Interaction of partially structured states of acidic fibroblast growth factor with phospholipid membranes

Intracellular partners of fibroblast growth factors 1 and 2 - implications for functions

Fibroblast growth factors 1 and 2 (FGF1 and FGF2) are mainly considered as ligands of surface receptors through which they regulate a broad spectrum of biological processes. They are secreted in non-canonical way and, unlike other growth factors, they are able to translocate from the endosome to the cell interior. These unique features, as well as the role of the intracellular pool of FGF1 and FGF2, are far from being fully understood. An increasing number of reports address this problem, focusing on the intracellular interactions of FGF1 and 2. Here, we summarize the current state of knowledge of the FGF1 and FGF2 binding partners inside the cell and the possible role of these interactions. The partner proteins are grouped according to their function, including proteins involved in secretion, cell signaling, nucleocytoplasmic transport, binding and processing of nucleic acids, ATP binding, and cytoskeleton assembly. An in-depth analysis of the network of these binding partners could indicate novel, non-classical functions of FGF1 and FGF2 and uncover an additional level of a fine control of the well-known FGF-regulated cellular processes.

Elucidating the Role of Val-Asn 95 and Arg-Gly 52 Mutations on Structure and Stability of Fibroblast Growth Factor Homologous Factor 2

Background:

Fibroblast growth Factor Homologous Factors (FHFs) belong to a subclass of Fibroblast Growth Factor (FGF) family owing to their high sequence and structural similarities with FGFs. However, despite these similarities, there are properties which set them apart from FGFs. FHFs lack the secretion signal sequence unlike other FGF members, except FGF1 and 2. Unlike FGFs, FHFs are not able to bind to FGF Receptors (FGFRs) and instead have been implicated in binding to Voltage-Gated Sodium Channels (VGSCs), neuronal MAP kinase scaffold protein and islet-brain-2 (IB2). The two amino acids Arg-52 and Val95 are conserved in all FHFs and mutation of these residues lead to its inability to bind with VGSC/IB2. However, it is not clear whether the loss of binding is due to destabilization of the protein on mutation or due to involvement of Arg52 and Val95 in conferring functionality to FHFs.

Objective:

In the present study, we have mutated these two conserved residues of FHF2 with its corresponding FGF counterpart amino acids and studied the effects of the mutations on the structure and stability of the protein.

Methods:

Several biophysical methods like isothermal equilibrium denaturation study, ANS fluorescence, intrinsic fluorescence, acrylamide quenching, circular dichroism studies as well as using computational approaches were employed.

Results:

The single mutations were found to affect the overall stability, conformation and functionality of the protein.

Conclusion:

Thus, the studies throw light on the role of specific amino acids in deciding the stability, structure and functionality of proteins and will be useful for development of therapeutically engineered proteins.

An S116R Phosphorylation Site Mutation in Human Fibroblast Growth Factor-1 Differentially Affects Mitogenic and Glucose-Lowering Activities

Fibroblast growth factor-1 (FGF-1), a potent human mitogen and insulin sensitizer, signals through both tyrosine kinase receptor-mediated autocrine/paracrine pathways as well as a nuclear intracrine pathway. Phosphorylation of FGF-1 at serine 116 (S116) has been proposed to regulate intracrine signaling. Position S116 is located within a ∼17 amino acid C-terminal loop that contains a rich set of functional determinants including heparin∖heparan sulfate affinity, thiol reactivity, nuclear localization, pharmacokinetics, functional half-life, nuclear ligand affinity, stability, and structural dynamics. Mutational targeting of specific functionality in this region without perturbing other functional determinants is a design challenge. S116R is a non-phosphorylatable variant present in bovine FGF-1 and other members of the human FGF family. We show that the S116R mutation in human FGF-1 is accommodated with no perturbation of biophysical or structural properties, and is therefore an attractive mutation with which to elucidate the functional role of phosphorylation. Characterization of S116R shows reduction in NIH 3T3 fibroblast mitogenic stimulation, increase in fibroblast growth factor receptor-1c activation, and prolonged duration of glucose lowering in ob/ob hyperglycemic mice. A novel FGF-1/fibroblast growth factor receptor-1c dimerization interaction combined with non-phosphorylatable intracrine signaling is hypothesized to be responsible for these observed functional effects.

Folding of Fibroblast Growth Factor 1 Is Critical for Its Nonclassical Release

Fibroblast growth factor 1 (FGF1), a ubiquitously expressed pro-angiogenic protein that is involved in tissue repair, carcinogenesis, and maintenance of vasculature stability, is released from the cells via a stress-dependent nonclassical secretory pathway. FGF1 secretion is a result of transmembrane translocation of this protein. It correlates with the ability of FGF1 to permeabilize membranes composed of acidic phospholipids. Like several other nonclassically exported proteins, FGF1 exhibits β-barrel folding. To assess the role of folding of FGF1 in its secretion, we applied targeted mutagenesis in combination with a complex of biophysical methods and molecular dynamics studies, followed by artificial membrane permeabilization and stress-induced release experiments. It has been demonstrated that a mutation of proline 135 located in the C-terminus of FGF1 results in (i) partial unfolding of FGF1, (ii) a decrease in FGF1's ability to permeabilize bilayers composed of phosphatidylserine, and (iii) drastic inhibition of stress-induced FGF1 export. Thus, folding of FGF1 is critical for its nonclassical secretion.

Clusters of isoleucine, leucine, and valine side chains define cores of stability in high-energy states of globular proteins: Sequence determinants of structure and stability

Measurements of protection against exchange of main chain amide hydrogens (NH) with solvent hydrogens in globular proteins have provided remarkable insights into the structures of rare high-energy states that populate their folding free-energy surfaces. Lacking, however, has been a unifying theory that rationalizes these high-energy states in terms of the structures and sequences of their resident proteins. The Branched Aliphatic Side Chain (BASiC) hypothesis has been developed to explain the observed patterns of protection in a pair of TIM barrel proteins. This hypothesis supposes that the side chains of isoleucine, leucine, and valine (ILV) residues often form large hydrophobic clusters that very effectively impede the penetration of water to their underlying hydrogen bond networks and, thereby, enhance the protection against solvent exchange. The linkage between the secondary and tertiary structures enables these ILV clusters to serve as cores of stability in high-energy partially folded states. Statistically significant correlations between the locations of large ILV clusters in native conformations and strong protection against exchange for a variety of motifs reported in the literature support the generality of the BASiC hypothesis. The results also illustrate the necessity to elaborate this simple hypothesis to account for the roles of adjacent hydrocarbon moieties in defining stability cores of partially folded states along folding reaction coordinates.

Oligomeric states of the Shigella translocator protein IpaB provide structural insights into formation of the type III secretion translocon

The Shigella flexneri Type III secretion system (T3SS) senses contact with human intestinal cells and injects effector proteins that promote pathogen entry as the first step in causing life threatening bacillary dysentery (shigellosis). The Shigella Type III secretion apparatus (T3SA) consists of an anchoring basal body, an exposed needle, and a temporally assembled tip complex. Exposure to environmental small molecules recruits IpaB, the first hydrophobic translocator protein, to the maturing tip complex. IpaB then senses contact with a host cell membrane, forming the translocon pore through which effectors are delivered to the host cytoplasm. Within the bacterium, IpaB exists as a heterodimer with its chaperone IpgC; however, IpaB's structural state following secretion is unknown due to difficulties isolating stable protein. We have overcome this by coexpressing the IpaB/IpgC heterodimer and isolating IpaB by incubating the complex in mild detergents. Interestingly, preparation of IpaB with n-octyl-oligo-oxyethylene (OPOE) results in the assembly of discrete oligomers while purification in N,N-dimethyldodecylamine N-oxide (LDAO) maintains IpaB as a monomer. In this study, we demonstrate that IpaB tetramers penetrate phospholipid membranes to allow a size-dependent release of small molecules, suggesting the formation of discrete pores. Monomeric IpaB also interacts with liposomes but fails to disrupt them. From these and additional findings, we propose that IpaB can exist as a tetramer having inherent flexibility, which allows it to cooperatively interact with and insert into host cell membranes. This event may then lay the foundation for formation of the Shigella T3SS translocon pore.

Protein-phospholipid interactions in nonclassical protein secretion: problem and methods of study

Extracellular proteins devoid of signal peptides use nonclassical secretion mechanisms for their export. These mechanisms are independent of the endoplasmic reticulum and Golgi. Some nonclassically released proteins, particularly fibroblast growth factors (FGF) 1 and 2, are exported as a result of their direct translocation through the cell membrane. This process requires specific interactions of released proteins with membrane phospholipids. In this review written by a cell biologist, a structural biologist and two membrane engineers, we discuss the following subjects: (i) Phenomenon of nonclassical protein release and its biological significance; (ii) Composition of the FGF1 multiprotein release complex (MRC); (iii) The relationship between FGF1 export and acidic phospholipid externalization; (iv) Interactions of FGF1 MRC components with acidic phospholipids; (v) Methods to study the transmembrane translocation of proteins; (vi) Membrane models to study nonclassical protein release.

An empirical phase diagram approach to investigate conformational stability of "second-generation" functional mutants of acidic fibroblast growth factor-1

Acidic fibroblast growth factor-1 (FGF-1) is an angiogenic protein which requires binding to a polyanion such as heparin for its mitogenic activity and physicochemical stability. To evaluate the extent to which this heparin dependence on solution stability could be reduced or eliminated, the structural integrity and conformational stability of 10 selected FGF-1 mutants were examined as a function of solution pH and temperature by a series of spectroscopic methods including circular dichroism, intrinsic and extrinsic fluorescence spectroscopy and static light scattering. The biophysical data were summarized in the form of colored empirical phase diagrams (EPDs). FGF-1 mutants were identified with stability profiles in the absence of heparin comparable to that of wild-type FGF-1 in the presence of heparin while still retaining their biological activity. In addition, a revised version of the EPD methodology was found to provide an information rich, high throughput approach to compare the effects of mutations on the overall conformational stability of proteins in terms of their response to environmental stresses such as pH and temperature.

Secretion without Golgi

A growing number of proteins devoid of signal peptides have been demonstrated to be released through the non-classical pathways independent of endoplasmic reticulum and Golgi. Among them are two potent proangiogenic cytokines FGF1 and IL1alpha. Stress-induced transmembrane translocation of these proteins requires the assembly of copper-dependent multiprotein release complexes. It involves the interaction of exported proteins with the acidic phospholipids of the inner leaflet of the cell membrane and membrane destabilization. Not only stress, but also thrombin treatment and inhibition of Notch signaling stimulate the export of FGF1. Non-classical release of FGF1 and IL1alpha presents a promising target for treatment of cardiovascular, oncologic, and inflammatory disorders.

Relevance of partially structured states in the non-classical secretion of acidic fibroblast growth factor

Acidic fibroblast growth factor (aFGF) is a signal peptide-less protein that is secreted into the extracellular compartment as part of a multiprotein release complex, consisting of aFGF, S100A13 (a calcium binding protein), and a 40 kDa (p40) form of synaptotagmin (Syt1), a protein that participates in the docking of a variety of secretory vesicles. p40 Syt1, and specifically its C2A domain, is believed to play a major role in the non-classical secretion of the aFGF release complex mediated by the interaction of aFGF and p40 Syt1with the phospholipids of the cell membrane inner leaflet. In the present study, we investigate the structural characteristics of aFGF and the C2A domain of p40 Syt1 under acidic conditions, using a variety of biophysical techniques including multidimensional NMR spectroscopy. Urea-induced equilibrium unfolding (at pH 3.4) of both aFGF and the C2A domain are non-cooperative and proceed with the accumulation of stable intermediate states. 1-Anilino-8-napthalene sulfonate (ANS) binding and size-exclusion chromatography results suggest that both aFGF and the C2A domain exist as partially structured states under acidic conditions (pH 3.4). Limited trypsin digestion analysis and 1H-15N chemical shift perturbation data reveal that the flexibility of certain portions of the protein backbone is increased in the partially structured state(s) of aFGF. The residues that are perturbed in the partially structured state(s) in aFGF are mostly located at the N- and C-terminal ends of the protein. In marked contrast, most of the interactions stabilizing the native secondary structure are preserved in the partially structured state of the C2A domain. Isothermal titration calorimetry data indicate that the binding affinity between aFGF and the C2A domain is significantly enhanced at pH 3.4. In addition, both aFGF and the C2A domain exhibit much higher lipid binding affinity in their partially structured states. The translocation of the multiprotein FGF release complex across the membrane appears to be facilitated by the formation of partially structured states of aFGF and the C2A domain of p40 Syt1.

Sphingosine kinase 1 is a critical component of the copper-dependent FGF1 export pathway

Sphingosine kinase 1 catalyzes the formation of sphingosine-1-phosphate, a lipid mediator involved in the regulation of angiogenesis. Sphingosine kinase 1 is constitutively released from cells, even though it lacks a classical signal peptide sequence. Because copper-dependent non-classical stress-induced release of FGF1 also regulates angiogenesis, we questioned whether sphingosine kinase 1 is involved in the FGF1 release pathway. We report that (i) the coexpression of sphingosine kinase 1 with FGF1 inhibited the release of sphingosine kinase 1 at 37 degrees C; (ii) sphingosine kinase 1 was released at 42 degrees C in complex with FGF1; (iii) sphingosine kinase 1 null cells failed to release FGF1 at stress; (iv) sphingosine kinase 1 is a high affinity copper-binding protein which formed a complex with FGF1 in a cell-free system, and (v) sphingosine kinase 1 over expression rescued the release of FGF1 from inhibition by the copper chelator, tetrathiomolybdate. We propose that sphingosine kinase 1 is a component of the copper-dependent FGF1 release pathway.

Effects of Solutes on Empirical Phase Diagrams of Human Fibroblast Growth Factor 1

A variety of solutes are commonly used to increase the stability of protein in therapeutic formulations. An empirical phase diagram approach is used to evaluate the effects of different types of additives on the solution behavior of a protein of pharmaceutical interest, human fibroblast growth factor 1 (FGF-1). A specific stabilizer, heparin, and a nonspecific stabilizer, sucrose, were used in this work. The protein was characterized as a function of pH (3-8) and temperature (10-85 degrees C) using Far-UV circular dichroism (Far-UV CD), intrinsic and extrinsic fluorescence as well as second derivative UV absorption spectroscopy. Empirical phase diagrams were constructed to summarize the biophysical characterization data obtained with FGF-1 alone, in the presence of a threefold weight excess of heparin (3x heparin) or 10% sucrose (w/v). Three phases are observed in the low temperature regions at pH 3, 4, and 5-8. Phase boundaries corresponding to major heat-induced transitions are detected in the physiological temperature range. The highest thermal stabilities are observed near neutral pH (pH 6 and 7). Both heparin and sucrose appear to enhance the thermal stability of FGF-1, although their effects on the phase diagram are quite distinct. The greatest stabilization is observed at pH 8. Only heparin appears to protect FGF-1 from acid-induced unfolding to any extent.

Effect of Polyanions on the Structure and Stability of Repifermin™ (Keratinocyte Growth Factor-2)

The interaction of several of the fibroblast growth factors (FGFs) with polyanions is thought to be of physiological significance and has been exploited to create more stable pharmaceutical formulations of FGF-1 and -2. The extent of such phenomena throughout the 23-member FGF family is, however, unknown. In these studies, we examine the effect of several polyanions on the structure and stability of keratinocyte growth factor 2 (KGF-2, FGF-10), a candidate for use as a wound-healing agent. Employing a variety of methods sensitive to the protein's structure including circular dichroism (CD), intrinsic fluorescence, derivative near-UV absorption spectroscopy, bis-ANS (4,4'-dianilino-1,1'-binaphthyl-5,5-disulfonic acid) fluorescence, differential scanning calorimetry (DSC), and dynamic light scattering (DLS), we find that a variety of polyanions (e.g., heparin, sucrose octasulfate (SOS), and inositol hexaphosphate (IHP)) stabilize KGF-2 by increasing the thermal-unfolding temperature by approximately 9-15 degrees C. Negatively charged liposomes produce a similar effect, arguing for relatively nonspecific interactions of polyanions with KGF-2. Unlike some other FGFs, no evidence for the presence of a molten globule state is found during thermal perturbation of this growth factor. The generality of this polyanion/protein interaction is discussed as well as its potential role in various cellular events such as protein folding and transport.

Effects of pH and Polyanions on the Thermal Stability of Fibroblast Growth Factor 20

Fibroblast growth factor 20 (FGF20) is a member of the FGF family with potential for use in several different therapeutic categories. In this work, we provide the first structural characterization of FGF20 using a wide variety of approaches. Like other members of the FGF family, FGF20 appears to possess a beta-trefoil structure. The effect of pH on the conformation and thermal stability of FGF20 is evaluated using far-UV circular dichroism (CD), intrinsic and ANS fluorescence, and high-resolution derivative UV absorption spectroscopy. Empirical phase diagrams are constructed to describe the solution behavior of FGF20 over a wide pH and temperature range. The protein appears to be unstable at pH <5, with aggregation and precipitation observed during dialysis. A major heat-induced conformational change also causes aggregation and precipitation of FGF20 at elevated temperatures. The highest thermal stability is observed near neutral pH (Tm ~55 degrees C at pH 7). The effect of several high- and low-molecular mass polyanions on the thermal stability of FGF20 is also examined using CD, intrinsic fluorescence, and DSC analysis. Among these ligands, heparin exhibits the greatest stabilizing effect on FGF20, increasing the Tm by more than 10 degrees C.

Characterization of the polyanion-induced molten globule-like state of cytochromec

Cytochrome c (cyt c) undergoes a poly(vinylsulphate) (PVS)-induced transition at slightly acidic pH into a molten globule-like state that resembles the effect that negatively charged membrane surfaces have on this protein. In this work, the thermodynamic properties of the molten globule-like state of cyt c in complex with PVS are studied using differential scanning calorimetry, circular dichroism, fluorescence, and absorbance spectroscopy. The temperature-induced transition of the molten globule-like state of cyt c in the complex with PVS is characterized by a significantly lower calorimetric enthalpy than in the "typical" molten globule state of cyt c, i.e. free protein at pH 2.0 in high ionic strength. Moreover, the thermally-denatured state of cyt c in the complex at pH < 6 contains nearly 50% of the native secondary structure. The dependence of the transition temperature on the pH indicates a role for histidine residues in the destabilization of the cyt c structure in the PVS complex and in stabilization of the denatured state with the residual secondary structure. A comparison of the effects of small anions and polyanions demonstrates the importance of cooperativity among the anions in the destabilization of cyt c. Predictably, other hydrophilic flexible polyanions such as heparin, polyglutamate, and polyadenylate also have a destabilizing effect on the structure of cyt c. However, a correlation between the properties of the polyanions and their effect on the protein stability is still unclear.

Probing protein structure and dynamics by second-derivative ultraviolet absorption analysis of cation-{pi} interactions

We describe an alternate approach for studying protein structure using the detection of ultraviolet (UV) absorbance peak shifts of aromatic amino acid side chains induced by the presence of salts. The method is based on the hypothesis that salt cations (Li+, Na+, and Cs+) of varying sizes can differentially diffuse through protein matrices and interact with benzyl, phenyl, and indole groups through cation-pi interactions. We have investigated the potential of this method to probe protein dynamics by measuring high resolution second-derivative UV spectra as a function of salt concentration for eight proteins of varying physical and chemical properties and the N-acetylated C-ethyl esterified amino acids to represent totally exposed side chains. We show that small shifts in the wavelength maxima for Phe, Tyr, and Trp in the presence of high salt concentrations can be reliably measured and that the magnitude and direction of the peak shifts are influenced by several factors, including protein size, charge, and the local environment and solvent accessibility of the aromatic groups. Evaluating the empirical UV spectral data in light of known protein structural information shows that probing cation-pi interactions in proteins reveals unique information about the influence of structure on aromatic side chain spectroscopic behavior.

Release of FGF1 and p40 synaptotagmin 1 correlates with their membrane destabilizing ability

Fibroblast growth factor (FGF)1 is released from cells as a constituent of a complex that contains the small calcium binding protein S100A13, and the p40 kDa form of synaptotagmin (Syt)1, through an ER-Golgi-independent stress-induced pathway. FGF1 and the other components of its secretory complex are signal peptide-less proteins. We examined their capability to interact with lipid bilayers by studying protein-induced carboxyfluorescein release from liposomes of different phospholipid (pL) compositions. FGF1, p40 Syt1, and S100A13 induced destabilization of liposomes composed of acidic but not of zwitterionic pL. We produced mutants of FGF1 and p40 Syt1, in which specific basic amino acid residues in the regions that bind acidic pL were substituted. The ability of these mutants to induce liposomes destabilization was strongly attenuated, and they exhibited drastically diminished spontaneous and stress-induced release. Apparently, the non-classical release of FGF1 and p40 Syt1 involves destabilization of membranes containing acidic pL.

Symmetric Primary and Tertiary Structure Mutations within a Symmetric Superfold: A Solution, not a Constraint, to Achieve a Foldable Polypeptide

In previous studies designed to increase the primary structure symmetry within the hydrophobic core of human acidic fibroblast growth factor (FGF-1) a combination of five mutations were accommodated, resulting in structure, stability and folding kinetic properties similar to wild-type (despite the symmetric constraint upon the set of core residues). A sixth mutation in the core, involving a highly conserved Met residue at position 67, appeared intolerant to substitution. Structural analysis suggested that the local packing environment of position 67 involved two regions of apparent insertions that distorted the tertiary structure symmetry inherent in the beta-trefoil architecture. It was postulated that a symmetric constraint upon the primary structure within the core could only be achieved after these insertions had been deleted (concomitantly increasing the tertiary structure symmetry). The deletion of these insertions is now shown to permit mutation of position 67, thereby increasing the primary structure symmetry relationship within the core. Furthermore, despite the imposed symmetric constraint upon both the primary and tertiary structure, the resulting mutant form of FGF-1 is substantially more stable. The apparent inserted regions are shown to be associated with heparin-binding functionality; however, despite a marked reduction in heparin-binding affinity the mutant form of FGF-1 is surprisingly approximately 70 times more potent in 3T3 fibroblast mitogenic assays. The results support the hypothesis that primary structure symmetry within a symmetric protein superfold represents a possible solution, rather than a constraint, to achieving a foldable polypeptide.

Protein deformation of lipid hybrid bilayer membranes studied by sum frequency generation vibrational spectroscopy

Structural deformations of lipid hybrid bilayer membranes induced by signal peptideless (SPL) proteins have been studied for the first time using the inherently surface specific nonlinear optical technique of sum frequency generation vibrational spectroscopy. Specifically, deformations of 1,2-distearoylphosphatidylglycerol(DSPG) membranes induced by interaction with FGF-1, a SPL protein which is released asa function of cellular stress through a nonclassical pathway, have been investigated. FGF-1 was found to induce lipid alkyl chain deformations in previously highly ordered DSPG membranes at the extremely low concentration of 1 nM at 60 degrees C. The deformation process was shown to exhibit a degree of reversibility upon removal of the protein by rinsing with buffer solution.

An Annexin 2 Phosphorylation Switch Mediates p11-dependent Translocation of Annexin 2 to the Cell Surface

Annexin 2 is a profibrinolytic co-receptor for plasminogen and tissue plasminogen activator that stimulates activation of the major fibrinolysin, plasmin, at cell surfaces. In human subjects, overexpression of annexin 2 in acute promyelocytic leukemia leads to a bleeding diathesis reflective of excessive cell surface annexin 2-dependent generation of plasmin (Menell, J. S., Cesarman, G. M., Jacovina, A. T., McLaughlin, M. A., Lev, E. A., and Hajjar, K. A. (1999) N. Engl. J. Med. 340, 994-1004). In addition, mice completely deficient in annexin 2 display fibrin accumulation within blood vessels and impaired clearance of injury-induced thrombi (Ling Q., Jacovina, A.T., Deora, A.B., Febbraio, M., Simantov, R., Silverstein, R. L., Hempstead, B. L., Mark, W., and Hajjar, K. A. (2004) J. Clin. Investig. 113, 38-48). Here, we show that endothelial cell annexin 2, a protein that lacks a typical signal peptide, translocates from the cytoplasm to the extracytoplasmic plasma membrane in response to brief temperature stress both in vitro and in vivo in the absence of cell death or cell lysis. This regulated response is independent of new protein or mRNA synthesis and does not require the classical endoplasmic reticulum-Golgi pathway. Temperature stress-induced annexin 2 translocation is dependent on both expression of protein p11 (S100A10) and tyrosine phosphorylation of annexin 2 because annexin 2 release is completely eliminated on depletion of p11, inactivation of tyrosine kinase, or mutation of tyrosine 23. Translocation of annexin 2 to the cell surface dramatically increases tissue plasminogen activator-dependent plasminogen activation potential and may represent a novel stress-induced protein secretion pathway.

The non-classical export routes: FGF1 and IL-1alpha point the way

Non-classical protein release independent of the ER-Golgi pathway has been reported for an increasing number of proteins lacking an N-terminal signal sequence. The export of FGF1 and IL-1alpha, two pro-angiogenic polypeptides, provides two such examples. In both cases, export is based on the Cu2+-dependent formation of multiprotein complexes containing the S100A13 protein and might involve translocation of the protein across the membrane as a 'molten globule'. FGF1 and IL-1alpha are involved in pathological processes such as restenosis and tumor formation. Inhibition of their export by Cu2+ chelators is thus an effective strategy for treatment of several diseases.

Polyanions and the proteome

The behavior of the proteome reflects spatial and temporal organization both within and without cells. We propose that various macromolecular entities possessing polyanionic character such as proteoglycans, lipid bilayer surfaces, microtubules, microfilaments, and polynucleotides may provide a functional network that mediates a variety of cellular phenomena. The interaction of proteins with this array of polyanions is characterized by a lower degree of specificity than seen with most commonly recognized macromolecular interactions. In this commentary, potential roles for this polyanion network in diverse functions such as protein/protein interactions, protein folding and stabilization, macromolecular transport, and various disease processes are all considered, as well as the use of polyanions as therapeutic agents. The role of small polyanions in the regulation of protein/polyanion interactions is also postulated. We provide preliminary experimental analysis of the extent to which proteins interact with polyanions inside cells using a combination of two-dimensional chromatographic and electrophoretic methods and antibody arrays. We conclude that many hundreds to thousands of such interactions are present in cells and argue that future understanding of the proteome will require that the "polyanion world" be taken into account.

Signaling, internalization, and intracellular activity of fibroblast growth factor

The fibroblast growth factor (FGF) family contains 23 members in mammals including its prototype members FGF-1 and FGF-2. FGFs have been implicated in regulation of many key cellular responses involved in developmental and physiological processes. These includes proliferation, differentiation, migration, apoptosis, angiogenesis, and wound healing. FGFs bind to five related, specific cell surface receptors (FGFRs). Four of these have intrinsic tyrosine kinase activity. Dimerization of the receptor is a prerequisite for receptor transphosphorylation and activation of downstream signaling molecules. All members of the FGF family have a high affinity for heparin and for cell surface heparan sulfate proteoglycans, which participate in formation of stable and active FGF-FGFR complexes. FGF-mediated signaling is an evolutionarily conserved signaling module operative in invertebrates and vertebrates. It seems that some members of the family have a dual mode of action. FGF-1, FGF-2, FGF-3, and FGF-11-14 have been found intranuclearly as endogenous proteins. Exogenous FGF-1 and FGF-2 are internalized by receptor-mediated endocytosis, in a clathrin-dependent and -independent way. Internalized FGF-1 and FGF-2 are able to cross cellular membranes to reach the cytosol and the nuclear compartment. The role of FGF internalization and the intracellular activity of some FGFs are discussed in the context of the known signaling induced by FGF.

Nonclassical transport proteins and peptides: an alternative to classical macromolecule delivery systems

The number of peptides and proteins known to exhibit nonclassical transport activity has increased significantly in recent years. In most cases, these entities have been studied in relation to their ability to deliver high molecular weight compounds, including proteins and DNA, for the ultimate purpose of developing new drug delivery strategies. In this review, an overview of the various types of vectors is presented. The in vitro and in vivo delivery successes of this technology, as well as preliminary therapeutic efforts, are described. Although a comprehensive mechanism of nonclassical transport has not yet been clearly established, we propose a straightforward model based on the cationic nature of the vectors and the need for lack of highly organized structure. In this hypothesis we suggest that the movement of polycations is mediated by a network of extra- and intracellular polyanions while transport across the bilayer is facilitated by cation-pi interactions between the vectors' basic groups and aromatic amino acid side chains in the bilayer spanning helices of membrane proteins.

S100A13 mediates the copper-dependent stress-induced release of IL-1alpha from both human U937 and murine NIH 3T3 cells

Copper is involved in the promotion of angiogenic and inflammatory events in vivo and, although recent clinical data has demonstrated the potential of Cu2+ chelators for the treatment of cancer in man, the mechanism for this activity remains unknown. We have previously demonstrated that the signal peptide-less angiogenic polypeptide, FGF1, uses intracellular Cu2+ to facilitate the formation of a multiprotein aggregate that enables the release of FGF1 in response to stress and that the expression of the precursor form but not the mature form of IL-1alpha represses the stress-induced export of FGF1 from NIH 3T3 cells. We report here that IL-1alpha is a Cu2+-binding protein and human U937 cells, like NIH 3T3 cells, release IL-1alpha in response to temperature stress in a Cu2+-dependent manner. We also report that the stress-induced export of IL-1alpha involves the intracellular association with the Cu2+-binding protein, S100A13. In addition, the expression of a S100A13 mutant lacking a sequence novel to this gene product functions as a dominant-negative repressor of IL-1alpha release, whereas the expression of wild-type S100A13 functions to eliminate the requirement for stress-induced transcription. Lastly, we present biophysical evidence that IL-1alpha may be endowed with molten globule character, which may facilitate its release through the plasma membrane. Because Cu2+ chelation also represses the release of FGF1, the ability of Cu2+ chelators to potentially serve as effective clinical anti-cancer agents may be related to their ability to limit the export of these proinflammatory and angiogenic signal peptide-less polypeptides into the extracellular compartment.

Copper chelation represses the vascular response to injury

The induction of an acute inflammatory response followed by the release of polypeptide cytokines and growth factors from peripheral blood monocytes has been implicated in mediating the response to vascular injury. Because the Cu2+-binding proteins IL-1alpha and fibroblast growth factor 1 are exported into the extracellular compartment in a stress-dependent manner by using intracellular Cu2+ to facilitate the formation of S100A13 heterotetrameric complexes and these signal peptideless polypeptides have been implicated as regulators of vascular injury in vivo, we examined the ability of Cu2+ chelation to repress neointimal thickening in response to injury. We observed that the oral administration of the Cu2+ chelator tetrathiomolybdate was able to reduce neointimal thickening after balloon injury in the rat. Interestingly, although immunohistochemical analysis of control neointimal sections exhibited prominent staining for MAC1, IL-1alpha, S100A13, and the acidic phospholipid phosphatidylserine, similar sections obtained from tetrathiomolybdate-treated animals did not. Further, adenoviral gene transfer of the IL-1 receptor antagonist during vascular injury also significantly reduced the area of neointimal thickening. Our data suggest that intracellular copper may be involved in mediating the response to injury in vivo by its ability to regulate the stress-induced release of IL-1alpha by using the nonclassical export mechanism employed by human peripheral blood mononuclear cells in vitro.

Structure-function analysis of invasion plasmid antigen C (IpaC) from Shigella flexneri

Shigella flexneri causes a self-limiting gastroenteritis in humans, characterized by severe localized inflammation and ulceration of the colonic mucosa. Shigellosis most often targets young children in underdeveloped countries. Invasion plasmid antigen C (IpaC) has been identified as the primary effector protein for Shigella invasion of epithelial cells. Although an initial model of IpaC function has been developed, no detailed structural information is available that could assist in a better understanding of the molecular basis for its interactions with the host cytoskeleton and phospholipid membrane. We have therefore initiated structural studies of IpaC, IpaC I', (residues 101-363 deleted), and IpaC Delta H (residues 63-170 deleted). The secondary and tertiary structure of the protein was examined as a function of temperature, employing circular dichroism and high resolution derivative absorbance techniques. ANS (8-anilino-1-napthalene sulfonic acid) was used to probe the exposure of the hydrophobic surfaces under different conditions. The interaction of IpaC and these mutants with a liposome model (liposomes with entrapped fluorescein) was also examined. Domain III (residues 261-363) was studied using linker-scanning mutagenesis. It was shown that domain III contains periodic, sequence-dependent activity, suggesting helical structure in this section of the protein. In addition to these structural studies, investigation into the actin nucleation properties of IpaC was conducted, and actin nucleation by IpaC and some of the mutants was exhibited. Structure-function relationships of IpaC are discussed.

Characterization of the structure and dynamics of a near-native equilibrium intermediate in the unfolding pathway of an all beta-barrel protein

The structure and dynamics of equilibrium intermediate in the unfolding pathway of the human acidic fibroblast growth factor (hFGF-1) are investigated using a variety of biophysical techniques including multidimensional NMR spectroscopy. Guanidinium hydrochloride (GdnHCl)-induced unfolding of hFGF-1 proceeds with the accumulation of a stable intermediate state. The transition from the intermediate state to the unfolded state(s) is cooperative without the accumulation of additional intermediate(s). The intermediate state induced maximally in 0.96 m GdnHCl is found to be obligatory in the folding/unfolding pathway of hFGF-1. Most of the native tertiary structure interactions are preserved in the intermediate state. (1)H-(15)N chemical shift perturbation data suggest that the residues in the C-terminal segment including those located in the beta-strands IX, X, and XI undergo the most discernible structural change(s) in the intermediate state in 0.96 m GdnHCl. hFGF-1 in the intermediate state (0.96 m GdnHCl) does not bind to its ligand, sucrose octasulfate. Limited proteolytic digestion experiments and hydrogen-deuterium exchange monitored by (15)N heteronuclear single quantum coherence (HSQC) spectra show that the conformational flexibility of the protein in the intermediate state is significantly higher than in the native conformation. (15)N spin relaxation experiments show that many residues located in beta-strands IX, X, and XI exhibit conformational motions in the micro- to millisecond time scale. Analysis of (15)N relaxation data in conjunction with the amide proton exchange kinetics suggests that residues in the beta-strands II, VIII, and XII possibly constitute the stability core of the protein in the near-native intermediate state.

The intracellular translocation of the components of the fibroblast growth factor 1 release complex precedes their assembly prior to export

The release of signal peptideless proteins occurs through nonclassical export pathways and the release of fibroblast growth factor (FGF)1 in response to cellular stress is well documented. Although biochemical evidence suggests that the formation of a multiprotein complex containing S100A13 and Synaptotagmin (Syt)1 is important for the release of FGF1, it is unclear where this intracellular complex is assembled. As a result, we employed real-time analysis using confocal fluorescence microscopy to study the spatio-temporal aspects of this nonclassical export pathway and demonstrate that heat shock stimulates the redistribution of FGF1 from a diffuse cytosolic pattern to a locale near the inner surface of the plasma membrane where it colocalized with S100A13 and Syt1. In addition, coexpression of dominant-negative mutant forms of S100A13 and Syt1, which both repress the release of FGF1, failed to inhibit the stress-induced peripheral redistribution of intracellular FGF1. However, amlexanox, a compound that is known to attenuate actin stress fiber formation and FGF1 release, was able to repress this process. These data suggest that the assembly of the intracellular complex involved in the release of FGF1 occurs near the inner surface of the plasma membrane and is dependent on the F-actin cytoskeleton.

Complex effects of molecular chaperones on the aggregation and refolding of fibroblast growth factor-1

Fibroblast growth factor one (FGF-1) exists in a molten globule (MG)-like state under physiological conditions (neutral pH, 37 degrees C). It has been proposed that this form of the protein may be involved in its atypical membrane transport properties. Macromolecular chaperones have been shown to bind to MG states of proteins as well as to be involved in protein membrane transport. We have therefore examined the effect of such proteins on the aggregation and refolding of FGF-1 to evaluate whether they might play a role in FGF-1 transport. The proposed chaperone alpha-crystallin was found to strongly inhibit the aggregation of the MG state of FGF-1. Curiously, two other proteins of similar size and charge (thyroglobulin and a monoclonal IgM immunoglobulin) with no previously reported chaperone properties were also found to have a related effect. In contrast, the chaperone GroEL/ES induced further aggregation of MG-like FGF-1 but had no effect on the native conformation. Both chaperones stimulated refolding to the native state (25 degrees C) but had no detectable effect when FGF-1 was refolded to the MG state (37 degrees C). This suggests that disordered intermediates are present in the folding pathways of the native and MG-like FGF conformations which differ from the MG-like state induced under physiological conditions. FGF-1 does, therefore, interact with molecular chaperones, although this may involve both the MG and the native states of the protein.

Identification and characterization of an equilibrium intermediate in the unfolding pathway of an all beta-barrel protein

The guanidinium hydrochloride (GdnHCl)-induced unfolding of an all beta-sheet protein, the human acidic fibroblast growth factor (hFGF-1), is studied using a variety of biophysical techniques including multidimensional NMR spectroscopy. The unfolding of hFGF-1 in GdnHCl is shown to involve the formation of a stable equilibrium intermediate. Size exclusion chromotagraphy using fast protein liquid chromatography shows that the intermediate accumulates maximally at 0.96 m GdnHCl. 1-Anilinonapthalene 8-sulfonate binding, one-dimensional (1)H NMR, and limited proteolytic digestion experiments suggest that the intermediate has characteristics resembling a molten globule state. Chemical shift perturbation and hydrogen-deuterium exchange monitored by (1)H-(15)N heteronuclear single quantum coherence spectra reveal that profound structural changes in the intermediate state (in 0.96 m GdnHCl) occur in the C-terminal, heparin binding region of the protein molecule. Additionally, results of the stopped flow fluorescence experiments suggest that the kinetic refolding of hFGF-1 proceeds through the accumulation of an intermediate at low concentrations of the denaturant. To our knowledge, the present study is the first report wherein an equilibrium intermediate is characterized in detail in an all beta-barrel protein.

Conformational lability of herpesvirus protein VP22

The herpesvirus protein VP22 traffics between cells, being exported from expressing cells in a non-Golgi-dependent manner and localizing in the nuclei of surrounding cells. This transport is retained in certain VP22 fusion proteins, making VP22 a candidate for use in macromolecular drug delivery. In an effort to understand the physical basis for this activity, we have initiated structural studies of VP22.C1, the C-terminal half of VP22, which possesses the full transport activity of the native protein. CD and Fourier transform infrared analyses indicate a secondary structure consisting of approximately 30% alpha-helix, 17% beta-sheet, and 51% disordered and turn structure. Unfolding studies conducted by CD, differential scanning calorimetry, and fluorescence reveal a series of discrete structural transitions in the range of 20-60 degrees C. CD and fluorescence studies of interactions between VP22.C1 and divalent cations and model polyanions indicate that Mg(2+), Zn(2+), oligonucleotides, and heparin interact with the protein, causing changes in secondary structure and thermal stability. Additionally, the interaction of VP22.C1 with model lipids was examined. Fluorescence titrations of the protein with trans-parinaric acid at various temperatures suggest the binding of one to two molecules of parinaric acid to VP22.C1 at temperatures >40 degrees C, suggesting the possibility of conformation dependent membrane interaction under physiological conditions.

Partly folded states of bovine carbonic anhydrase interact with zwitterionic and anionic lipid membranes

The acidic, partly folded states of bovine carbonic anhydrase II (BCAII) were used as an experimental system to study the interactions of partly denatured proteins with lipid membranes. The pH dependence of their interactions with palmitoyloleoyl phosphatidylcholine (POPC) and palmitoyloleoyl phosphatidylglycerol (POPG) membranes was studied. A filtration binding assay shows that acidic partly folded states of BCAII bind to POPC membranes. Fluorescence emission spectra from Trp residues of the bound protein are slightly shifted to shorter wavelength and can be quenched by a water-soluble quencher of fluorescence, indicating that the binding occurs without deep penetration of Trp residues into the membrane. The content of beta-structures of the protein in solution, as revealed by FT-IR spectroscopy, decreases in the partly folded states and the binding to POPC membrane occurs without further changes of secondary structure. In the presence of 0.1 M NaCl, a partly folded state self-aggregates and does not bind to POPC membrane. At acidic pH, BCAII binds to POPG membranes both at high and low ionic strength. The binding to the anionic lipid occurs with protein self-aggregation within the lipid-protein complexes and with changes in the secondary structure; large blue shifts in the fluorescence emission spectra and the decrease in the exposure to water-soluble acrylamide quencher of Trp fluorescence strongly suggest that BCAII penetrates the hydrocarbon domain in the POPG-protein complexes.

Potential use of non-classical pathways for the transport of macromolecular drugs

Since an increasing number of drug delivery strategies utilising proteins and peptides exhibiting 'non-classical' transport activities have been proposed, studies have begun to establish underlying functional relationships between different vectors. These attempts to find common factors have been hampered by a lack of biophysical data for the various potential protein and peptide transporters, as well as by the structural and functional diversity of the group as a whole. We describe the various types of vectors being considered for use and the preliminary therapeutic successes that have been achieved. Additionally, the various models that have been proposed for non-classical import and export are outlined and discussed in relation to therapeutic delivery. Possible future developments are also discussed.

Interactions of the HIV-1 fusion peptide with large unilamellar vesicles and monolayers. A cryo-TEM and spectroscopic study

We have examined the interaction of the human immunodeficiency virustype 1 fusion peptide (23 amino acid residues) and of a Trp-containing analog with vesicles composed of dioleoylphosphatidylcholine, dioleoylphosphatidylethanolamine and cholesterol (molar ratio, 1:1:1). Both the native and the Trp-substituted peptides bound the vesicles to the same extent and induced intervesicular lipid mixing with comparable efficiency. Infrared reflection-absorption spectroscopy data are compatible with the adoption by the peptide of a main beta-sheet structure in a cospread lipid/peptide monolayer. Cryo-transmission electron microscopy observations of peptide-treated vesicles reveal the existence of a peculiar morphology consisting of membrane tubular elongations protruding from single vesicles. Tryptophan fluorescence quenching by brominated phospholipids and by water-soluble acrylamide further indicated that the peptide penetrated into the acyl chain region closer to the interface rather than into the bilayer core. We conclude that the differential partition and shallow penetration of the fusion peptide into the outer monolayer of a surface-constrained bilayer may account for the detected morphological effects. Such single monolayer-restricted interaction and its structural consequences are compatible with specific predictions of current theories on viral fusion.

Transport of human growth hormone across Caco-2 cells with novel delivery agents: evidence for P-glycoprotein involvement

Emisphere Technologies, Inc. has synthesized a series of small molecules which have been shown to improve protein absorption through mucosal tissue. This enhancement is specific between protein and a particular delivery agent. Despite the specificity of interaction, the mechanism of enhanced tissue penetration is still unclear. The purpose of this work is to understand the enhancement mechanism(s) of these delivery agents by using Caco-2 cells as a model membrane. It was found that the bidirectional transepithelial fluxes of human growth hormone (hGH) in the presence of these delivery agents across human intestinal epithelial Caco-2 cell line showed marked asymmetry. Average permeability coefficient values obtained in the apical (AP) to basolateral (BL) direction were lower than those of the reverse (BL to AP) direction. On the other hand, the fluxes for human growth hormone alone were symmetric. When P-glycoprotein inhibitors were included in the transport medium, the permeability coefficient values of BL to AP direction were significantly decreased while the transport was increased in the reverse direction in the presence of delivery agents. P-glycoprotein inhibitors had no effect on the transport of human growth hormone alone. This study shows that human growth hormone alone can be transported across Caco-2 cells in very limited quantities by passive diffusion, but in the presence of delivery agents, human growth hormone can be effluxed in a P-glycoprotein-mediated fashion. This also indirectly shows that the human growth hormone has become more lipophilic in the presence of delivery agents.

Transcellular and lipophilic complex-enhanced intestinal absorption of human growth hormone

PURPOSE

To evaluate the transcellular mechanism of novel enhancers absorption enhancement of human growth hormone (hGH), by examining the involvement of a P-glycoprotein-like efflux system, changes in membrane fluidity, and membrane damage.

METHODS

Caco-2 cell monolayers were grown on Snapwell filter supports and placed in a side-by-side diffusion apparatus. Transport in both the apical to basolateral (AP to BL) and basolateral to apical (BL to AP) direction was measured at different temperatures and in the presence of potential inhibitors. Fluorescence anisotropy measurement was used to measure membrane fluidity. The fluorescence anisotropy of DPH- and TMA-DPH-labeled cell suspensions was measured at room temperature. LDH (a measure of cytosolic lactate dehydrogenase) leakage assay was used to evaluate cytotoxicity.

RESULTS

The bi-directional transepithelial fluxes of hGH in the presence of these novel enhancers across Caco-2 cells showed marked asymmetry. Average permeability coefficient values obtained in the apical to basolateral (AP to BL) direction were lower than those of the reverse (BL to AP) direction. On the other hand, the fluxes for hGH alone were symmetric. When P-gp-like efflux inhibitors were included in the transport medium, the permeability coefficient value of BL to AP direction was significantly decreased while the transport was increased in the reverse direction in the presence of novel enhancers. In addition, lowering the temperature to 25 degrees C completely eliminated the asymmetry of hGH transport in the presence of novel enhancers. It was also shown by fluorescence anisotropy that these novel enhancers alone only slightly increased membrane fluidity. On the other hand, upon addition of hGH to the novel enhancers, the cell membrane showed a dramatic change as compared to treatment with novel enhancers alone. The results from the LDH assay showed that the novel enhancers and/or hGH did not cause cell damage, at least up to 1 hour, and the damage seen at the 2 hour point is also much lower than other known enhancers.

CONCLUSIONS

This study shows that human growth hormone alone cannot be transported across Caco-2 cells, except in small quantities, by passive diffusion, but in the presence of novel enhancers, human growth hormone permeation is substantial. In addition, the asymmetry of transport of the complexed hGH appears to be due to a P-gp-like efflux system. Assuming that the present substrate specificity of the P-gp-like efflux system shows the same preference for hydrophobic molecules as p-gp, the present work also indirectly shows that human growth hormone has become more lipophilic in the presence of these novel enhancers. Furthermore, membrane fluidity data also supports the premise that these novel enhancers interact and stabilize hGH, to make them more hydrophobic and easier to be transported through cell membranes.

Reversible thermal denaturation of human FGF-1 induced by low concentrations of guanidine hydrochloride

Human acidic fibroblast growth factor (FGF-1) is a powerful mitogen and angiogenic factor with an apparent melting temperature (Tm) in the physiological range. FGF-1 is an example of a protein that is regulated, in part, by stability-based mechanisms. For example, the low Tm of FGF-1 has been postulated to play an important role in the unusual endoplasmic reticulum-independent secretion of this growth factor. Despite the close relationship between function and stability, accurate thermodynamic parameters of unfolding for FGF-1 have been unavailable, presumably due to effects of irreversible thermal denaturation. Here we report the determination of thermodynamic parameters of unfolding (DeltaH, DeltaG, and DeltaCp) for FGF-1 using differential scanning calorimetry (DSC). The thermal denaturation is demonstrated to be two-state and reversible upon the addition of low concentrations of added guanidine hydrochloride (GuHCl). DeltaG values from the DSC studies are in excellent agreement with values from isothermal GuHCl denaturation monitored by fluorescence and circular dichroism (CD) spectroscopy. Furthermore, the results indicate that irreversible denaturation is closely associated with the formation of an unfolding intermediate. GuHCl appears to promote reversible two-state denaturation by initially preventing aggregation of this unfolding intermediate, and at subsequently higher concentrations, by preventing formation of the intermediate.

The extravesicular domain of synaptotagmin-1 is released with the latent fibroblast growth factor-1 homodimer in response to heat shock

The heparin-binding fibroblast growth factor (FGF) prototypes lack a classical signal sequence, yet their presence is required in the extracellular compartment for the activation of cell-surface receptor-dependent signaling. Early studies with FGF-1 demonstrated its presence in bovine brain as a novel high molecular weight complex, and subsequent studies identified a second heparin-binding protein that co-purified with FGF-1. Polypeptide sequence analysis revealed that this heparin-binding protein corresponded to the extravesicular domain of bovine synaptotagmin (Syn)-1, a transmembrane component of synaptic vesicles involved in the regulation of organelle traffic. Since FGF-1 is released in response to heat shock as a mitogenically inactive Cys-30 homodimer, we sought to determine whether this heparin-binding protein was involved in the release of FGF-1. We report that a proteolytic fragment of the extravesicular domain of Syn-1 is associated with FGF-1 in the extracellular compartment of FGF-1-transfected NIH 3T3 cells following temperature stress. By using heparin-Sepharose affinity to discriminate between the monomer and homodimer forms of FGF-1 and resolution by conventional and limited denaturant gel shift immunoblot analysis, it was possible to identify FGF-1 and Syn-1 as potential components of a denaturant- and reducing agent-sensitive extracellular complex. It was also possible to demonstrate that the expression of an antisense-Syn-1 gene represses the release of FGF-1 in response to heat shock. These data indicate that FGF-1 may be able to utilize the cytosolic face of conventional exocytotic vesicles to traffic to the inner surface of the plasma membrane where it may gain access to the extracellular compartment as a complex with Syn-1.

Synaptotagmin-1 is required for fibroblast growth factor-1 release

By using p65 synaptotagmin-1 and fibroblast growth factor (FGF)-1:beta-galactosidase (beta-gal) NIH 3T3 cell co-transfectants, we demonstrate that a proteolytic fragment consisting of the extravesicular domain of synaptotagmin-1 is released into the extracellular compartment in response to temperature stress with similar kinetics and pharmacological properties as FGF-1:beta-gal. Using a deletion mutant that lacks 95 amino acids from the extravesicular domain of synaptotagmin-1, neither synaptotagmin-1 nor FGF-1:beta-gal are able to access the stress-induced release pathway. Furthermore, the p40 extravesicular fragment of synaptotagmin-1 is constitutively released in p40 synaptotagmin-1 NIH 3T3 cell transfectants, and this release is potentiated when the cells are subjected to temperature stress. These data demonstrate that the p40 fragment derived from synaptotagmin-1 is able to utilize the FGF-1 non-classical exocytotic pathway and that the release of FGF-1 is dependent on synaptotagmin-1.

Interaction of recombinant interleukin-2 with liposomal bilayers

Liposomes have been employed as a delivery system for recombinant interleukin-2 (rIL-2) in cancer immunotherapy. In this study the effects of the rIL-2-bilayer interaction on protein structure were investigated. It was shown that rIL-2 adsorbs to liposomal membranes when added to preformed liposomes. Polarized fluorescence decay studies showed that the single tryptophan in "native" rIL-2 has a relatively large motional freedom, although iodide quenching of this residue's fluorescence was relatively ineffective. However, adsorption of rIL-2 to liposomes alters this situation dramatically- fluorescence intensity increased 2-fold and the residue became more susceptible to iodide quenching. At the same time, the average fluorescence lifetime of the fluorophore is extended. Interestingly, circular dichroism studies showed that no major conformational changes occurred in rIL-2's secondary structure upon adsorption. These observations support the hypothesis that intramolecular quenching takes place in the native rIL-2 molecule, which is abrogated upon adsorption to the liposomal membrane, resulting in a higher fluorescence intensity. Fluorescence anisotropy decay experiments indicate that the protein forms self-aggregates under the low-ionic strength conditions used, confirming the earlier observations on the tendency of the protein to precipitate in salt-containing media.

SKN-1 domain folding and basic region monomer stabilization upon DNA binding

The SKN-1 transcription factor specifies early embryonic cell fates in Caenorhabditis elegans. SKN-1 binds DNA at high affinity as a monomer, by means of a basic region like those of basic-leucine zipper (bZIP) proteins, which bind DNA only as dimers. We have investigated how the SKN-1 DNA-binding domain (the Skn domain) promotes stable binding of a basic region monomer to DNA. A flexible arm at the Skn domain amino terminus binds in the minor groove, but a support segment adjacent to the carboxy-terminal basic region can independently stabilize basic region-DNA binding. Off DNA, the basic region and arm are unfolded and, surprisingly, the support segment forms a molten globule of four alpha-helices. On binding DNA, the Skn domain adopts a tertiary structure in which the basic region helix extends directly from a support segment alpha-helix, which is required for binding. The remainder of the support segment anchors this uninterrupted helix on DNA, but leaves the basic region exposed in the major groove. This is similar to how the bZIP basic region extends from the leucine zipper, indicating that positioning and cooperative stability provided by helix extension are conserved mechanisms that promote binding of basic regions to DNA.