Crystal structure of calcium-free human sorcin: a member of the penta-EF-hand protein family

Solution NMR backbone resonance assignment of the full-length resistance-related calcium-binding protein Sorcin

Sorcin is a penta-EF hand calcium-binding protein that confers multidrug resistance in cancer cells. It regulates cellular Ca2+ homeostasis by interacting with calcium channels such as Ryanodine receptor 2 and Sarcoplasmic/endoplasmic reticulum Ca2+-ATPase in a calcium-dependent manner. The crystal structure of the Sorcin has been determined in both calcium-free and calcium-bound states to understand calcium-binding induced conformational change. However, due to its flexibility, most of the N-terminal domain is invisible in these crystal structures. Here we report the 1H, 13C, and 15N backbone resonance assignments of full-length Sorcin in the calcium-free state using solution NMR. The protein secondary structure was predicted based on the assigned backbone chemical shifts using TALOS+ and CSI 3.0. Our backbone resonance assignment of the full-length Sorcin provides a foundation for future NMR spectroscopic studies to uncover the mechanism of Ca2+ sensing by Sorcin.

Uncovering allostery and regulation in SORCIN through molecular dynamics simulations

Soluble resistance-related calcium-binding protein or Sorcin is an allosteric, calcium-binding Penta-EF hand (PEF) family protein implicated in multi-drug resistant cancers. Sorcin is known to bind chemotherapeutic molecules such as Doxorubicin. This study uses in-silico molecular dynamics simulations to explore the dynamics and allosteric behavior of Sorcin in the context of Ca2+ uptake and Doxorubicin binding. The results show that Ca2+ binding induces large, but reversible conformational changes in the Sorcin structure which manifest as rigid body reorientations that preserve the local secondary structure. A reciprocal allosteric handshake centered around the EF5 hand is found to be key in Sorcin dimer formation and stabilization. Binding of Doxorubicin results in rearrangement of allosteric communities which disrupts long-range allosteric information transfer from the N-terminal domain to the middle lobe. However, this binding does not result in secondary structure destabilization. Sorcin does not appear to have a distinct Ca2+ activated mode of Doxorubicin binding.Communicated by Ramaswamy H. Sarma.

Sorcin regulate pyroptosis by interacting with NLRP3 inflammasomes to facilitate the progression of hepatocellular carcinoma

A high recurrence rate and easy metastasis are two prominent clinical features of hepatocellular carcinoma (HCC), which is also the most common cause of cancer-related death. However, the molecular pathogenesis of HCC remains unclear. Soluble resistance-related calcium-binding protein (Sorcin) is highly expressed in a variety of tumor cell lines and multidrug-resistant cell lines and participates in the malignant progression of tumors by regulating apoptosis. Pyroptosis is also a form of programmed cell death that plays a crucial role in exerting tumor suppression function and evoking anti-tumor immune responses. However, there is no consensus that Sorcin promotes HCC progression by regulating pyroptosis. Our study manifested that Sorcin was considerably upregulated, whereas pyroptosis-associated proteins were significantly decreased in HCC tissues and cells. Sorcin silencing attenuated the proliferation, migration, and invasion of HCC cells. Knockdown of Sorcin activates pyroptosis, and overexpression of Sorcin inhibits pyroptosis, yet has no significant effect on apoptosis, ferroptosis, and autophagy in HCC cells. Furthermore, coimmunoprecipitation and immunofluorescence assays revealed that Sorcin interacted with NLRP3 inflammasome to regulate pyroptosis in HCC cells. Then, the NLRP3 inhibitor MCC950 inhibited the activation of Sorcin knockdown-induced pyroptosis and reversed the effect of Sorcin silencing-induced weakening of malignant biological behavior in HCC. Similarly, suppression of Caspase-1 reversed the inhibitory effect of Sorcin knockdown on the malignant progression of HCC via knockdown of Caspase-1 or the inhibitor VX765. Consistent with the in vitro results, the nude mouse experiment showed that Sorcin knockdown inhibited the growth of HCC by activating pyroptosis, while Caspase-1 knockdown partially restored the growth inhibition caused by Sorcin knockdown. Collectively, high Sorcin expression in HCC negatively regulates pyroptosis by interacting with the NLRP3 inflammasome to promote HCC proliferation, migration, and invasion. The results of this study provide a scientific basis for Sorcin as a new biomarker and potential therapeutic target for HCC.

Sorcin a Potential Molecular Target for Cancer Therapy

Sorcin (Soluble resistance related calcium binding protein) is a small soluble penta EF family (PEF) of calcium (Ca2+) binding protein (22,000 Da). It has been reported to play crucial roles in the regulation of calcium homeostasis, apoptosis, vesicle trafficking, cancer development, and multidrug resistance (MDR). Overexpression of sorcin has been reported to be associated with different cancers such as breast cancer, colorectal cancer, gastric cancer, leukemia, lung cancer, nasopharyngeal cancer, ovarian cancer, etc. Essentially, expression of sorcin has been found to be elevated in cancer cells as compared to normal cells, indicating that it has prominent role in cancer. Moreover, sorcin was found to be the regulator of various proteins that has an association with carcinogenesis including NF-κB, STAT3, Akt, ERK1/2, VEGF, MMPs, caspases, etc. Sorcin was also found to regulate apoptosis, as silencing of the same resulted in increased levels of proapoptotic genes and induced mitochondrial apoptotic pathway in cancer. Interestingly, mutations in the sorcin gene have been closely linked with poor overall survival in bladder cancer, brain lower-grade glioma, glioblastoma, glioblastoma multiforme, kidney renal clear cell carcinoma, and stomach adenocarcinoma. Additionally, overexpression of sorcin was also found to induce MDR against different chemotherapeutic drugs. All these findings mark the importance of sorcin in cancer development and MDR. Therefore, there is urgent need to explore the functional mechanism of sorcin and to analyze whether silencing of sorcin would able to chemosensitize MDR cells. The current review summarizes the structure, expression, and functions of sorcin and its importance in the regulation of various malignancies and MDR.

Sorcin Predicts Poor Prognosis and Promotes Metastasis by Facilitating Epithelial-mesenchymal Transition in Hepatocellular Carcinoma

Metastasis-associated recurrence is the main cause for the poor prognosis of hepatocellular carcinoma (HCC). However, the detailed molecular mechanisms underlying HCC metastasis remain elusive. Though some data indicated the oncogenic role of Sorcin in tumors, the prognostic value and biological role of Sorcin in HCC is still unknown. In this study, it demonstrated that Sorcin expression levels were significantly upregulated in HCC tumor tissues compared with matched adjacent nontumorous liver tissues and normal liver tissues, and such expression level correlated with HCC metastasis. High Sorcin expression was significantly correlated with aggressive clinicopathological characteristics such as multiple tumor nodules, high Edmondson-Steiner grade, microvascular invasion, advanced TNM stage and advanced BCLC stage (all P < 0.05). HCC patients with high Sorcin expression had both shorter survival and higher recurrence than those with low Sorcin expression (all P < 0.05). Sorcin expression was an independent and significant risk factor for survival and recurrence of HCC patients. Results of functional experiments showed that Sorcin could promote HCC cell proliferation, migration, and invasion in vitro, and facilitate HCC growth and metastasis in vivo. Mechanistically, Sorcin exerted its role by activating extracellular signal-regulated kinase (ERK) pathway and promoted metastasis by facilitating epithelial-mesenchymal transition (EMT) in HCC.

Structural insights into Ca(2+)-activated long-range allosteric channel gating of RyR1

Ryanodine receptors (RyRs) are a class of giant ion channels with molecular mass over 2.2 mega-Daltons. These channels mediate calcium signaling in a variety of cells. Since more than 80% of the RyR protein is folded into the cytoplasmic assembly and the remaining residues form the transmembrane domain, it has been hypothesized that the activation and regulation of RyR channels occur through an as yet uncharacterized long-range allosteric mechanism. Here we report the characterization of a Ca²⁺-activated open-state RyR1 structure by cryo-electron microscopy. The structure has an overall resolution of 4.9 Å and a resolution of 4.2 Å for the core region. In comparison with the previously determined apo/closed-state structure, we observed long-range allosteric gating of the channel upon Ca²⁺ activation. In-depth structural analyses elucidated a novel channel-gating mechanism and a novel ion selectivity mechanism of RyR1. Our work not only provides structural insights into the molecular mechanisms of channel gating and regulation of RyRs, but also sheds light on structural basis for channel-gating and ion selectivity mechanisms for the six-transmembrane-helix cation channel family.

Nonstructural 5A Protein of Hepatitis C Virus Regulates Soluble Resistance-Related Calcium-Binding Protein Activity for Viral Propagation

Hepatitis C virus (HCV) is a major cause of chronic liver disease and is highly dependent on cellular proteins for virus propagation. To identify the cellular factors involved in HCV propagation, we recently performed protein microarray assays using the HCV nonstructural 5A (NS5A) protein as a probe. Of 90 cellular protein candidates, we selected the soluble resistance-related calcium-binding protein (sorcin) for further characterization. Sorcin is a calcium-binding protein and is highly expressed in certain cancer cells. We verified that NS5A interacted with sorcin through domain I of NS5A, and phosphorylation of the threonine residue 155 of sorcin played a crucial role in protein interaction. Small interfering RNA (siRNA)-mediated knockdown of sorcin impaired HCV propagation. Silencing of sorcin expression resulted in a decrease of HCV assembly without affecting HCV RNA and protein levels. We further demonstrated that polo-like kinase 1 (PLK1)-mediated phosphorylation of sorcin was increased by NS5A. We showed that both phosphorylation and calcium-binding activity of sorcin were required for HCV propagation. These data indicate that HCV modulates sorcin activity via NS5A protein for its own propagation.

IMPORTANCE

Sorcin is a calcium-binding protein and regulates intracellular calcium homeostasis. HCV NS5A interacts with sorcin, and phosphorylation of sorcin is required for protein interaction. Gene silencing of sorcin impaired HCV propagation at the assembly step of the HCV life cycle. Sorcin is phosphorylated by PLK1 via protein interaction. We showed that sorcin interacted with both NS5A and PLK1, and PLK1-mediated phosphorylation of sorcin was increased by NS5A. Moreover, calcium-binding activity of sorcin played a crucial role in HCV propagation. These data provide evidence that HCV regulates host calcium metabolism for virus propagation, and thus manipulation of sorcin activity may represent a novel therapeutic target for HCV.

Structural basis of Sorcin-mediated calcium-dependent signal transduction

Sorcin is an essential penta-EF hand calcium binding protein, able to confer the multi-drug resistance phenotype to drug-sensitive cancer cells and to reduce Endoplasmic Reticulum stress and cell death. Sorcin silencing blocks cell cycle progression in mitosis and induces cell death by triggering apoptosis. Sorcin participates in the modulation of calcium homeostasis and in calcium-dependent cell signalling in normal and cancer cells. The molecular basis of Sorcin action is yet unknown. The X-ray structures of Sorcin in the apo (apoSor) and in calcium bound form (CaSor) reveal the structural basis of Sorcin action: calcium binding to the EF1-3 hands promotes a large conformational change, involving a movement of the long D-helix joining the EF1-EF2 sub-domain to EF3 and the opening of EF1. This movement promotes the exposure of a hydrophobic pocket, which can accommodate in CaSor the portion of its N-terminal domain displaying the consensus binding motif identified by phage display experiments. This domain inhibits the interaction of sorcin with PDCD6, a protein that carries the Sorcin consensus motif, co-localizes with Sorcin in the perinuclear region of the cell and in the midbody and is involved in the onset of apoptosis.

Overexpression of sorcin in multidrug-resistant human breast cancer

Sorcin is a soluble resistance-related calcium-binding protein, which is expressed in normal mammalian tissues, such as the liver, lungs and heart. It has been observed to be elevated in a number of cancer types, including colorectal, gastric and breast cancer. Its upregulation is usually associated with the development of chemotherapeutic drug resistance. The aim of this study was to evaluate the sorcin expression levels in human serum samples of breast cancer subjects at various stages, and subsequently compare the outcome of neoadjuvant chemotherapy when the sorcin levels fluctuated. In total, 50 subjects were recruited from patients who were admitted to Yantai Yuhunagding Hospital (Yantai, China) and diagnosed with breast cancer. Blood samples prior to and following chemotherapy were assessed using two-dimensional gel electrophoresis (2-DE) and western blot analysis. The 2-DE analysis of the serum samples revealed that sorcin was upregulated in six out of 29 neoadjuvant chemotherapy (NAC)-sensitive patients and, in those who developed multidrug resistance, sorcin was upregulated in 15 out of 21 patients (P<0.01). The differential expression levels of sorcin were confirmed by western blot and immunohistochemical analysis. In conclusion, sorcin expression in the human serum of breast cancer patients who are resistant to NAC was elevated when compared with that of NAC-sensitive patients.

Sorcin, a calcium binding protein involved in the multidrug resistance mechanisms in cancer cells

Sorcin is a penta-EF hand calcium binding protein, which participates in the regulation of calcium homeostasis in cells. Sorcin regulates calcium channels and exchangers located at the plasma membrane and at the endo/sarcoplasmic reticulum (ER/SR), and allows high levels of calcium in the ER to be maintained, preventing ER stress and possibly, the unfolded protein response. Sorcin is highly expressed in the heart and in the brain, and overexpressed in many cancer cells. Sorcin gene is in the same amplicon as other genes involved in the resistance to chemotherapeutics in cancer cells (multi-drug resistance, MDR) such as ABCB4 and ABCB1; its overexpression results in increased drug resistance to a number of chemotherapeutic agents, and inhibition of sorcin expression by sorcin-targeting RNA interference leads to reversal of drug resistance. Sorcin is increasingly considered a useful marker of MDR and may represent a therapeutic target for reversing tumor multidrug resistance.

Calpain system and its involvement in myocardial ischemia and reperfusion injury

Calpains are ubiquitous non-lysosomal Ca²⁺-dependent cysteine proteases also present in myocardial cytosol and mitochondria. Numerous experimental studies reveal an essential role of the calpain system in myocardial injury during ischemia, reperfusion and postischemic structural remodelling. The increasing Ca²⁺-content and Ca²⁺-overload in myocardial cytosol and mitochondria during ischemia and reperfusion causes an activation of calpains. Upon activation they are able to injure the contractile apparatus and impair the energy production by cleaving structural and functional proteins of myocytes and mitochondria. Besides their causal involvement in acute myocardial dysfunction they are also involved in structural remodelling after myocardial infarction by the generation and release of proapoptotic factors from mitochondria. Calpain inhibition can prevent or attenuate myocardial injury during ischemia, reperfusion, and in later stages of myocardial infarction.

A technique for high-throughput protein crystallization in ionically cross-linked polysaccharide gel beads for X-ray diffraction experiments

A simple technique for high-throughput protein crystallization in ionically cross-linked polysaccharide gel beads has been developed for contactless handling of crystals in X-ray crystallography. The method is designed to reduce mechanical damage to crystals caused by physical contact between crystal and mount tool and by osmotic shock during various manipulations including cryoprotection, heavy-atom derivatization, ligand soaking, and diffraction experiments. For this study, protein crystallization in alginate and κ-carrageenan gel beads was performed using six test proteins, demonstrating that proteins could be successfully crystallized in gel beads. Two complete diffraction data sets from lysozyme and ID70067 protein crystals in gel beads were collected at 100 K without removing the crystals; the results showed that the crystals had low mosaicities. In addition, crystallization of glucose isomerase was carried out in alginate gel beads in the presence of synthetic zeolite molecular sieves (MS), a hetero-epitaxic nucleant; the results demonstrated that MS can reduce excess nucleation of this protein in beads. To demonstrate heavy-atom derivatization, lysozyme crystals were successfully derivatized with K₂PtBr₆ within alginate gel beads. These results suggest that gel beads prevent serious damage to protein crystals during such experiments.

Molecular basis for defect in Alix-binding by alternatively spliced isoform of ALG-2 (ALG-2DeltaGF122) and structural roles of F122 in target recognition

Background

ALG-2 (a gene product of PDCD6) belongs to the penta-EF-hand (PEF) protein family and Ca²⁺-dependently interacts with various intracellular proteins including mammalian Alix, an adaptor protein in the ESCRT system. Our previous X-ray crystal structural analyses revealed that binding of Ca²⁺ to EF3 enables the side chain of R125 to move enough to make a primary hydrophobic pocket (Pocket 1) accessible to a short fragment of Alix. The side chain of F122, facing a secondary hydrophobic pocket (Pocket 2), interacts with the Alix peptide. An alternatively spliced shorter isoform, designated ALG-2^ΔGF122, lacks Gly¹²¹Phe¹²² and does not bind Alix, but the structural basis of the incompetence has remained to be elucidated.

Results

We solved the X-ray crystal structure of the PEF domain of ALG-2^ΔGF122 in the Ca²⁺-bound form and compared it with that of ALG-2. Deletion of the two residues shortened α-helix 5 (α5) and changed the configuration of the R125 side chain so that it partially blocked Pocket 1. A wall created by the main chain of 121-GFG-123 and facing the two pockets was destroyed. Surprisingly, however, substitution of F122 with Ala or Gly, but not with Trp, increased the Alix-binding capacity in binding assays. The F122 substitutions exhibited different effects on binding of ALG-2 to other known interacting proteins, including TSG101 (Tumor susceptibility gene 101) and annexin A11. The X-ray crystal structure of the F122A mutant revealed that removal of the bulky F122 side chain not only created an additional open space in Pocket 2 but also abolished inter-helix interactions with W95 and V98 (present in α4) and that α5 inclined away from α4 to expand Pocket 2, suggesting acquirement of more appropriate positioning of the interacting residues to accept Alix.

Conclusions

We found that the inability of the two-residue shorter ALG-2 isoform to bind Alix is not due to the absence of bulky side chain of F122 but due to deformation of a main-chain wall facing pockets 1 and 2. Moreover, a residue at the position of F122 contributes to target specificity and a smaller side chain is preferable for Alix binding but not favored to bind annexin A11.

Molecular basis for the impaired function of the natural F112L sorcin mutant: X-ray crystal structure, calcium affinity, and interaction with annexin VII and the ryanodine receptor

The penta-EF hand protein sorcin participates in the modulation of Ca2+-induced calcium-release in the heart through the interaction with several Ca2+ channels such as the ryanodine receptor. The modulating activity is impaired in the recently described natural F112L mutant. The F112 residue is located at the end of the D helix next to Asp113, one of the calcium ligands in the EF3 hand endowed with the highest affinity for the metal. The F112L-sorcin X-ray crystal structure at 2.5 A resolution displays marked alterations in the EF3 hand, where the hydrogen bonding network established by Phe112 is disrupted, and in the EF1 region, which is tilted in both monomers that give rise to the dimer, the stable form of the molecule. In turn, the observed tilt is indicative of an increased flexibility of the N-terminal part of the molecule. The structural alterations result in a 6-fold decrease in calcium affinity with respect to the wild-type protein and to an even larger impairment of the interaction with annexin VII and of the ability of sorcin to interact with and inhibit ryanodine receptors. These results provide a plausible structural and functional framework that helps elucidate the phenotypic alterations of mice overexpressing F112L-sorcin.

Expression of a sorcin missense mutation in the heart modulates excitation-contraction coupling

Sorcin is a Ca2+ binding protein implicated in the regulation of intracellular Ca2+ cycling and cardiac excitation-contraction coupling. Structural and human genetic studies suggest that a naturally occurring sequence variant encoding L112-sorcin disrupts an E-F hand Ca2+ binding domain and may be responsible for a heritable form of hypertension and hypertrophic heart disease. We generated transgenic mice overexpressing L112-sorcin in the heart and characterized the effects on Ca2+ regulation and cardiac function both in vivo and in dissociated cardiomyocytes. Hearts of sorcin(F112L) transgenic mice were mildly dilated but ventricular function was preserved and systemic blood pressure was normal. Sorcin(F112L) myocytes were smaller than control cells and displayed complex alterations in Ca2+ regulation and contractility, including a slowed inactivation of L-type Ca2+ current, enhanced Ca2+ spark width, duration, and frequency, and increased Na+-Ca2+ exchange activity. In contrast, mice with cardiac-specific overexpression of wild-type sorcin displayed directionally opposite effects on L-type Ca2+ channel function and Ca2+ spark behavior. These data further define the role of sorcin in cardiac excitation-contraction coupling and highlight its negative regulation of SR calcium release. Our results also suggest that additional factors may be responsible for the development of cardiac hypertrophy and hypertension in humans expressing the L112-sorcin sequence variant.

Calretinin and calbindin D28k have different domain organizations

The domain organization of calretinin (CR) was predicted to involve all six EF-hand motifs (labeled I to VI) condensed into a single domain, as characterized for calbindin D28k (Calb), the closest homolog of calretinin. Unperturbed (1)H,(15)N HSQC NMR spectra of a (15)N-labeled calretinin fragment (CR III-VI, residues 100-271) in the presence of the unlabeled complimentary fragment (CR I-II, residues 1-100) show that these fragments do not interact. Size exclusion chromatography and affinity chromatography data support this conclusion. The HSQC spectrum of (15)N-labeled CR is similar to the overlaid spectra of individual (15)N-labeled CR fragments (CR I-II and CR III-VI), also suggesting that these regions do not interact within intact CR. In contrast to these observations, but in accordance with the Calb studies, we observed interactions between other CR fragments: CR I (1-60) with CR II-VI (61-271), and CR I-III (1-142) with CR IV-VI (145-271). We conclude that CR is formed from at least two independent domains consisting of CR I-II and CR III-VI. The differences in domain organization of Calb and CR may explain the specific target interaction of Calb with caspase-3. Most importantly, the comparison of CR and Calb domain organizations questions the value of homologous modeling of EF-hand proteins, and perhaps of other protein families.