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

S100 A16 promotes the progression of osteosarcoma by activating the PI3 K/AKT signaling pathway through ANXA2

Osteosarcoma is a common primary malignant bone tumor. S100A16 gene was reported to highly expressed in several tumor tissues while the relationship between S100A16 and osteosarcoma remains less well-understood. This study aimed to investigate the expression characteristics of S100A16 in osteosarcoma and the mechanism by which it promotes osteosarcoma progression. Firstly, by analyzing databases and assessing mRNA and protein level, we found that the expression of S100A16 was significantly promoted in osteosarcoma, as compared with normal tissue. Then transfection techniques were employed to upregulate and downregulate S100A16 in osteosarcoma cells, the results demonstrated that S100A16 can increase osteosarcoma cell viability, migration and invasion capacities, while decline osteosarcoma cell apoptosis. GSEA (gene set enrichment analysis) revealed that increased expression of S100A16 was enriched in the PI3K/AKT pathway. Cellular experiments showed that the S100A16 promoted osteosarcoma progression by activating the PI3K/AKT signaling pathway, and upregulated expression of ANXA2, a crucial protein in occurrence and development of tumors. We also found that overexpression of ANXA2 can restore the decreased levels of p-PI3K and p-AKT induced by S100A16 inhibition, which indicated that S100A16 stimulates PI3K/AKT pathway activation via ANXA2. To sum up, S100A16 can promotes osteosarcoma progression by activating the PI3K/AKT signaling pathway through ANXA2, suggesting that the S100A16/ANXA2 axis may represent a novel therapeutic target for osteosarcoma.

Construction of an immune-related gene prognostic model with experimental validation and analysis of immune cell infiltration in lung adenocarcinoma

There is a correlation between tumors and immunity with the degree of immune cell infiltration in tumors being closely related to tumor growth and progression. Therefore, the present study identified immune-related prognostic genes and evaluated the immune infiltration level in lung adenocarcinoma (LUAD). This study performed Kyoto Encyclopedia of Genes and Genomes, Gene Ontology, and Gene Set Enrichment Analysis (GSEA) enrichment analyses on differential immune-associated genes. A risk model was created and validated using six immune-related prognostic genes. Reverse transcription-quantitative PCR was used to assess the prognostic gene expression in non-small cell lung cancer cells. Immune cell infiltration in LUAD was analyzed using the CIBERSORT method. Single sample GSEA was used to compare Tumor Immune Dysfunction and Exclusion (TIDE) scores between high and low-risk groups and to assess the activation of thirteen immune-related pathways. Multifactor Cox proportional hazards model analysis identified six prognostic risk genes (S100A16, FURIN, FGF2, LGR4, TNFRSF11A and VIPR1) to construct a risk model. The survival and receiver operating characteristic curves indicated that patients with higher risk scores had lower overall survival rates. The expression levels of prognostic genes S100A16, FURIN, LGR4, TNFRSF11A and VIPR1 were significantly increased in LUAD. B cells naive, plasma cells, T cells CD4 memory activated, T cells follicular helper, T cells regulatory, NK cells activated, macrophages M1, macrophages M2, and Dendritic cells resting cells showed elevated expression in LUAD. The prognostic genes were differentially associated with individual immune cells. Immune-related function scores, such as those for antigen presenting cell (APC) co-stimulation, APC co-inhibition, check-point, Cytolytic-activity, chemokine receptor, parainflammation, major histocompatibility complex-class-I, type-I-IFN-reponse and T-cell-co-inhibition, were higher in the high-risk group compared with the low-risk group. Furthermore, the TIDE score of the high-risk group was significantly lower than the low-risk group. This immune-related gene prognostic model has the potential to predict the prognosis of LUAD patients, supporting the development of a personalized clinical diagnosis and treatment plan.

An Update on S100A16 in Human Cancer

S100A16 is a member of the S100 protein family. S100A16 is expressed in a variety of human tissues, although at varying levels. S100A16 expression is especially high in tissues rich in epithelial cells. mRNA and protein levels of S100A16 have been reported to be differentially expressed in the majority of human cancers. Functionally, S100A16 has been linked to several aspects of tumorigenesis, for example, cell proliferation, differentiation, migration, invasion, and epithelial-mesenchymal transition (EMT). Accordingly, S100A16 has been suggested to have both tumour-promoting and suppressive roles in human cancers. S100A16-mediated cellular functions are suggested to be mediated by the regulation of various signaling pathways/proteins including EMT-related proteins E-cadherin and Vimentin, PI3K-AKT, p53, MMP1-1, MMP-2, MMP-9, JNK/p38, etc. In addition to the functional roles, expression of S100A16 has been suggested to have prognostic potential in various cancer types. The aims of this review are to summarise the expression profile, identify common molecular partners and functional roles, and explore the prognostic potential of S100A16 in human cancers.

New progress with calcium-binding protein S100A16 in digestive system disease

INTRODUCTION

This review summarizes and analyzes the abnormal expression and mechanism of S100A16 in digestive system diseases, which is expected to provide new ideas and methods for adjuvant treatment and prognosis evaluation of digestive system diseases.

AREAS COVERED

Based on original publications found in database systems (PubMed, Cochrane), we introduce the mechanism and research progress of S100A16 in digestive system tumors, inflammatory bowel disease and fatty liver.

EXPERT OPINION

S100A16 is closely related to the proliferation, migration, and invasion of digestive system tumor cells. Further, it plays an important role in inflammatory bowel disease and fatty liver.

RAGE Inhibitors for Targeted Therapy of Cancer: A Comprehensive Review

The receptor for advanced glycation end products (RAGE) is a member of the immunoglobulin family that is overexpressed in several cancers. RAGE is highly expressed in the lung, and its expression increases proportionally at the site of inflammation. This receptor can bind a variety of ligands, including advanced glycation end products, high mobility group box 1, S100 proteins, adhesion molecules, complement components, advanced lipoxidation end products, lipopolysaccharides, and other molecules that mediate cellular responses related to acute and chronic inflammation. RAGE serves as an important node for the initiation and stimulation of cell stress and growth signaling mechanisms that promote carcinogenesis, tumor propagation, and metastatic potential. In this review, we discuss different aspects of RAGE and its prominent ligands implicated in cancer pathogenesis and describe current findings that provide insights into the significant role played by RAGE in cancer. Cancer development can be hindered by inhibiting the interaction of RAGE with its ligands, and this could provide an effective strategy for cancer treatment.

S100a16 deficiency prevents hepatic stellate cells activation and liver fibrosis via inhibiting CXCR4 expression

INTRODUCTION

Liver fibrosis caused by hepatic stellate cells (HSCs) activation is implicated in the pathogenesis of liver diseases. To date, there has been no effective intervention means for this process. S100 proteins are calcium-binding proteins that regulate cell growth and differentiation. This study aimed to investigate whether S100A16 induces HSCs activation and participates in liver fibrosis progression.

METHODS

HSCs were isolated, and the relationship between S100A16 expression and HSCs activation was studied. S100a16 knockdown and transgenic mice were generated and subjected to HSCs activation and liver fibrosis stimulated by different models. Clinical samples were collected for further confirmation. Alterations in gene expression in HSCs were investigated, using transcriptome sequencing to determine the underlying mechanisms.

RESULTS

We observed increased S100A16 levels during HSCs activation. Genetic silencing of S100a16 prevented HSCs activation in vitro. Furthermore, S100a16 silencing exhibited obvious protective effects against HSCs activation and fibrosis progression in mice. In contrast, S100a16 transgenic mice exhibited spontaneous liver fibrosis. S100A16 was also upregulated in the HSCs of patients with fibrotic liver diseases. RNA sequencing revealed that C-X-C motif chemokine receptor 4 (Cxcr4) gene was a crucial regulator of S100A16 induction during HSCs activation. Mechanistically, S100A16 bound to P53 to induce its degradation; this augmented CXCR4 expression to activate ERK 1/2 and AKT signaling, which then promoted HSCs activation and liver fibrosis.

CONCLUSIONS

These data indicate that S100a16 deficiency prevents liver fibrosis by inhibiting Cxcr4 expression. Targeting S100A16 may provide insight into the pathogenesis of liver fibrosis and pave way for the design of novel clinical therapeutic strategies.

A Novel S100 Family-Based Signature Associated with Prognosis and Immune Microenvironment in Glioma

Background

Glioma is the most common central nervous system (CNS) cancer with a short survival period and a poor prognosis. The S100 family gene, comprising 25 members, relates to diverse biological processes of human malignancies. Nonetheless, the significance of S100 genes in predicting the prognosis of glioma remains largely unclear. We aimed to build an S100 family-based signature for glioma prognosis.

Methods

We downloaded 665 and 313 glioma patients, respectively, from The Cancer Genome Atlas (TCGA) and Chinese Glioma Genome Atlas (CGGA) database with RNAseq data and clinical information. This study established a prognostic signature based on the S100 family genes through multivariate COX and LASSO regression. The Kaplan-Meier curve was plotted to compare overall survival (OS) among groups, whereas Receiver Operating Characteristic (ROC) analysis was performed to evaluate model accuracy. A representative gene S100B was further verified by in vitro experiments.

Results

An S100 family-based signature comprising 5 genes was constructed to predict the glioma that stratified TCGA-derived cases as a low- or high-risk group, whereas the significance of prognosis was verified based on CGGA-derived cases. Kaplan-Meier analysis revealed that the high-risk group was associated with the dismal prognosis. Furthermore, the S100 family-based signature was proved to be closely related to immune microenvironment. In vitro analysis showed S100B gene in the signature promoted glioblastoma (GBM) cell proliferation and migration.

Conclusions

We constructed and verified a novel S100 family-based signature associated with tumor immune microenvironment (TIME), which may shed novel light on the glioma diagnosis and treatment.

ADAMTS19 Suppresses Cell Migration and Invasion by Targeting S100A16 via the NF-κB Pathway in Human Gastric Cancer

A Disintegrin and Metalloproteinase with Thrombospondin motifs 19 (ADAMTS19) has been reported to participate in the pathogenesis of solid cancers. However, its role in gastric cancer (GC) remains undocumented. Using immunohistochemistry (IHC) staining and quantitative real-time polymerase chain reaction (qRT-PCR) on GC tissues and adjacent normal tissues, we found that ADAMTS19 was downregulated in GC tissues (IHC: p < 0.001; qRT-PCR: p = 0.017). Further investigation revealed that ADAMTS19 correlated with distant metastasis (p = 0.008) and perineural invasion (p = 0.018) and that patients with low ADAMTS19 had worse overall survival (p = 0.021). Gain- and loss-of-function assays showed that ADAMTS19 suppressed cell migration and invasion in vitro. Using bioinformatics analysis and co-immunoprecipitation, immunofluorescence, and dual-luciferase reporter gene assays, we confirmed that ADAMTS19 binds with cytoplasm P65, decreasing the nucleus phosphorylation of P65, a crucial transcription factor in the nuclear factor kappa-B (NF-κB) pathway, thereby downregulating S100 calcium-binding protein A16 (S100A16) expression. S100A16 acted as the downstream of ADAMTS19, reversing the suppression of cell migration and invasion by ADAMTS19 in vitro. A combination of ADAMTS19 and S100A16 expression provided the optimal prognostic indicator for GC. Patients with ADAMTS19^high-S100A16^low had better overall survival than ADAMTS19^low-S100A16^high patients (p = 0.006). These results suggest that ADAMTS19 suppresses cell migration and invasion by targeting S100A16 via the NF-κB pathway and that ADAMTS19 and S100A16 are potential metastasis and survival biomarkers for GC.

S100A16 suppresses the proliferation, migration and invasion of colorectal cancer cells in part via the JNK/p38 MAPK pathway

S100 calcium binding protein A16 (S100A16) is the most recent member of the S100 calcium-binding protein family. The function of S100A16 has been associated with various types of cancer; however, its role in colorectal cancer (CRC) remains unknown. Therefore, the aim of the present study was to investigate the role of S100A16 in CRC progression. The Oncomine dataset used in the current study revealed that the expression of S100A16 was decreased in CRC compared with normal colorectal tissues. Similar results were also determined via immunohistochemistry. In addition, a negative association was identified between S100A16 expression and the prognosis of patients with CRC. Further functional experiments revealed that S100A16 knockdown promoted the proliferation, migration and invasion of HCT116 and SW480 cells, and vice versa in Lovo cells. Epithelial-mesenchymal transition (EMT) was promoted and the JNK/p38 MAPK pathway was activated in HCT116 cells following S100A16 knockdown, as determined via western blotting. Furthermore, S100A16 silencing promoted the migration and invasion of cells. EMT was also reversed when cells were treated with the JNK inhibitor (SP600125) or the p38 inhibitor (SB203580). In summary, the results of the present study demonstrated that S100A16 suppressed the proliferation, migration and invasion of CRC cells partially via the JNK/p38 MAPK signalling pathway and subsequent EMT mediation.

S100A16 promotes metastasis and progression of pancreatic cancer through FGF19-mediated AKT and ERK1/2 pathways

The S100 protein family genes play a crucial role in multiple stages of tumorigenesis and progression. Most of S100 genes are located at chromosome locus 1q21, which is a region frequently rearranged in cancers. Here, we examined the expression of the S100 family genes in paired pancreatic ductal adenocarcinoma (PDAC) samples and further validated the expression of S100A16 by immunohistochemistry staining. We found that S100A16 is significantly upregulated in clinical PDAC samples. However, its roles in PDAC are still unclear. We next demonstrated that S100A16 promotes PDAC cell proliferation, migration, invasion, and metastasis both in vitro and in vivo. Knockdown of S100A16 induces PDAC cell cycle arrest in the G2/M phase and apoptosis. Furthermore, we also demonstrated that S100A16 promotes PDAC cell proliferation, migration, and invasion via AKT and ERK1/2 signaling in a fibroblast growth factor 19 (FGF19)-dependent manner. Taken together, our results reveal that S100A16 is overexpressed in PDAC and promotes PDAC progression through FGF19-mediated AKT and ERK1/2 signaling, suggesting that S100A16 may be a promising therapeutic target for PDAC. S100A16 was upregulated in PDAC and associated with prognosis of PDAC patients. S100A16 regulates apoptosis and the cell cycle of pancreatic cancer cells. S100A16 promotes the progression of pancreatic cancer by AKT-ERK1/2 signaling. S100A16 may be a promising therapeutic target for PDAC.

Identification of Prognostic Immune-Related Genes by Integrating mRNA Expression and Methylation in Lung Adenocarcinoma

Background

There is plenty of evidence showing that immune-related genes (IRGs) and epigenetic modifications play important roles in the biological process of cancer. The purpose of this study is to establish novel IRG prognostic markers by integrating mRNA expression and methylation in lung adenocarcinoma (LUAD).

Methods and Results

The transcriptome profiling data and the RNA-seq data of LUAD with the corresponding clinical information of 543 LUAD cases were downloaded from The Cancer Genome Atlas (TCGA) database, which were analyzed by univariate Cox proportional regression and multivariate Cox proportional regression to develop an independent prognostic signature. On the basis of this signature, we could divide LUAD patients into the high-risk, medium-risk, and low-risk groups. Further survival analyses demonstrated that high-risk patients had significantly shorter overall survival (OS) than low-risk patients. The signature, which contains 8 IRGs (S100A16, FGF2, IGKV4-1, CX3CR1, INHA, ANGPTL4, TNFRSF11A, and VIPR1), was also validated by data from the Gene Expression Omnibus (GEO) database. We also conducted analyses of methylation levels of the relevant IRGs and their CpG sites. Meanwhile, their associations with prognosis were examined and validated by the GEO database, revealing that the methylation levels of INHA, S100A16, the CpG site cg23851011, and the CpG site cg06552037 may be used as the potential regulators for the treatment of LUAD.

Conclusion

Collectively, INHA, S100A16, the CpG site cg23851011, and the CpG site cg06552037 are promising biomarkers for monitoring the outcomes of LUAD.

Reviewing the Crystal Structure of S100Z and Other Members of the S100 Family: Implications in Calcium-Regulated Quaternary Structure

This paper takes the cue from the previously solved crystal structure of human apo-S100Z and compares it with that of the calcium-bound S100Z from zebrafish in order to stress, for this particular S100, the significant role of the presence of calcium in promoting supramolecular assemblies with likely biological meaning. This consideration is then expanded through a wider review on analogous situations concerning all other S100s for which there is crystallographic o biochemical evidence of how the presence of calcium promotes the formation of quaternary complexes.The paper also deals with some considerations on the quality of the crystals obtained for the solved members of this family and on the need for experimental phasing for solving some of the structures where the good general sequence homology among the members of the family would have suggested molecular replacement (MR) as the easiest way to solve them.These considerations, along with the PCA analysis carried out on all the known S100s, further demonstrate that calcium plays a fundamental role in triggering quaternary structure formation for several members of this family of proteins.

S100A16 is a prognostic marker for colorectal cancer

BACKGROUND

S100 is a superfamily of calcium-binding proteins that regulate multiple biological processes and are involved in many diseases. S100A16 has recently been identified to be involved in several cancers such as bladder cancer, lung cancer, and oral squamous cell carcinoma. However, the role of S100A16 expression in the colorectal cancer (CRC) has not been investigated.

METHODS

S100A16 protein expression was detected by immunohistochemistry in 296 cases of CRC. Kaplan-Meier survival analysis and Cox regression analysis were performed to evaluate the prognostic significance of S100A16.

RESULT

The results showed that the overall survival (OS) of patients with low membrane S100A16 expression was significantly shorter than patients with high expression (P < 0.05). Chi-square analysis showed that S100A16 expression had a positive correlation with tumor grade (P = 0.02). Multivariate analysis identified membrane S100A16 expression as an independent prognostic marker for OS in CRC patients. (P < 0.05). Univariate analysis showed no significant association between cytoplasmic/nuclear S100A16 expression and OS.

CONCLUSION

Membrane S100A16 is associated with the prognosis of CRC patients, indicating that S100A16 may be a potential prognostic biomarker and therapeutic target for CRC.

Solving the crystal structure of human calcium-free S100Z: the siege and conquer of one of the last S100 family strongholds

The X-ray structure of human apo-S100Z has been solved and compared with that of the zebrafish calcium-bound S100Z, which is the closest in sequence. Human apo-S100A12, which shows only 43% sequence identity to human S100Z, has been used as template model to solve the crystallographic phase problem. Although a significant buried surface area between the two physiological dimers is present in the asymmetric unit of human apo-S100Z, the protein does not form the superhelical arrangement in the crystal as observed for the zebrafish calcium-bound S100Z and human calcium-bound S100A4. These findings further demonstrate that calcium plays a fundamental role in triggering quaternary structure formation in several S100s. Solving the X-ray structure of human apo-S100Z by standard molecular replacement procedures turned out to be a challenge and required trying different models and different software tools among which only one was successful. The model that allowed structure solution was that with one of the lowest sequence identity with the target protein among the S100 family in the apo state. Based on the previously solved zebrafish holo-S100Z, a putative human holo-S100Z structure has been then calculated through homology modeling; the differences between the experimental human apo and calculated holo structure have been compared to those existing for other members of the family.

Multiple Evolutionary Origins of Ubiquitous Cu2+ and Zn2+ Binding in the S100 Protein Family

The S100 proteins are a large family of signaling proteins that play critical roles in biology and disease. Many S100 proteins bind Zn2+, Cu2+, and/or Mn2+ as part of their biological functions; however, the evolutionary origins of binding remain obscure. One key question is whether divalent transition metal binding is ancestral, or instead arose independently on multiple lineages. To tackle this question, we combined phylogenetics with biophysical characterization of modern S100 proteins. We demonstrate an earlier origin for established S100 subfamilies than previously believed, and reveal that transition metal binding is widely distributed across the tree. Using isothermal titration calorimetry, we found that Cu2+ and Zn2+ binding are common features of the family: the full breadth of human S100 paralogs-as well as two early-branching S100 proteins found in the tunicate Oikopleura dioica-bind these metals with μM affinity and stoichiometries ranging from 1:1 to 3:1 (metal:protein). While binding is consistent across the tree, structural responses to binding are quite variable. Further, mutational analysis and structural modeling revealed that transition metal binding occurs at different sites in different S100 proteins. This is consistent with multiple origins of transition metal binding over the evolution of this protein family. Our work reveals an evolutionary pattern in which the overall phenotype of binding is a constant feature of S100 proteins, even while the site and mechanism of binding is evolutionarily labile.

Two structural motifs within canonical EF-hand calcium-binding domains identify five different classes of calcium buffers and sensors

Proteins with EF-hand calcium-binding motifs are essential for many cellular processes, but are also associated with cancer, autism, cardiac arrhythmias, and Alzheimer's, skeletal muscle and neuronal diseases. Functionally, all EF-hand proteins are divided into two groups: (1) calcium sensors, which function to translate the signal to various responses; and (2) calcium buffers, which control the level of free Ca²⁺ ions in the cytoplasm. The borderline between the two groups is not clear, and many proteins cannot be described as definitive buffers or sensors. Here, we describe two highly-conserved structural motifs found in all known different families of the EF-hand proteins. The two motifs provide a supporting scaffold for the DxDxDG calcium binding loop and contribute to the hydrophobic core of the EF hand domain. The motifs allow more precise identification of calcium buffers and calcium sensors. Based on the characteristics of the two motifs, we could classify individual EF-hand domains into five groups: (1) Open static; (2) Closed static; (3) Local dynamic; (4) Dynamic; and (5) Local static EF-hand domains.

Up-regulation of S100A16 expression promotes epithelial-mesenchymal transition via Notch1 pathway in breast cancer

BACKGROUND

Our previous studies demonstrated that S100A16 promotes adipogenesis and is involved in weight gain attenuation induced by dietary calcium. Till now, the function of S100A16 in the breast cancer remains to be elucidated.

RESULTS

In this study, we observed that S100A16 was expressed in higher levels in human breast cancer tissues compared with paired adjacent non-cancerous tissues. Further examination showed that overexpression of S100A16 in MCF-7 cells could increase cell proliferation and colony formation. One major mechanistic change was that S100A16 was able to up-regulate the transcription factors Notch1, ZEB1, and ZEB2, which had the capacities to directly repress the expression of epithelial markers E-cadherin and β-catenin but increase mesenchymal markers N-cadherin and vimentin, a characterized phenotype of epithelial-mensenchymal transition (EMT). In addition to display with morphologic change, migration and invasion were increased in S100A16 over-expressed MCF-7 cells. Importantly, knockdown of Notch1 by specific siRNA could reverse the EMT induced by S100A16 overexpression, which confirmed that Notch1 played a critical role in the process of EMT induced by S100A16.

CONCLUSIONS

All together, our data indicated that S100A16 had a potential function to regulate some embryonic transcription factors to promote EMT in breast cancer cells which may be an important target site for the therapy of breast cancer.

S100A14 interacts with S100A16 and regulates its expression in human cancer cells

Both S100A14 and S100A16 are members of the multifunctional S100 protein family. Formation of homo/heterodimers is considered to be one of the major mechanisms for S100 proteins to execute their diverse cellular functions. By employing a classical Yeast two hybrid (Y-2 H) screen, we identified S100A16 as the single interaction partner of S100A14. This interaction was verified by co-immunoprecipitation, double indirect immunofluorescence and double immunostaining in specimens of oral squamous cell carcinoma and normal oral mucosa. The functional significance of this interaction was examined by employing retroviral mediated over-expression and knock-down of these proteins in several cancer cell-lines. Over-expression and knock-down of S100A14 led to concomitant up- and down-regulation of S100A16 protein in the cell-lines examined. However, there was no up-regulation of S100A16 mRNA upon S100A14 over-expression, indicating that modulation of S100A16 expression was not due to enhanced transcriptional activity but possibly by post-transcriptional regulation. In contrary, over-expression of S100A16 was associated neither with the up-regulation of S100A14 mRNA nor its protein, suggesting a unidirectional regulation between S100A14 and S100A16. Cellular treatment with protein synthesis inhibitor cycloheximide demonstrated a time-dependent intracellular degradation of both S100A16 and S100A14 proteins. Additionally, regulation of S100A16 and S100A14 degradation was found to be independent of the classical proteasomal and lysosomal pathways of protein degradation. Further studies will therefore be necessary to understand the functional significance of this interaction and the mechanisms on how S100A14 is involved in the regulation of S100A16 expression.

Solution structure and dynamics of human S100A14

Human S100A14 is a member of the EF-hand calcium-binding protein family that has only recently been described in terms of its functional and pathological properties. The protein is overexpressed in a variety of tumor cells and it has been shown to trigger receptor for advanced glycation end products (RAGE)-dependent signaling in cell cultures. The solution structure of homodimeric S100A14 in the apo state has been solved at physiological temperature. It is shown that the protein does not bind calcium(II) ions and exhibits a "semi-open" conformation that thus represents the physiological structure of the S100A14. The lack of two ligands in the canonical EF-hand calcium(II)-binding site explains the negligible affinity for calcium(II) in solution, and the exposed cysteines and histidine account for the observed precipitation in the presence of zinc(II) or copper(II) ions.