Three-dimensional solution structure of a unique S100 protein

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.

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.

S100A13-C2A binary complex structure-a key component in the acidic fibroblast growth factor for the non-classical pathway

Fibroblast growth factors (FGFs) are key regulators of cell proliferation, differentiation, tumor-induced angiogenesis and migration. FGFs are essential for early embryonic development, organ formation and angiogenesis. They play important roles in tumor formation, inflammation, wound healing and restenosis. The biological effects of FGFs are mediated through the activation of the four transmembrane phosphotyrosine kinase receptors (FGFRs) in the presence of heparin sulfate proteoglycans (HSPGs) and therefore require the release of FGFs into the extracellular space. However, FGF-1 lacks the signal peptide required for the releasing of these proteins through the classical endoplasmic reticulum (ER)-Golgi secretary pathway. Maciag et al. demonstrated that FGF-1 is exported through a non-classical release pathway involving the formation of a specific multiprotein complex [M. Landriscina, R. Soldi, C. Bagala, I. Micucci, S. Bellum, F. Tarantini, I. Prudovsky, T. Maciag, S100A13 participates in the release of fibroblast growth factor 1 in response to heat shock in vitro, J. Biol. Chem. 276 (2001) 22544-22552; C.M. Carreira, T.M. LaVallee, F. Tarantini, A. Jackson, J.T. Lathrop, B. Hampton, W.H. Burgess, T. Maciag, S100A13 is involved in the regulation of fibroblast growth factor-1 and p40 synaptotagmin-1 release in vitro, J. Biol. Chem. 273 (1998) 22224-22231; T.M. LaValle, F. Tarantini, S. Gamble, C.M. Carreira, A. Jackson, T. Maciag, Synaptotagmin-1 is required for fibroblast growth factor-1 release, J. Biol. Chem. 273 (1998) 22217-22223; C. Bagalá, V. Kolev, A. Mandinova, R. Soldi, C. Mouta, I. Graziani, I, Prudovsky, T. Maciag, The alternative translation of synaptotagmin 1 mediates the non-classical release of FGF1, Biochem. Biophys. Res. Commun. 310 (2003) 1041-1047]. The protein constituents of this complex include FGF-1, S100A13 (a Ca(2+)-binding protein), and the p40 form of synaptotagmin 1 (Syt1). To understand the molecular events in the FGF-1 releasing pathway, we have studied the interactions of S100A13 with C2A by (1)H-(15)N HSQC titration and 3D-filtered NOESY experiments. We characterized the binary complex structure of S100A13-C2A by using a variety of multi-dimensional NMR experiments. This complex acts as a template for FGF-1 dimerization and multiprotein complex formation.

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.

S100A13-lipid interactions-role in the non-classical release of the acidic fibroblast growth factor

S100A13 is a 98-amino acid, calcium binding protein. It is known to participate in the non-classical secretion of signal peptide-less proteins, such as the acidic fibroblast growth factor. In this study, we investigate the lipid binding properties of S10013 using a number of biophysical techniques, including multidimensional NMR spectroscopy. Isothermal titration calorimetry and steady state fluorescence experiments show that apoS100A13 exhibits preferential binding to small unilamelar vesicles of l-phosphatidyl serine (pS). In comparison, Ca2+-bound S100A13 is observed to bind weakly to unilamelar vesicles (SUVs) of pS. Equilibrium thermal unfolding and limited trypsin digestion analysis reveal that apoS100A13 is significantly destabilized upon binding to SUVs of pS. Results of the far UV circular dichroism and ANS (8-anilino-1-napthalene sufonate) binding experiments indicate a subtle conformational change resulting in the increase in the solvent-accessible hydrophobic surface in the protein. Availability of the solvent-exposed hydrophobic surface(s) in apoS10013 facilitates its interaction with the lipid vesicles. Our data suggest that Ca2+ binding dictates the membrane binding affinity of S100A13. Based on the results of this study, a model describing the sequence of molecular events that possibly can occur during the non-classical secretion of FGF-1 is presented.

Crystal structure study on human S100A13 at 2.0 A resolution

The S100 protein family is the largest group of calcium-binding protein families, which consists of at least 25 members. S100A13, which is widely expressed in a variety of tissues, is a unique member of the S100 protein family. Previous reports showed that S100A13 might be involved in the stress-induced release of some signal peptide-less proteins (such as FGF-1 and IL-1alpha) and also associated with inflammatory functions. It was also reported that S100A13 is a new angiogenesis marker. Here we report the crystal structure of the Ca(2+)-bound form of S100A13 at 2.0 A resolution. S100A13 is a homodimer with four EF-hand motifs in an asymmetric unit, displaying a folding pattern similar to other S100 members. However, S100A13 has the unique structural feature with all alpha-helices being amphiphilic, which was not found in other members of S100s. We propose that this characteristic structure of S100A13 might be related to its ability to mediate the release of FGF-1 and IL-1alpha.

Pathologies involving the S100 proteins and RAGE

The S100 proteins are exclusively expressed in vertebrates and are the largest subgroup within the superfamily of EF-hand Ca2(+)-binding proteins Generally, S100 proteins are organized as tight homodimers (some as heterodimers). Each subunit is composed of a C-terminal, 'canonical' EF-hand, common to all EF-hand proteins, and a N-terminal, 'pseudo' EF-hand, characteristic of S100 proteins. Upon Ca2(+)-binding, the C-terminal EF-hand undergoes a large conformational change resulting in the exposure of a hydrophobic surface responsible for target binding A unique feature of this protein family is that some members are secreted from cells upon stimulation, exerting cytokine- and chemokine-like extracellular activities via the Receptor for Advanced Glycation Endproducts, RAGE. Recently, larger assemblies of some S100 proteins (hexamers, tetramers, octamers) have been also observed and are suggested to be the active extracellular species required for receptor binding and activation through receptor multimerization Most S100 genes are located in a gene cluster on human chromosome 1q21, a region frequently rearranged in human cancer The functional diversification of S100 proteins is achieved by their specific cell- and tissue-expression patterns, structural variations, different metal ion binding properties (Ca2+, Zn2+ and Cu2+) as well as their ability to form homo-, hetero- and oligomeric assemblies Here, we review the most recent developments focussing on the biological functions of the S100 proteins and we discuss the presently available S100-specific mouse models and their possible use as human disease models In addition, the S100-RAGE interaction and the activation of various cellular pathways will be discussed. Finally, the close association of S100 proteins with cardiomyopathy, cancer, inflammation and brain diseases is summarized as well as their use in diagnosis and their potential as drug targets to improve therapies in the future.

Survey of the year 2005: literature on applications of isothermal titration calorimetry

Isothermal titration calorimetry (ITC) can provide a full thermodynamic characterization of an interaction. Its usage does not suffer from constraints of molecular size, shape or chemical constitution. Neither is there any need for chemical modification or attachment to solid support. This ease of use has made it an invaluable instrumental resource and led to its appearance in many laboratories. Despite this, the value of the thermodynamic parameterization has, only quite recently, become widely appreciated. Although our understanding of the correlation between thermodynamic data and structural details continues to be somewhat naïve, a large number of publications have begun to improve the situation. In this overview of the literature for 2005, we have attempted to highlight works of interest and novelty. Furthermore, we draw attention to those works which we feel have provided a route to better analysis and increased our ability to understand the meaning of thermodynamic change on binding.

Copper binding affinity of S100A13, a key component of the FGF-1 nonclassical copper-dependent release complex

S100A13 is a member of the S100 protein family that is involved in the copper-dependent nonclassical secretion of signal peptideless proteins fibroblast growth factor 1 and interleukin 1 lpha. In this study, we investigate the effects of interplay of Cu2+ and Ca2+ on the structure of S100A13 using a variety of biophysical techniques, including multi-dimensional NMR spectroscopy. Results of the isothermal titration calorimetry experiments show that S100A13 can bind independently to both Ca2+ and Cu2+ with almost equal affinity (Kd in the micromolar range). Terbium binding and isothermal titration calorimetry data reveal that two atoms of Cu2+/Ca2+ bind per subunit of S100A13. Results of the thermal denaturation experiments monitored by far-ultraviolet circular dichroism, limited trypsin digestion, and hydrogen-deuterium exchange (using 1H-15N heteronuclear single quantum coherence spectra) reveal that Ca2+ and Cu2+ have opposite effects on the stability of S100A13. Binding of Ca2+ stabilizes the protein, but the stability of the protein is observed to decrease upon binding to Cu2+. 1H-15N chemical shift perturbation experiments indicate that S100A13 can bind simultaneously to both Ca2+ and Cu2+ and the binding of the metal ions is not mutually exclusive. The results of this study suggest that the Cu2+-binding affinity of S100A13 is important for the formation of the FGF-1 homodimer and the subsequent secretion of the signal peptideless growth factor through the nonclassical release pathway.

Evidence for serum-deprivation-induced co-release of FGF-1 and S100A13 from astrocytes

Since fibroblast growth factor (FGF)-1 lacks conventional amino-terminal signal peptide essential for endoplasmic reticulum (ER)-Golgi pathway, the mode of release of this polypeptide remains to be fully understood. We attempted to characterize the non-classical (non-vesicular) mode of FGF-1 release in the analyses using immunocytochemistry and immunoblot of conditioned medium (CM) from astrocytes. FGF-1 was completely released from astrocytes upon serum-deprivation stress in a Brefeldin A-insensitive manner. In the immunoprecipitation study using anti-FGF-1 IgG, S100A13 was identified to be the major protein co-eluted with FGF-1. The interaction between GST-FGF-1 and Strep-tag II-S100A13 was found to be Ca(2+)-sensitive, and to require the C-terminal 11 amino acid peptide sequence of S100A13. The overexpression of Delta88-98 mutant of S100A13 selectively inhibited the serum-deprivation stress-induced release of FGF-1, but not the release of S100A13 mutant from C6 glioma cells. However, amlexanox, anti-allergic drug whose target is S100A13, completely inhibited the stress-induced release of FGF-1 as well as S100A13. The stress-induced release of both proteins was also abolished by BAPTA-AM, an intracellular Ca(2+) chelating agent. The serum-deprivation caused Ca(2+) spikes in omega-conotoxin GVIA and thapsigargin-sensitive manner. All these results suggest that S100A13 is a cargo molecule for the serum-deprivation stress-induced non-classical release of FGF-1, and that its driving force of protein-protein interaction and release is possibly mediated by Ca(2+)-induced Ca(2+) release (CICR) coupled to N-type Ca(2+) channel activity.