of the microsomal ω6 fatty-acid desaturase fad2 gene in resistance and acclimation of plants to low temperatures.     

Potassium-selective channel in the red beet vacuolar membrane

In higher plants the vacuolar K(+)-selective (VK) channel was identified solely in guard cells. This patch-clamp study describes a 40 pS homologue of the VK channel in Beta vulgaris taproot vacuoles. This voltage-independent channel is activated by submicromolar Ca(2+), and is ideally selective for K(+) over Cl(-) and Na(+).     

CD95 signaling via ceramide-rich membrane rafts

Clustering seems to be employed by many receptors for transmembrane signaling. Here, we show that acid sphingomyelinase (ASM)-released ceramide is essential for clustering of CD95. In vitro and in vivo, extracellularly orientated ceramide, released upon CD95-triggered translocation of ASM to the plasma membrane outer surface, enabled clustering of CD95 in sphingolipid-rich membrane rafts and apoptosis induction. Whereas ASM deficiency, destruction of rafts, or neutralization of surface ceramide prevented CD95 clustering and apoptosis, natural ceramide only rescued ASM-deficient cells. The data suggest CD95-mediated clustering by ceramide is prerequisite for signaling and death.     

Chloride transport of yeast vacuolar membrane vesicles: a study of in vitro vacuolar acidification.

Effects of various solutes on acidification inside the vacuolar membrane vesicles of the yeast Saccharomyces cerevisiae were examined. ATP-dependent acidification was stimulated by the presence of chloride salts. There was essentially no difference in the stimulatory effects of NaCl, KCl, LiCl, and choline chloride. The membrane potential across the vacuolar membrane was reduced by the presence of Cl- salts. Transport of 36Cl- is driven by the protonmotive force across the vacuolar membrane. Kinetic analyses have revealed that the stimulatory effect of Cl- on internal acidification depends on two distinct components. One shows linear dependency on chloride concentration and is inhibited by 4,4'-diisothiocyano-2,2'-stilbenedisulphonic acid (DIDS). The other exhibits saturable kinetics with an apparent Km for chloride of 15-20 mM. We conclude that the vacuolar membrane of yeast is equipped with Cl- transport systems contributing to the formation of a chemical gradient of protons across the vacuolar membrane by shunting the membrane potential generated by proton translocation.     

Subversion of membrane transport pathways by vacuolar pathogens

Mammalian phagocytes control bacterial infections effectively through phagocytosis, the process by which particles engulfed at the cell surface are transported to lysosomes for destruction. However, intracellular pathogens have evolved mechanisms to avoid this fate. Many bacterial pathogens use specialized secretion systems to deliver proteins into host cells that subvert signaling pathways controlling membrane transport. These bacterial effectors modulate the function of proteins that regulate membrane transport and alter the phospholipid content of membranes. Elucidating the biochemical function of these effectors has provided a greater understanding of how bacteria control membrane transport to create a replicative niche within the host and provided insight into the regulation of membrane transport in eukaryotic cells.     

Protein sorting to the vacuolar membrane.

The vacuolar membrane (tonoplast) of plant cells contains a polytopic integral membrane protein with six membrane-spanning domains and cytoplasmically oriented amino-terminal and carboxy-terminal domains. This protein, tonoplast intrinsic protein (TIP), is a member of the membrane intrinsic protein (MIP) family of proteins, a family of channel proteins found in a variety of organisms. In bean seeds, alpha-TIP is synthesized on the rough endoplasmic reticulum and its transport to the tonoplast is mediated by the secretory system. In this study, we report that a polypeptide segment that includes the sixth membrane domain and the cytoplasmic tail of 18 amino acids of alpha-TIP is sufficient to target the reporter protein phosphinotricine acetyltransferase to the tonoplast of stably transformed tobacco cells. To determine if the carboxy-terminal cytoplasmic tail of alpha-TIP contains important tonoplast targeting information, a deletion construct lacking the 15 carboxy-terminal amino acids was introduced for transient expression in tobacco cells; we found that the slightly truncated protein still accumulated in the tonoplast. From these results, we concluded that a transmembrane domain of a tonoplast protein probably contains sufficient information for transport to the tonoplast. Whether such transport occurs by bulk flow or involves specific cellular machinery remains to be determined.