Many genetically encoded biosensors use Förster resonance energy transfer (FRET) between fluorescent proteins to report biochemical phenomena in living cells. Most commonly, the enhanced cyan fluorescent protein (ECFP) is used as the donor fluorophore, coupled with one of several yellow fluorescent protein (YFP) variants as the acceptor. ECFP is used despite several spectroscopic disadvantages, namely a low quantum yield, a low extinction coefficient and a fluorescence lifetime that is best fit by a double exponential. To improve the characteristics of ECFP for FRET measurements, we used a site-directed mutagenesis approach to overcome these disadvantages. The resulting variant, which we named Cerulean (ECFP/S72A/Y145A/H148D), has a greatly improved quantum yield, a higher extinction coefficient and a fluorescence lifetime that is best fit by a single exponential. Cerulean is 2.5-fold brighter than ECFP and replacement of ECFP with Cerulean substantially improves the signal-to-noise ratio of a FRET-based sensor for glucokinase activation.

The serine/threonine kinase Raf-1 is an essential component of the MAPK cascade. Activation of Raf-1 by extracellular signals is initiated by association with intracellular membranes. Recruitment of Raf-1 to membranes has been reported to be mediated by direct association with Ras and by the phospholipase D product phosphatidic acid (PA). Here we report that insulin stimulation of HIRcB fibroblasts leads to accumulation of Ras, Raf-1, phosphorylated MEK, phosphorylated MAPK, and PA on endosomal membranes. Mutations that disrupt Raf-PA interactions prevented recruitment of Raf-1 to membranes, whereas disruption of Ras-Raf interactions did not affect agonistdependent translocation. Expression of a dominantnegative Ras mutant did not prevent insulin-dependent Raf-1 translocation, but inhibited phosphorylation of MAPK. Finally, the PA-binding region of Raf-1 was sufficient to target green fluorescent protein to membranes, and its overexpression blocked recruitment of Raf-1 to membranes and disrupted insulin-dependent MAPK phosphorylation. These results indicate that agonist-dependent Raf-1 translocation is primarily mediated by a direct interaction with PA and is independent of association with Ras.The MAPK 1 cascade is a signaling pathway essential for the regulation of mitogenesis by extracellular signals (1). One of the critical regulatory points in this cascade is the activation of the serine/threonine kinase Raf-1. Inactive Raf-1 exists in a large cytoplasmic complex with molecular chaperone proteins (2). Upon stimulation of cell-surface receptors, Raf-1 becomes associated with membranes and undergoes a complex series of activation steps modulated by the small GTPase Ras (3-5), 14-3-3 proteins (6 -11), and phosphorylation (12-15). Furthermore, the association of Raf-1 with membranes appears to be essential for its activation. Forced membrane recruitment through attachment of the Ras prenylation moiety to the C terminus of Raf-1 has been shown to induce kinase activation by a mechanism reportedly independent of association with Ras (16,17). It was also found that overexpression of constitutively activated Ras proteins results in the recruitment of Raf-1 to membranes (16,17). Although it was clear that Raf-1 associates with activated Ras in vivo, in vitro activation of Raf-1 by Ras has been difficult to demonstrate (18,19). Therefore, it was proposed that Ras mediates recruitment of Raf-1 to membranes (20).The recruitment of Raf-1 to membranes has also been reported to be dependent on its association with phosphatidic acid (21, 24). Disruption of agonist-dependent PLD activity either pharmacologically or by expression of catalytically inactive mutants blocks recruitment of Raf-1 to membranes and Raf-1 activation. Furthermore, addition of exogenous PA reverses the effects of PLD inhibition on Raf-1 translocation and MAPK phosphorylation. Although PA does not activate Raf-1 in vitro or in vivo, addition of exogenous PA can induce recruitment of Raf-1 to membranes in HIRcB cells or Ras-transformed cells (21). Theref...

The primary known function of phospholipase D (PLD) is to generate phosphatidic acid (PA) via the hydrolysis of phosphatidylcholine. However, the functional role of PA is not well understood. We report here evidence that links the activation of PLD by insulin and the subsequent generation of PA to the activation of the Raf-1-mitogen-activated protein kinase (MAPK) cascade. Brefeldin A (BFA), an inhibitor of the activation of ADP-ribosylation factor proteins, inhibited insulin-dependent production of PA and MAPK phosphorylation. The addition of PA reversed the inhibition of MAPK activation by BFA. Overexpression of a catalytically inactive variant of PLD2, but not PLD1, blocked insulindependent activation of PLD and phosphorylation of MAPK. Real time imaging analysis showed that insulin induced Raf-1 translocation to cell membranes by a process that was inhibited by BFA. PA addition reversed the effects of BFA on Raf-1 translocation. However, PA did not activate Raf-1 in vitro or in vivo, suggesting that the primary function of PA is to enhance the recruitment of Raf-1 to the plasma membrane where other factors may activate it. Finally, we found that the recruitment of Raf-1 to the plasma membrane was transient, but Raf-1 remained bound to endocytic vesicles.Growth factor-mediated activation of PLD 1 has been well documented and occurs in response to a broad class of mitogens, including insulin, platelet-derived growth factor, epidermal growth factor, vasopressin, and phorbol esters (1-4). Activation of PLD occurs through interaction with the small G-proteins of the ADP-ribosylation factor (ARF) (5, 6) and Rac/Rho families (7) as well as with protein kinase C (PKC) (8, 9). The relative contribution of these factors to the activation of PLD is highly dependent on the cell type and signaling model examined. For example, stimulation of Rat-1 fibroblasts overexpressing the human insulin receptor (HIRcB cells) with insulin activates PLD exclusively through the ARF pathway (10), whereas the activation of PLD by insulin in adipocytes appears to be primarily Rho-mediated (11). Activation of PLD has been implicated in a wide variety of intracellular and extracellular processes, including actin polymerization, coatomer assembly, vesicle transport, neutrophil activation, and platelet aggregation (12-16).Activated PLD catalyzes the hydrolysis of phosphatidylcholine to generate PA. However, the downstream consequences of PA generation are not well understood. Although it is clear that the principal effects of PA in some systems may be mediated by its conversion to diacylglycerol (DAG) or lysophosphatidic acid (LPA), PA may also be a potent second messenger. Several laboratories have identified putative targets for PA in growth factor signal transduction, including a protein tyrosine phosphatase (17), phospholipase C-␥ (18), and Ras-GAP (19). However, the physiological relevance of these interactions has not been established.Recently, Ghosh et al. (20) reported that PA interacts directly with the serine-threonine kinase Raf-1...

A previous study has shown an efficient, systemic transinclusion of cholesterol as a helper lipid increased the in gene expression in mice via intravenous administration of vivo transfection efficiency of LPD and more importantly, a LPD formulation composed of DOTAP liposomes, protadecrease the amount of cationic lipid required for the maximine sulfate and plasmid DNA. In this study, factors affectmal level of gene expression. Studies on the interaction ing the in vivo performance of this formulation were further between mouse serum and LPD showed that LPD became evaluated. A protocol in which liposomes were mixed with negatively charged after exposure to serum, and LPDs protamine before the addition of plasmid DNA was shown containing different helper lipids varied in the amount of to produce small condensed particles with a diameter of associated serum proteins. LPD containing DOPE was about 135 nm. These particles were stable over time and more enriched in a protein corresponding to albumin in gave a high level of gene expression in all tissues exammolecular weight. These results suggest that the mechined including lung, heart, spleen, liver and kidney with the anism of LPD-mediated intravenous gene delivery might highest level of expression in the lung. Inclusion of dioleoylbe different from that of in vitro lipofection and that serum phosphatidylethanolamine (DOPE) as a helper lipid sigprotein association might be a major factor limiting the in nificantly decreased the in vivo activity of LPD. In contrast, vivo transfection by LPD.

Cyan fluorescent proteins (CFPs), such as Cerulean, are widely used as donor fluorophores in Förster resonance energy transfer (FRET) experiments. Nonetheless, the most widely used variants suffer from drawbacks that include low quantum yields and unstable flurorescence. To improve the fluorescence properties of Cerulean, we used the X-ray structure to rationally target specific amino acids for optimization by site-directed mutagenesis. Optimization of residues in strands 7 and 8 of the β-barrel improved the quantum yield of Cerulean from 0.48 to 0.60. Further optimization by incorporating the wild-type T65S mutation in the chromophore improved the quantum yield to 0.87. This variant, mCerulean3, is 20% brighter and shows greatly reduced fluorescence photoswitching behavior compared to the recently described mTurquoise fluorescent protein in vitro and in living cells. The fluorescence lifetime of mCerulean3 also fits to a single exponential time constant, making mCerulean3 a suitable choice for fluorescence lifetime microscopy experiments. Furthermore, inclusion of mCerulean3 in a fusion protein with mVenus produced FRET ratios with less variance than mTurquoise-containing fusions in living cells. Thus, mCerulean3 is a bright, photostable cyan fluorescent protein which possesses several characteristics that are highly desirable for FRET experiments.

Lipolysis is a critical metabolic pathway contributing to energy homeostasis through degradation of triacylglycerides stored in lipid droplets (LDs), releasing fatty acids. Neutral lipid lipases act at the oil/water interface. In mammalian cells, LD surfaces are coated with one or more members of the perilipin protein family, which serve important functions in regulating lipolysis. We investigated mechanisms by which three perilipin proteins control lipolysis by adipocyte triglyceride lipase (ATGL), a key lipase in adipocytes and non-adipose cells. Using a cell culture model, we examined interactions of ATGL and its co-lipase CGI-58 with perilipin 1 (perilipin A), perilipin 2 (adipose differentiation-related protein), and perilipin 5 (LSDP5) using multiple techniques as follows: anisotropy Forster resonance energy transfer, coimmunoprecipitation, [32 P]orthophosphate radiolabeling, and measurement of lipolysis. The results show that ATGL interacts with CGI-58 and perilipin 5; the latter is selectively expressed in oxidative tissues. Both proteins independently recruited ATGL to the LD surface, but with opposite effects; interaction of ATGL with CGI-58 increased lipolysis, whereas interaction of ATGL with perilipin 5 decreased lipolysis. In contrast, neither perilipin 1 nor 2 interacted directly with ATGL. Activation of protein kinase A (PKA) increased [ 32 P]orthophosphate incorporation into perilipin 5 by 2-fold, whereas neither ATGL nor CGI-58 was labeled under the incubation conditions. Cells expressing both ectopic perilipin 5 and ATGL showed a 3-fold increase in lipolysis following activation of PKA. Our studies establish perilipin 5 as a novel ATGL partner and provide evidence that the protein composition of perilipins at the LD surface regulates lipolytic activity of ATGL. Hydrolysis of triacylglycerol (TAG)3 stored in the lipid droplet (LD) compartment provides a convenient source of cellular fuel for energy production during conditions such as fasting. However, lipolysis can contribute to the build up of toxic lipid intermediates and/or oxidized lipids in pathological conditions when cellular lipid homeostasis is altered, e.g. with obesity (1). Therefore, TAG hydrolysis must be carefully controlled to meet tissue-specific requirements for energy or lipid substrates in both adipose and non-adipose tissues. A better understanding of the mechanisms by which cells control lipid mobilization is needed to design novel approaches for intervention and prevention of the pathophysiological consequences of obesity.During the past 10 years, key players in the lipolytic pathway of adipocytes were identified through the study of transgenic mouse models (2-8). Phenotypic analysis of hormone-sensitive lipase null mice suggested the existence of another lipase (7), leading to the identification of adipose triglyceride lipase (ATGL) (6, 9, 10). Characterization of ATGL null mice helped to establish the respective roles of these two lipases in the lipolytic cascade (7). Complete lipolysis requires three enzyme reactions t...

To perform quantitative live cell imaging, investigators require fluorescent reporters that accurately report protein localization and levels, while minimally perturbing the cell. Yet, within the biochemically distinct environments of cellular organelles, popular fluorescent proteins (FPs), including EGFP, can be unreliable for quantitative imaging, resulting in underestimation of protein levels and incorrect localization. Specifically, within the secretory pathway, significant populations of FPs misfold and fail to fluoresce due to non-native disulphide bond formation. Furthermore, transmembrane FP fusion constructs can disrupt organelle architecture due to oligomerizing tendencies of numerous common FPs. Here, we describe a powerful set of bright and inert FPs optimized for use in multiple cellular compartments, especially oxidizing environments and biological membranes. Also, we provide new insights into use of red FPs in the secretory pathway. Our monomeric "oxFPs" finally resolve long standing, underappreciated, and important problems of cell biology and should be useful for a number of applications.

Lipid droplets are cellular organelles, structurally similar to lipoprotein particles. Lipid droplets include a neutral lipid core composed largely of triglycerides, surrounded by a phospholipid monolayer and coated with surface proteins that provide an interface for various aspects of lipid metabolism, including lipid transport, lipogenesis, and lipolysis (1-5). Lipolysis is an important mechanism by which cells release energy stored in lipid droplets; its impairment has been linked to cellular lipotoxicity and insulin resistance (6). Studies are needed to gain an understanding of the underlying molecular mechanisms regulating lipolysis. Although all cells are equipped to perform lipolysis, the extent of lipid accumulation and specific components of the lipolytic pathway are variable, depending on the type of cell.Numerous recent studies have led to consensus that members of the PAT family of proteins, originally named for Perilipin, Adipose differentiation-related protein (ADFP) 4 and Tail Interacting Protein 47 (TIP47), play conserved structural and functional roles on lipid droplets (6 -9). Proteomic studies have identified a "signature" composition for lipid droplets from a variety of types of cells that includes at least one PAT family member. In mammalian cells, the PAT family includes perilipin