Calcium-driven changes in S100A11 structure revealed

Zebrafish do not have calprotectin

The protein heterodimer calprotectin and its component proteins, S100A8 and S100A9, play important antibacterial and proinflammatory roles in the mammalian innate immune response. Gaining mechanistic insights into the regulation and biological function of calprotectin will help facilitate patient diagnostics and therapy and further our understanding of the host-microbe interface. Recent literature has identified zebrafish s100a10b as zebrafish calprotectin based on sequence similarity, genomic context, and transcriptional upregulation during the immune response to bacterial infections. The field would benefit from expanding the breadth of calprotectin studies into a zebrafish innate immunity model. Here, we carefully evaluated the possibility that zebrafish possess a calprotectin. We found that zebrafish do not possess an ortholog of mammalian S100A8 or S100A9. We then identified four zebrafish s100 proteins- including s100a10b-that are expressed in immune cells and upregulated during the immune response. We recombinantly expressed and purified these proteins and measured the antimicrobial and proinflammatory characteristics. We found that none of the zebrafish proteins exhibited activity comparable to mammalian calprotectin. Our work demonstrates conclusively that zebrafish have no ortholog of calprotectin, and the most plausible candidate proteins have not convergently evolved similar functions.

Interaction of S100A13 with C2 domain of receptor for advanced glycation end products (RAGE)

S100A13 is involved in several key biological functions like angiogenesis, tumor formation and cell apoptosis. It is a homodimeric protein that belongs to the S100 protein family. S100A13 is co-expressed with acidic fibroblast growth factor (FGF1) and interleukin-1α which are key angiogenesis inducers. The S100 proteins have been shown to be involved in several cellular functions such as calcium homeostasis, cell growth and differentiation dynamic of cytoskeleton. Its biological functions are mainly mediated through the receptor for advanced glycation end products (RAGE) signaling. RAGE is involved in inflammatory processes and is associated with diabetic complications, tumor outgrowth, and neurodegenerative disorders. RAGE induces cellular signaling upon binding of different ligands, such as S100 proteins, glycated proteins, and HMGB1. RAGE signaling is complex, and it depends on the cell type and concentration of the ligand. Molecular level interactions of RAGE and S100 proteins are useful to understand the RAGE signaling diversity. In this report we focus on the molecular level interactions of S100A13 and RAGE C2 domain. The binding between RAGE C2 and S100A13 is moderately strong (Kd~1.3μM). We have solved the solution structure of the S100A13-RAGE C2 complex and pronounce the interface regions in S100A13-RAGE C2 complex which are helpful for drug development of RAGE induced diseases.

The IL1alpha-S100A13 heterotetrameric complex structure: a component in the non-classical pathway for interleukin 1alpha secretion

Interleukin 1α (IL1α) plays an important role in several key biological functions, such as angiogenesis, cell proliferation, and tumor growth in several types of cancer. IL1α is a potent cytokine that induces a wide spectrum of immunological and inflammatory activities. The biological effects of IL1α are mediated through the activation of transmembrane receptors (IL1Rs) and therefore require the release of the protein into the extracellular space. IL1α is exported through a non-classical release pathway involving the formation of a specific multiprotein complex, which includes IL1α and S100A13. Because IL1α plays an important role in cell proliferation and angiogenesis, inhibiting the formation of the IL1α-S100A13 complex would be an effective strategy to inhibit a wide range of cancers. To understand the molecular events in the IL1α release pathway, we studied the structure of the IL1α-S100A13 tetrameric complex, which is the key complex formed during the non-classical pathway of IL1α release.

Structural characterization of human S100A16, a low-affinity calcium binder

The homodimeric structure of human S100A16 in the apo state has been obtained both in the solid state and in solution, resulting in good agreement between the structures with the exception of two loop regions. The homodimeric solution structure of human S100A16 was also calculated in the calcium(II)-bound form. Differently from most S100 proteins, the conformational rearrangement upon calcium binding is minor. This characteristic is likely to be related to the weak binding affinity of the protein for the calcium(II) ions. In turn, this is ascribed to the lack of the glutamate residue at the end of the S100-specific N-domain binding site, which in most S100 proteins provides two important side chain oxygen atoms as calcium(II) ligands. Furthermore, the presence of hydrophobic interactions stronger than for other S100 proteins, present in the closed form of S100A16 between the third and fourth helices, likely make the closed structure of the second EF-hand particularly stable, so even upon calcium(II) binding such a conformation is not disrupted.

The heterohexameric complex structure, a component in the non-classical pathway for fibroblast growth factor 1 (FGF1) secretion

Fibroblast growth factors (FGFs) are key regulators of cell proliferation, tumor-induced angiogenesis, and migration. FGFs are essential for early embryonic development, organ formation, and angiogenesis. FGF1 also plays an important role in inflammation, wound healing, and restenosis. The biological effects of FGF1 are mediated through the activation of the four transmembrane phosphotyrosine kinase fibroblast growth factor receptors in the presence of heparin sulfate proteoglycans and, therefore, require the release of the protein into the extracellular space. FGF1 is exported through a non-classical release pathway involving the formation of a specific multiprotein complex. The protein constituents of this complex include FGF1, S100A13, and the p40 form of synaptotagmin 1 (Syt1). Because FGF1 plays an important role in tumor formation, it is clear that preventing the formation of the multiprotein complex would be an effective strategy to inhibit a wide range of cancers. To understand the molecular events in the FGF1 release pathway, we studied the FGF1-S100A13 tetrameric and FGF1-S100A13-C2A hexameric complex structures, which are both complexes possibly formed during the non-classical pathway of FGF1 release.

Solution structure and dynamics of S100A5 in the apo and Ca2+-bound states

S100A5 is a calcium binding protein of the S100 family, with one canonical and one S100-specific EF-hand motif per subunit. Although its function is still unknown, it has recently been reported to be one of the S100 proteins able to interact with the receptor for advanced glycation end products. The homodimeric solution structures of S100A5 in both the apo and the calcium(II)-loaded forms have been obtained, and show a conformational rearrangement upon calcium binding. This rearrangement involves, in particular, the hinge loop connecting the N-terminal and the C-terminal EF-hand domains, the reorientation of helix III with respect to helix IV, as common to several S100 proteins, and the elongation of helix IV. The details of the structural changes are important because they must be related to the different functions, still largely unknown, of the different members of the S100 family. For the first time for a full-length S100 protein, relaxation measurements were performed on both the apo and the calcium-bound forms. A quite large mobility was observed in the hinge loop, which is not quenched in the calcium form. The structural differences resulting upon calcium binding change the global shape and the distribution of hydrophobic and charged residues of the S100A5 homodimer in a modest but significantly different manner with respect to the closest homologues S100A4 and S100A6.

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.