Basal localization of the presumptive auxin transport carrier in pea stem cells

By means of an indirect immunofluorescence technique with the use of monoclonal antibodies, the location of the presumptive auxin transport carrier of pea stem tissue was identified in the plasma membranes at the basal ends of parenchyma cells sheathing the vascular bundles. The results represent what is believed to be the first direct evidence for the hypothesized basal efflux carrier conferring polarity to auxin transport in plant stems.     

Direct evidence for a colchicine-induced impairment in the mobility of membrane components

Freeze-etch electron microscopy was used to show that colchicine interacts with membranes of the ciliate protozoan Tetrahymena pyriformis. Colchicine impairs the temperature-induced translational and vertical mobility of the membrane-intercalating particles of the freeze-fractured alveolar membranes lying just below the plasma membranes.     

Influenza virus effects on cell membrane proteins

During infection by influenza virus, viral proteins become firmly attached to (or part of) host cell plasma membrane, nuclear membranes, mitochondrial, and microsomal membranes. Purified virus particles contain less than I percent host cell protein. Virus envelope proteins completely replace host membrane proteins in those discrete spots on the plasma membrane from which progeny virions bud out.     

Activation of apical chloride channels in the gastric oxyntic cell

Oxyntic cells that retain distinct morphological polarity between apical and basolateral membranes were isolated from the gastric mucosa of the amphibian Necturus. Patch-clamp techniques were applied to these cells to identify apical membrane ion channels associated with hydrochloric acid secretion. A single class of voltage-dependent, inwardly rectifying chloride channels was observed in the apical membranes of both resting and stimulated (acid-secreting) oxyntic cells. Stimulation of the cells with dibutyryladenosine 3',5'-monophosphate and isobutylmethylxanthine increased channel open probability and simultaneously increased apical membrane surface area. This chloride channel is probably responsible for electrogenic chloride secretion by the gastric mucosa and may also participate in the fluid- and enzyme-secretory functions of the oxyntic cell, analogous to the chloride channels found in the apical membranes of other exocrine cells.     

Tissue factor (thromboplastin): localization to plasma membranes by peroxidase-conjugated antibodies

Peroxidase-conjugated antibodies were used to determine the histologic and cytologic localization of bovine and human tissue factor (thromboplastin). Tissue factor antigen was found in highest concentration in the intima of blood vessels, particularly in the plasma membranes of endothelial cells and in human atheromatous plaques. Tissue factor was also found limited to the plasma membranes of many cell types. The presence of tissue factor in the plasma membranes of endothelial cells and atheromata suggests that it may play a significant role in hemostasis and thrombosis.     

Plasma membranes: isolation from naturally fused and lysolecithin-treated muscle cells

A medium containing bicarbonate and calcium was used to isolate plasma membranes of cultured muscle cells. Membranes from differentiated myotubes, as well as the labile, largely unfused, lysolecithin-treated cells from the same culture could be isolated by identical manipulations. Adenylate cyclase of high specific activity was assayed in plasma membranes from both types of cells. Lysolecithin treatment apparently interferes with the metabolism and turnover of membrane phospholipids and thus prevents the natural fusion of muscle cells.     

Formation of virus-like particles by bone cells in mice with a high incidence of spontaneous leukemia

Bone samples from potentially leukemic and leukemic mice revealed numerous 90-to 110-nanometer particles morphologically identical to murine leukemia virus. Particles were observed budding from plasma membranes of osteocytes and osteoblasts but were most numerous in osteocyte lacunae. Particles were not observed in bone samples from mice which rarely develop leukemia.     

Freeze-fractured Acholeplasma laidlawii membranes: nature of particles observed

Freeze-fracture of Acholeplasma laidlawii membranes from cells incubated in the presence of puromycin or omission of amino acids reveals a decrease in the number of particles between 50 and 100 angstroms in the hydrophobic fracture plane, which strongly suggests that these particles are protein. Additional evidence indicates that they may be involved in substrate transport.     

Composition and formation of intercellular junctions in epithelial cells

The polarized nature of epithelial cells is manifested by the nonrandom partitioning of organelles within the cells, the concentration of intercellular junctions at one pole, and the asymmetric distribution of proteins and lipids within the plasma membrane. These features allow epithelia to fulfill their specific tasks, such as targeted uptake and secretion of molecules and the segregation of different tissue compartments. The accessibility of Drosophila melanogaster and Caenorhabditis elegans to genetic and cell biological analyses, combined with the study of mammalian cells in culture, provides an ideal basis for understanding the mechanisms that control the establishment and maintenance of epithelial cell polarity and tissue integrity. Here, we focus on some of the best-studied junctions and membrane-associated protein complexes and their relation to cell polarity. Comparisons between fly, worm, and vertebrate epithelia reveal marked similarities with respect to the molecules used, and pronounced differences in the organization of the junctions themselves.     

Cell biology. Travel bulletin--traffic jams cause tumors

All animal cells have a polarity, that is, different proteins are clustered in distinct domains of the plasma membrane and these regions carry out different jobs. As Peifer discusses in a lively Perspective, new work (Bilder et al.) identifies some of the molecular characters that direct proteins to their different cellular destinations.     

钙离子在植物生理调节中的作用

钙是植物必须的营养元素,同时也是植物体内转导多种生理过程的胞内胞外信号物质之一。胞外Ca2+通过Ca2+通道内流进入胞质,并通过Ca2+-ATPase和Ca2+/H+反向转运蛋白外流,以保持胞质内低Ca2+浓度。同时为了应对植物发育和环境胁迫信号,Ca2+由质膜、液泡膜和内质网膜的Ca2+通道内流进入胞质,导致胞质Ca2+浓度迅速增加,产生钙瞬变和钙振荡,传递到钙信号靶蛋白(如钙调素、钙依赖型蛋白激酶及钙调磷酸酶B类蛋白,引起特异的生理生化反应),这一系列钙信号调节、应答机制构成了植物的钙信号系统。对钙转运系统、钙信号调节和放大及应答方式进行了综述。     

Plasma membrane compartmentalization in yeast by messenger RNA transport and a septin diffusion barrier

Asymmetric localization of proteins plays a key role in many cellular processes, including cell polarity and cell fate determination. Using DNA microarray analysis, we identified a plasma membrane protein-encoding mRNA (IST2) that is transported to the bud tip by an actomyosin-based process. mRNA localization created a higher concentration of IST2 protein in the bud compared with that of the mother cell, and this asymmetry was maintained by a septin-mediated membrane diffusion barrier at the mother-bud neck. These results indicate that yeast creates distinct plasma membrane compartments, as has been described in neurons and epithelial cells.     

Brush border particulates of renal tissue

Particulates containing a large part of the alkaline phosphatase activity of renal tissue were separated from homogenates and from ribosomal preparations by zonal centrifugation. The particles had a high content of phospholipid and cholesterol that was not removed by treatment with I percent deoxycholate. Enzymatic activities concentrated with the particles were the alkaline phosphatase, a peptidase resistant to proteolysis, glucose-6-phosphatase, inorganic pyrophos-phatase, and adenosine triphosphatase. The particles accumulated leucine with no stimulation from soluble factors and with inhibition by other amino acids; the accumulation was stimulated by adenosine triphosphate and was not inhibited by puromycin. The particles appear to be derived from the membranes of the brush borders of tubular cells.     

The cytoskeletal protein vinculin contains transformation-sensitive, covalently bound lipid

Vinculin, which is associated with the cytoskeleton of many cells, has been suggested as a possible linker between microfilament bundles and the plasma membrane. Here it will be shown that fatty acid is covalently attached to vinculin in vivo. Furthermore, in chicken embryo fibroblasts infected with a temperature-sensitive mutant of Rous sarcoma virus, tsNY68, the acylation of vinculin at the permissive temperature was less than one-third that at the nonpermissive temperature. Thus, the covalent binding of lipid to vinculin is a transformation-sensitive event. The covalent modification of vinculin by lipids could be directly or indirectly involved in its reversible association with membranes. This modification may also provide a mechanism to alter the organization of vinculin within cells and thereby play a regulatory role in anchoring or stabilizing microfilament bundles at plasma membranes.     

Multiple mechanisms of protein insertion into and across membranes

Protein localization in cells is initiated by the binding of characteristic leader (signal) peptides to specific receptors on the membranes of mitochondria or endoplasmic reticulum or, in bacteria, to the plasma membrane. There are differences in the timing of protein synthesis and translocation into or across the bilayer and in the requirement for a transmembrane electrochemical potential. Comparisons of protein localization in these different membranes suggest underlying common mechanisms.     

Integrins regulate Rac targeting by internalization of membrane domains

Translocation of the small GTP-binding protein Rac1 to the cell plasma membrane is essential for activating downstream effectors and requires integrin-mediated adhesion of cells to extracellular matrix. We report that active Rac1 binds preferentially to low-density, cholesterol-rich membranes, and specificity is determined at least in part by membrane lipids. Cell detachment triggered internalization of plasma membrane cholesterol and lipid raft markers. Preventing internalization maintained Rac1 membrane targeting and effector activation in nonadherent cells. Regulation of lipid rafts by integrin signals may regulate the location of membrane domains such as lipid rafts and thereby control domain-specific signaling events in anchorage-dependent cells.     

Membrane phosphatidylserine regulates surface charge and protein localization

Electrostatic interactions with negatively charged membranes contribute to the subcellular targeting of proteins with polybasic clusters or cationic domains. Although the anionic phospholipid phosphatidylserine is comparatively abundant, its contribution to the surface charge of individual cellular membranes is unknown, partly because of the lack of reagents to analyze its distribution in intact cells. We developed a biosensor to study the subcellular distribution of phosphatidylserine and found that it binds the cytosolic leaflets of the plasma membrane, as well as endosomes and lysosomes. The negative charge associated with the presence of phosphatidylserine directed proteins with moderately positive charge to the endocytic pathway. More strongly cationic proteins, normally associated with the plasma membrane, relocalized to endocytic compartments when the plasma membrane surface charge decreased on calcium influx.     

Cell surface P-glycoprotein associated with multidrug resistance in mammalian cell lines

The plasma membranes of hamster, mouse, and human tumor cell lines that display multiple resistance to drugs were examined by gel electrophoresis and immunoblotting. In every case, increased expression of a 170,000-dalton surface antigen was found to be correlated with multidrug resistance. This membrane component is of identical molecular size and shares some immunogenic homology with the previously characterized P-glycoprotein of colchicine-resistant Chinese hamster ovary cells. This finding may have application to cancer therapy.     

Plasmalemmal and subsurface complexes in human leukemic cells: membrane bonding by zipperlike junctions

After a brief exposure to agents that provoke phagocytosis, monocytic cells from patients with acute leukemia exhibit pentalaminar and septate membranous complexes. These structures connect plasma membranes of adjacent cells and join surfaces of approximating pseudopodia on the same cell; they also appear to be present in cortical areas of the cytoplasm.     

Polar PIN localization directs auxin flow in plants

Polar flow of the phytohormone auxin requires plasma membrane-associated PIN proteins and underlies multiple developmental processes in plants. Here we address the importance of the polarity of subcellular PIN localization for the directionality of auxin transport in Arabidopsis thaliana. Expression of different PINs in the root epidermis revealed the importance of PIN polar positions for directional auxin flow and root gravitropic growth. Interfering with sequence-embedded polarity signals directly demonstrates that PIN polarity is a primary factor in determining the direction of auxin flow in meristematic tissues. This finding provides a crucial piece in the puzzle of how auxin flow can be redirected via rapid changes in PIN polarity.