Interactions of c-Raf-1 with phosphatidylserine and 14-3-3.
(41/2206)Activation of Raf-1 occurs at the plasma membrane. We recently showed that 14-3-3 must be complexed with Raf-1 for efficient recruitment to the plasma membrane and activation by Ras, but that 14-3-3 is completely displaced from Raf-1 following plasma membrane binding. We show here that the Raf-1 zinc finger is not absolutely required for 14-3-3 binding but is required to stabilize the interaction between Raf-1 and 14-3-3. Incubation of Raf-1 with phosphatidylserine, an inner plasma membrane phospholipid, results in removal of 14-3-3 and an increase in Raf-1 kinase activity, whereas removal of 14-3-3 from Raf-1 using specific phosphopeptides substantially reduces Raf-1 basal kinase activity. Displacement of 14-3-3 from activated Raf-1 by phosphopeptides has no effect on kinase activity if Raf-1 is first removed from solution, but completely eradicates kinase activity of soluble activated Raf-1. These results suggest a mechanism for the removal of 14-3-3 from Raf-1 at the plasma membrane and show that removal of 14-3-3 from Raf-1 has markedly different effects depending on experimental conditions. (+info)
Brownian ratchets: molecular separations in lipid bilayers supported on patterned arrays.
(42/2206)Brownian ratchets use a time-varying asymmetric potential that can be applied to separate diffusing particles or molecules. A new type of Brownian ratchet, a geometrical Brownian ratchet, has been realized. Charged, fluorescently labeled phospholipids in a two-dimensional fluid bilayer were driven in one direction by an electric field through a two-dimensional periodic array of asymmetric barriers to lateral diffusion fabricated from titanium oxide on silica. Diffusion spreads the phospholipid molecules in the orthogonal direction, and the asymmetric barriers rectify the Brownian motion, causing a directional transport of molecules. The geometrical ratchet can be used as a continuous molecular sieve to separate mixtures of membrane-associated molecules that differ in electrophoretic mobility and diffusion coefficient. (+info)
Control of phosphatidylserine synthase II activity in Chinese hamster ovary cells.
(43/2206)Phosphatidylserine (PtdSer) in Chinese hamster ovary (CHO) cells is synthesized through the action of PtdSer synthase (PSS) I and II, which catalyzes the exchange of L-serine with the base moiety of phosphatidylcholine and phosphatidylethanolamine, respectively. The PtdSer synthesis in a CHO cell mutant, PSA-3, which lacks PSS I but has normal PSS II activity, was almost completely inhibited by the addition of PtdSer to the culture medium, like that in the wild-type CHO-K1 cells. In contrast, the PtdSer synthesis in a PSS II-overproducing stable transformant of CHO-K1, K1/wt-pssB, was reduced by only 35% upon addition of PtdSer. The serine exchange activity in a membrane fraction of K1/wt-pssB cells was not inhibited by PtdSer at all, whereas those of PSA-3 and CHO-K1 cells were inhibited by >95%. These results indicated that PSS II activity in PSA-3 and CHO-K1 cells is inhibited by exogenous PtdSer and that overproduction of PSS II leads to the loss of normal control of PSS II activity by exogenous PtdSer. Although overproduced PSS II in K1/wt-pssB cells was not normally controlled by exogenous PtdSer, K1/wt-pssB cells cultivated without exogenous PtdSer exhibited a normal PtdSer biosynthetic rate similar to that in CHO-K1 cells. In contrast to K1/wt-pssB cells, another stable transformant of CHO-K1, K1/R97K-pssB, which overproduces R97K mutant PSS II, exhibited a approximately 4-fold higher PtdSer biosynthetic rate compared with that in CHO-K1 cells. These results suggested that for maintenance of a normal PtdSer biosynthetic rate, the activity of overproduced wild-type PSS II in K1/wt-pssB cells is depressed by an as yet unknown post-translational mechanisms other than those for the exogenous PtdSer-mediated inhibition and that Arg-97 of PSS II is critical for this depression of overproduced PSS II activity. When the cDNA-directed wild-type and R97K mutant PSS II activities were expressed at nonoverproduction levels in a PSS I- and PSS II-defective mutant of CHO-K1 cells, expression of the mutant PSS II activity but not that of the wild-type PSS II activity induced the PtdSer-resistant PtdSer biosynthesis. This suggested that Arg-97 of PSS II is critical also for the exogenous PtdSer-mediated inhibition of PSS II. (+info)
Lipid translocation across the plasma membrane of mammalian cells.
(44/2206)The plasma membrane, which forms the physical barrier between the intra- and extracellular milieu, plays a pivotal role in the communication of cells with their environment. Exchanging metabolites, transferring signals and providing a platform for the assembly of multi-protein complexes are a few of the major functions of the plasma membrane, each of which requires participation of specific membrane proteins and/or lipids. It is therefore not surprising that the two leaflets of the membrane bilayer each have their specific lipid composition. Although membrane lipid asymmetry has been known for many years, the mechanisms for maintaining or regulating the transbilayer lipid distribution are still not completely understood. Three major players have been presented over the past years: (1) an inward-directed pump specific for phosphatidylserine and phosphatidylethanolamine, known as aminophospholipid translocase; (2) an outward-directed pump referred to as 'floppase' with little selectivity for the polar headgroup of the phospholipid, but whose actual participation in transport of endogenous lipids has not been well established; and (3) a lipid scramblase, which facilitates bi-directional migration across the bilayer of all phospholipid classes, independent of the polar headgroup. Whereas a concerted action of aminophospholipid translocase and floppase could, in principle, account for the maintenance of lipid asymmetry in quiescent cells, activation of the scramblase and concomitant inhibition of the aminophospholipid translocase causes a collapse of lipid asymmetry, manifested by exposure of phosphatidylserine on the cell surface. In this article, each of these transporters will be discussed, and their physiological importance will be illustrated by the Scott syndrome, a bleeding disorder caused by impaired lipid scrambling. Finally, phosphatidylserine exposure during apoptosis will be briefly discussed in relation to inhibition of translocase and simultaneous activation of scramblase. (+info)
Increased cell surface exposure of phosphatidylserine on propidium iodide negative thymocytes undergoing death by necrosis.
(45/2206)Phosphatidylserine (PS) exposure on propidium iodide negative cells using FITC labelled annexin-V has been used to quantify apoptosis in vitro and in vivo. Detection of PS within cells undergoing necrosis is also possible if labelled annexin-V specific for PS enters the cell following early membrane damage. Necrotic or late apoptotic cells can be excluded from flow cytometric analysis using propidium iodide which enters and stains cells with compromised membrane integrity. Here we show that thymocytes undergoing death exclusively by necrosis show early exposure of PS prior to loss of membrane integrity. This early exposure of PS occurs in cells treated with agents which both raise intracellular calcium levels and are also capable of interacting with protein thiol groups. We also demonstrate that PS exposure in thymocytes induced to undergo apoptosis by three different agents does not correlate with calcium rises but correlates with and precedes DNA fragmentation. (+info)
Anionic phospholipids decrease the rate of slippage on the Ca(2+)-ATPase of sarcoplasmic reticulum.
(46/2206)Accumulation of Ca(2+) by the Ca(2+)-ATPase of skeletal-muscle sarcoplasmic reticulum has been measured in reconstituted, sealed vesicles as a function of lipid composition. Measurements were performed in the presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) to eliminate any effects of H(+) transport; in the presence of FCCP, addition of valinomycin had no effect on the level or rate of accumulation of Ca(2+) showing that, in the presence of FCCP, no electrical potential built up across the membrane. Levels of accumulation were low when the phospholipid was dioleoylphosphatidylcholine (DOPC), even though DOPC supports high ATPase activity. Inclusion of 10 mol% anionic phospholipid [dioleoylphosphatidic acid (DOPA) or dioleoylphosphatidylserine (DOPS)] led to higher levels of accumulation of Ca(2+), 10 mol% being the optimum concentration. Cardiolipin or phosphatidylinositol 4-phosphate were more effective than DOPA or DOPS in increasing accumulation of Ca(2+). Effects of anionic phospholipids were seen in the presence of an ATP-regenerating system to remove ADP, and in the presence of phosphate within the reconstituted vesicles to precipitate calcium phosphate. Rates of passive leak of Ca(2+) from the reconstituted vesicles were slow. The Ca(2+)-accumulation process was simulated assuming either simple passive leak of Ca(2+) from the vesicles or assuming slippage on the ATPase, a process in which the phosphorylated intermediate of the ATPase releases bound Ca(2+) on the cytoplasmic rather than the lumenal side of the membrane. The experimental data fitted to a slippage model, with anionic phospholipids decreasing the rate of slippage. (+info)
Curcumin mediated apoptosis in AK-5 tumor cells involves the production of reactive oxygen intermediates.
(47/2206)Curcumin, the active ingredient of the rhizome of Curcuma longa has anti-inflammatory, antioxidant and antiproliferative activities. Although its precise mode of action remains elusive, studies have shown that chemopreventive action of curcumin might be due to its ability to induce apoptosis in cancer cells. Curcumin was shown to be responsible for the inhibition of AK-5 tumor (a rat histiocytoma) growth by inducing apoptosis in AK-5 tumor cells via caspase activation. This study was designed to investigate the mechanism leading to the induction of apoptosis in AK-5 tumor cells. Curcumin treatment resulted in the hyperproduction of reactive oxygen species (ROS), loss of mitochondrial membrane potential (delta psi(m)) and cytochrome c release to the cytosol, with the concomitant exposure of phosphatidylserine (PS) residues on the cell surface. This study suggests redox signalling and caspase activation as the mechanisms responsible for the induction of curcumin mediated apoptosis in AK-5 tumor cells. (+info)
Ceramides modulate protein kinase C activity and perturb the structure of Phosphatidylcholine/Phosphatidylserine bilayers.
(48/2206)We studied the effects of natural ceramide and a series of ceramide analogs with different acyl chain lengths on the activity of rat brain protein kinase C (PKC) and on the structure of bovine liver phosphatidylcholine (BLPC)/dipalmitoylphosphatidylcholine (DPPC)/dipalmitoylphosphatidylserine (DPPS) (3:1:1 molar ratio) bilayers using (2)H-NMR and specific enzymatic assays in the absence or presence of 7.5 mol % diolein (DO). Only a slight activation of PKC was observed upon addition of the short-chain ceramide analogs (C(2)-, C(6)-, or C(8)-ceramide); natural ceramide or C(16)-ceramide had no effect. In the presence of 7.5 mol % DO, natural ceramide and C(16)-ceramide analog slightly attenuated DO-enhanced PKC activity. (2)H-NMR results demonstrated that natural ceramide and C(16)-ceramide induced lateral phase separation of gel-like and liquid crystalline domains in the bilayers; however, this type of membrane perturbation has no direct effect on PKC activity. The addition of both short-chain ceramide analogs and DO had a synergistic effect in activating PKC, with maximum activity observed with 20 mol % C(6)-ceramide and 15 mol % DO. Further increases in C(6)-ceramide and/or DO concentrations led to decreased PKC activity. A detailed (2)H-NMR investigation of the combined effects of C(6)-ceramide and DO on lipid bilayer structure showed a synergistic effect of these two reagents to increase membrane tendency to adopt nonbilayer structures, resulting in the actual presence of such structures in samples exceeding 20 mol % ceramide and 15 mol % DO. Thus, the increased tendency to form nonbilayer lipid phases correlates with increased PKC activity, whereas the actual presence of such phases reduced the activity of the enzyme. Moreover, the results show that short-chain ceramide analogs, widely used to study cellular effects of ceramide, have biological effects that are not exhibited by natural ceramide. (+info)