The crystal structure of zebrafish S100Z: implications for calcium-promoted S100 protein oligomerisation.

The S100 family, with about 20 members in humans, is composed of EF-hand calcium-regulated proteins and is linked to a range of serious human diseases, including cancer and autoimmune and neurological disorders. The oldest S100 family members are found in teleosts (bony fish). The zebrafish, Danio rerio, was suggested as a promising model system for in vivo studies on S100 family functions, and we chose to investigate zebrafish S100Z as the closest homologue of the metastasis-promoting human S100A4. Here, we report the first crystal structure of an S100 protein from this organism, the calcium-bound state of S100Z to 2.03 Å resolution. Crystal packing suggests higher-order oligomerisation of S100Z dimers, with a tetramerisation interface very similar to, but even more extensive than, that reported for S100A4. The interactions are primarily through the C-terminal αIV helices from adjacent dimers in an antiparallel orientation. Structural comparisons between known S100 multimeric assemblies together with analysis of calcium-driven changes to the dimerisation cores suggest a mechanism for calcium-promoted oligomerisation of S100 proteins.

The role of zinc in the S100 proteins: insights from the X-ray structures.

We here aim to summarise the present knowledge on zinc binding by S100 proteins. While the importance of modulation of the function of the S100 family of EF-hand proteins by calcium is well established, a substantial proportion is also regulated by zinc or copper. Indeed regulation by zinc in addition to calcium was suggested almost as soon as the first S100 protein was discovered and has been confirmed for many family members by numerous experiments. For the first, "His-Zn", group, zinc-binding sites composed of three histidines and an aspartic acid were first proposed based on sequence comparisons and later confirmed by structural studies. A second, "Cys-Zn", group lacks such well-defined zinc-binding motifs and for these cysteines were suggested as the main zinc ligands. There is no three-dimensional structure for a Cys-Zn S100 in the presence of zinc. However, analysis of their sequences together with their X-ray structures in the absence of zinc suggests the possibility of two zinc-binding sites: a conserved site with a degree of similarity to those of the His-Zn group and a less-defined site with a Cys interdimer-binding motif. Some S100 protein-mediated events, such as signalling in the extracellular space, where the levels of calcium are already high, are most unlikely to be calcium regulated. Therefore, a broader knowledge of the role of zinc in the functioning of the S100 proteins will add significantly to the understanding how they propagate their signals.

The crystal structures of human S100A12 in apo form and in complex with zinc: new insights into S100A12 oligomerisation.

The functions of the members of the S100 family of EF-hand proteins are modulated by calcium and, in a number of cases, by zinc or copper. One such protein is S100A12, which is implicated in inflammation and host-parasite responses. Previously, we reported the structures of human S100A12 in both low (dimeric) and high (hexameric) calcium forms and, in addition, that of a complex with copper and calcium. Here we report the crystal structures of the metal-free apo form of human S100A12 at 1.77 A resolution and of the zinc complex in two crystal forms (P2(1)2(1)2(1) and F222) to 1.88 A and 1.73 A resolution, respectively. These are the first structures of a zinc-only complex of an S100 protein to be determined. The zinc complex structure shows significant differences from those of both calcium-loaded and apo-S100A12 structures, and comparisons suggest an explanation for the zinc-induced 1500-fold increase in calcium affinity. In addition, the new structures provide insight into the role of zinc-calcium interplay in the transition of S100A12 from a dimer through a tetramer to a hexamer. The role of both zinc and calcium in target binding by S100A12 during host-parasite responses is confirmed by experiments with paramyosin from the tropical parasites Onchocerca volvulus and Brugia malayi.

Both Ca2+ and Zn2+ are essential for S100A12 protein oligomerization and function.

BACKGROUND: Human S100A12 is a member of the S100 family of EF-hand calcium-modulated proteins that are associated with many diseases including cancer, chronic inflammation and neurological disorders. S100A12 is an important factor in host/parasite defenses and in the inflammatory response. Like several other S100 proteins, it binds zinc and copper in addition to calcium. Mechanisms of zinc regulation have been proposed for a number of S100 proteins e.g. S100B, S100A2, S100A7, S100A8/9. The interaction of S100 proteins with their targets is strongly dependent on cellular microenvironment. RESULTS: The aim of the study was to explore the factors that influence S100A12 oligomerization and target interaction. A comprehensive series of biochemical and biophysical experiments indicated that changes in the concentration of calcium and zinc led to changes in the oligomeric state of S100A12. Surface plasmon resonance confirmed that the presence of both calcium and zinc is essential for the interaction of S100A12 with one of its extracellular targets, RAGE--the Receptor for Advanced Glycation End products. By using a single-molecule approach we have shown that the presence of zinc in tissue culture medium favors both the oligomerization of exogenous S100A12 protein and its interaction with targets on the cell surface. CONCLUSION: We have shown that oligomerization and target recognition by S100A12 is regulated by both zinc and calcium. Our present work highlighted the potential role of calcium-binding S100 proteins in zinc metabolism and, in particular, the role of S100A12 in the cross talk between zinc and calcium in cell signaling.

Amyloid fibrils of mammalian prion protein induce axonal degeneration in NTERA2-derived terminally differentiated neurons.

Defects in axonal transport and synaptic dysfunctions are associated with early stages of several neurodegenerative diseases including Alzheimer's, Huntington's, Parkinson's, and prion diseases. Here, we tested the effect of full-length mammalian prion protein (rPrP) converted into three conformationally different isoforms to induce pathological changes regarded as early subcellular hallmarks of prion disease. We employed human embryonal teratocarcinoma NTERA2 cells (NT2) that were terminally differentiated into neuronal and glial cells and co-cultured together. We found that rPrP fibrils but not alpha-rPrP or soluble beta-sheet rich oligomers caused degeneration of neuronal processes. Degeneration of processes was accompanied by a collapse of microtubules and aggregation of cytoskeletal proteins, formation of neuritic beads, and a dramatic change in localization of synaptophysin. Our studies demonstrated the utility of NT2 cells as valuable human model system for elucidating subcellular events of prion pathogenesis, and supported the emerging hypothesis that defects in neuronal transport and synaptic abnormalities are early pathological hallmarks associated with prion diseases.

Hexameric calgranulin C (S100A12) binds to the receptor for advanced glycated end products (RAGE) using symmetric hydrophobic target-binding patches.

Calgranulin C (S100A12) is a member of the S100 family of proteins that undergoes a conformational change upon calcium binding allowing them to interact with target molecules and initiate biological responses; one such target is the receptor for advanced glycation products (RAGE). The RAGE-calgranulin C interaction mediates a pro-inflammatory response to cellular stress and can contribute to the pathogenesis of inflammatory lesions. The soluble extracellular part of RAGE (sRAGE) was shown to decrease the inflammation response possibly by scavenging RAGE-activating ligands. Here, by using high resolution NMR spectroscopy, we identified the sRAGE-calgranulin C interaction surface. Ca2+ binding creates two symmetric hydrophobic surfaces on Ca2+-calgranulin C that allow calgranulin C to bind to the C-type immunoglobulin domain of RAGE. Apo-calgranulin C also binds to sRAGE using a completely different surface and with substantially lower affinity, thus underscoring the role of Ca2+ binding to S100 proteins as a molecular switch. By using native gel electrophoresis, chromatography, and fluorescence spectroscopy, we established that sRAGE forms tetramers that bind to hexamers of Ca2+-calgranulin C. This arrangement creates a large platform for effectively transmitting RAGE-dependent signals from extracellular S100 proteins to the cytoplasmic signaling complexes.

Probing the conformation of the prion protein within a single amyloid fibril using a novel immunoconformational assay.

The coexistence of multiple strains or subtypes of the disease-related isoform of prion protein (PrP) in natural isolates, together with the observed conformational heterogeneity of PrP amyloid fibrils generated in vitro, indicates the importance of probing the conformation of single particles within heterogeneous samples. Using an array of PrP-specific antibodies, we report the development of a novel immunoconformational assay. Uniquely, application of this new technology allows the conformation of multimeric PrP within a single fibril or particle to be probed without pretreatment of the sample with proteinase K. Using amyloid fibrils prepared from full-length recombinant PrP, we demonstrated the utility of this assay to define (i) PrP regions that are surface-exposed or buried, (ii) the susceptibility of defined PrP regions to GdnHCl-induced denaturation, and (iii) the conformational heterogeneity of PrP fibrils as measured for either the entire fibrillar population or for individual fibrils. Specifically, PrP regions 159-174 and 224-230 were shown to be buried and were the most resistant to denaturation. The 132-156 segment of PrP was found to be cryptic under native conditions and solvent-exposed under partially denaturing conditions, whereas the region 95-105 was solvent-accessible regardless of the solvent conditions. Remarkably, a subfraction of fibrils showed immunoreactivity to PrPSc-specific antibodies designated as IgGs 89-112 and 136-158. The immunoreactivity of the conformational epitopes was reduced upon exposure to partially denaturing conditions. Unexpectedly, PrPSc -specific antibodies revealed conformational polymorphisms even within individual fibrils. Our studies provide valuable new insight into fibrillar substructure and offer a new tool for probing the conformation of single PrP fibrils.

Human DNA topoisomerase I: An anticancer drug target present in human sarcomas.

New anticancer drugs targeting DNA topoisomerase I (topo I) are showing activity against human sarcomas. Laboratory studies have indicated that cells responsive to topo I-targeted drugs have elevated levels of topo I, require active DNA replication, and may require a functional apoptotic pathway. In this study, we evaluated these potential markers of topo I-targeted drug sensitivity in 55 cases of human sarcoma (42 high grade, 4 intermediate grade, and 9 low grade). By immunohistochemical staining, we observed elevated topo I expression in 20 of 55 neoplasms (36%). Immunohistochemical staining for the proliferation marker DNA topoisomerase II-alpha (topo II-alpha), showed that 15 of 55 neoplasms (27%) had topo II-alpha indices >50, indicating a large number of actively cycling tumor cells. Abnormal p53 expression was observed in 19 of the 55 cases (35%). None of the cases were interpreted as positive for ALK-1. To complement our immunohistochemical staining of topo I, we isolated functionally active topo I from extracts of a human sarcoma. These isolates demonstrated that sarcoma topo I is sensitive to topo I-targeted anticancer drugs. Of the 55 cases of human sarcoma, 7 (13%) had high levels of topo I, a large number of cycling tumor cells, and normal p53 expression. These are the molecular parameters that might suggest responsiveness to drugs targeting topo I.