Effect of pH on stability, conformation, and chaperone activity of erythroid & non-erythroid spectrin

Bovine serum albumin prevents human hemoglobin aggregation and retains its chaperone-like activity

This study investigates the ability of bovine serum albumin (BSA) to act as an extracellular chaperone (EC) on human hemoglobin (Hb) at a pH of 7.4. The best temperature for studying this behavior was determined by analyzing Hb's aggregation kinetics at multiple temperatures. 55 °C was chosen as the optimal temperature for forming Hb amyloids. BSA was then tested at various concentrations (20-100 μM) to assess its chaperone-like activity on Hb at 55 °C. At a concentration of 100 μM, BSA exhibits chaperone-like activity with a client protein:BSA ratio of 1:10. The high ratio implies that the chaperone activity of BSA is favored by the effects of macromolecular crowding. The results showed that BSA has the potential to inhibit Hb's dissociation into alpha and beta subunits and protein aggregation by inhibiting secondary nucleation. BSA also causes the depolymerization of fibrils over time. The results were validated using molecular docking and all-atom molecular dynamics simulations. MD analysis such as RMSD, RMSF, Rg, SASA, Hydrogen bond, PCA, Free energy landscape (FEL) revealed that the stability of hemoglobin is greater when it is bound to BSA compared to unbound state. The study suggests that BSA can potentially bind to Hb dimers and reduce excitonic interactions, which reduces Hb aggregation. These results are consistent with the aggregation kinetics experiments.Communicated by Ramaswamy H. Sarma.

Clinical significance of nonerythrocytic spectrin Beta 1 (SPTBN1) in human kidney renal clear cell carcinoma and uveal melanoma: a study based on Pan-Cancer Analysis

BACKGROUND

Nonerythrocytic spectrin beta 1 (SPTBN1) is an important cytoskeletal protein that involves in normal cell growth and development via regulating TGFβ/Smad signaling pathway, and is aberrantly expressed in various cancer types. But, the exact role of SPTBN1 in pan-cancer is still unclear. This report aimed to display expression patterns and prognostic landscapes of SPTBN1 in human cancers, and further assess its prognostic/therapeutic value and immunological role in kidney renal carcinoma (KIRC) and uveal melanoma (UVM).

METHODS

We firstly analyzed expression patterns and prognostic landscapes of SPTBN1 in human cancers using various databases and web-based tools. The relationships between SPTBN1 expression and survival/tumor immunity in KIRC and UVM were further investigated via R packages and TIMER 2.0 platform. The therapeutic roles of SPTBN1 in KIRC and UVM were also explored via R software. Following this, the prognostic value and cancer immunological role of SPTBN1 in KIRC and UVM were validated in our cancer patients and GEO database.

RESULTS

Overall, cancer tissue had a lower expression level of SPTBN1 frequently in pan-cancer, compared with those in adjacent nontumor one. SPTBN1 expression often showed a different effect on survival in pan-cancer; upregulation of SPTBN1 was protective to the survival of KIRC individuals, which was contrary from what was found in UVM patients. In KIRC, there were significant negative associations between SPTBN1 expression and pro-tumor immune cell infiltration, including Treg cell, Th2 cell, monocyte and M2-macrophage, and expression of immune modulator genes, such as tumor necrosis factor superfamily member 9 (TNFSF9); while, in UVM, these correlations exhibited opposite patterns. The following survival and expression correlation analysis in our cancer cohorts and GEO database confirmed these previous findings. Moreover, we also found that SPTBN1 was potentially involved in the resistance of immunotherapy in KIRC, and the enhance of anti-cancer targeted treatment in UVM.

CONCLUSIONS

The current study presented compelling evidence that SPTBN1 might be a novel prognostic and therapy-related biomarker in KIRC and UVM, shedding new light on anti-cancer strategy.

Structural Flexibility of Proteins Dramatically Alters Membrane Stability─A Novel Aspect of Lipid-Protein Interaction

Protein isoforms are structural variants with changes in the overall flexibility predominantly at the tertiary level. For membrane associated proteins, such structural flexibility or rigidity affects membrane stability by playing modulatory roles in lipid-protein interaction. Herein, we investigate the protein chain flexibility mediated changes in the mechanistic behavior of phospholipid model membranes in the presence of two well-known isoforms, erythroid (ER) and nonerythroid (NER) spectrin. We show dramatic alterations of membrane elasticity and stability induced by spectrin in the Langmuir monolayers of phosphatidylocholine (PC) and phosphatidylethanolamine (PE) by a combination of isobaric relaxation, surface pressure-area isotherm, X-ray scattering, and microscopy measurements. The NER spectrin drives all monolayers to possess an approximately equal stability, and that required 25-fold increase and 5-fold decrease of stability in PC and PE monolayers, respectively. The untilting transition of the PC membrane in the presence of NER spectrin observed in X-ray measurements can explain better membrane packing and stability.

Comparative Analysis of Tryptophan Dynamics in Spectrin and Its Constituent Domains: Insights from Fluorescence

Spectrin is a cytoskeletal protein ubiquitous in metazoan cells that acts as a liaison between the plasma membrane and the cellular interior and imparts mechanical stability to the plasma membrane. Spectrin is known to be highly dynamic, with an appreciable degree of torsional and segmental mobility. In this context, we have earlier utilized the red edge excitation shift (REES) approach to report the retention of restricted solvation dynamics and local structure in the vicinity of spectrin tryptophans on urea denaturation and loss of spectrin secondary structure. As a natural progression of our earlier work, in this work, we carried out a biophysical dissection of tryptophan solvation and rotational dynamics in spectrin and its constituent domains, in order to trace the origin of local structure retention observed in denatured spectrin. Our results show that the ankyrin binding domain (and, to a lesser extent, the β-tetramerization domain) is capable of retention of local structure, similar to that observed for intact spectrin. However, all α-chain domains studied exhibit negligible retention of local structure on urea denaturation. Such a stark chain-specific retention of local structure could originate from the fact that the β-chain domains possess specialized functions, where conservation of local (structural) integrity may be a prerequisite for optimum cellular function. To the best of our knowledge, these observations represent one of the first systematic biophysical dissections of spectrin dynamics in terms of its constituent domains and add to emerging literature on comprehensive domain-based analysis of spectrin organization, dynamics, and function.

βII spectrin (SPTBN1): biological function and clinical potential in cancer and other diseases

βII spectrin, the most common isoform of non-erythrocyte spectrin, is a cytoskeleton protein present in all nucleated cells. Interestingly, βII spectrin is essential for the development of various organs such as nerve, epithelium, inner ear, liver and heart. The functions of βII spectrin include not only establishing and maintaining the cell structure but also regulating a variety of cellular functions, such as cell apoptosis, cell adhesion, cell spreading and cell cycle regulation. Notably, βII spectrin dysfunction is associated with embryonic lethality and the DNA damage response. More recently, the detection of altered βII spectrin expression in tumors indicated that βII spectrin might be involved in the development and progression of cancer. Its mutations and disorders could result in developmental disabilities and various diseases. The versatile roles of βII spectrin in disease have been examined in an increasing number of studies; nonetheless, the exact mechanisms of βII spectrin are still poorly understood. Thus, we summarize the structural features and biological roles of βII spectrin and discuss its molecular mechanisms and functions in development, homeostasis, regeneration and differentiation. This review highlight the potential effects of βII spectrin dysfunction in cancer and other diseases, outstanding questions for the future investigation of therapeutic targets. The investigation of the regulatory mechanism of βII spectrin signal inactivation and recovery may bring hope for future therapy of related diseases.

Effects of free soluble iron on thermal aggregation of hemoglobin

Aggregation of hemoglobin is implicated in the presentation of diseases like sickle cell disease and thalassemia. Hallmark of the disease being imbalance in the production of globin chains leading to aggregation of excess globin chains and aberrant hemoglobins associated with the disease, broadly categorized as hemoglobinopathy. We have studied thermal aggregation of hemoglobin at 70 °C and pH 6.5 using light scattering, flow cytometry and optical microscopy and tried to investigate effects of few abundant soluble metal ions on such aggregation. Our study indicate that only iron, both in Fe²⁺ and Fe³⁺ forms, could inhibit hemoglobin aggregation and the extent of inhibition was 60% in presence of 100 mgL^-1 FeCl₃. Similar effect was not seen in lysozyme aggregation. Metal ions such as, Cu²⁺, Zn²⁺ and Ni²⁺ also did not have any significant effects on hemoglobin aggregation. Results show this important chaperone like behavior of free iron affecting the kinetics and yield of the aggregation process which could have important consequence in the extent of severity of such hematological diseases.

Multiple Functions of Spectrin: Convergent Effects

Spectrin is a multifunctional, multi-domain protein most well known in the membrane skeleton of mature human erythrocytes. Here we review the literature on the crosstalk of the chaperone activity of spectrin with its other functionalities. We hypothesize that the chaperone activity is derived from the surface exposed hydrophobic patches present in individual "spectrin-repeat" domains and show a competition between the membrane phospholipid binding functionality and chaperone activity of spectrin. Moreover, we show that post-translational modifications such as glycation which shield these surface exposed hydrophobic patches, reduce the chaperone function. On the other hand, oligomerization which is linked to increase of hydrophobicity is seen to increase it. We note that spectrin seems to prefer haemoglobin as its chaperone client, binding with it preferentially over other denatured proteins. Spectrin is also known to interact with unstable haemoglobin variants with a higher affinity than in the case of normal haemoglobin. We propose that chaperone activity of spectrin could be important in the cellular biochemistry of haemoglobin, particularly in the context of diseases.

Chaperone and Immunoglobulin-Binding Activities of Skp Protein from Yersinia pseudotuberculosis

Here, we determined qualitative and quantitative characteristics of the chaperone and immunoglobulin-binding activities of recombinant Skp protein (rSkp) from Yersinia pseudotuberculosis using the methods of dynamic light scattering and surface plasmon resonance. Commercial human polyclonal IgG and Fc and Fab fragments of human IgG were used as substrate proteins. The activity of rSkp strongly depended on the medium pH. The most stable low-molecular-weight complexes with a hydrodynamic radius up to 10 nm were formed by rSkp and protein substrates at acidic pH values. Under these conditions, rSkp exhibited the lowest propensity to self-association and the highest affinity for human IgG and its Fc and Fab fragments, as well as prevented their aggregation most efficiently (i.e., demonstrated the maximal chaperone activity). As the medium pH increased, the affinity of rSkp for IgG and its fragments decreased; rSkp was not able to completely prevent the aggregation of protein substrates, but significantly slowed it down. The obtained information may be of practical interest, since the stability of therapeutic IgG preparations affects their safety and efficacy in medical applications.

Chaperone potential of erythroid spectrin: Effects of hemoglobin interaction, macromolecular crowders, phosphorylation and glycation

Spectrin, the major protein component of the erythrocyte membrane skeleton has chaperone like activity and is known to bind membrane phospholipids and hemoglobin. We have probed the chaperone activity of spectrin in presence of hemoglobin and phospholipid SUVs of different compositions to elucidate the effect of phospholipid/hemoglobin binding on chaperone function. It is seen that spectrin displays a preference for hemoglobin over other substrates leading to a decrease in chaperone activity in presence of hemoglobin. A competition is seen to exist between phospholipid binding and chaperone function of spectrin, in a dose dependent manner with the greatest extent of decrease being seen in case of phospholipid vesicles containing aminophospholipids e.g. PS and PE which may have implications in diseases like hereditary spherocytosis where mutation in spectrin is implicated in its detachment from cell membrane. To gain a clearer understanding of the chaperone like activity of spectrin under in-vivo like conditions we have investigated the effect of macromolecular crowders as well as phosphorylation and glycation states on chaperone activity. It is seen that the presence of non-specific, protein and non-protein macromolecular crowders do not appreciably affect chaperone function. Phosphorylation also does not affect the chaperone function unlike glycation which progressively diminishes chaperone activity. We propose a model where chaperone clients adsorb onto spectrin's surface and processes that bind to and occlude these surfaces decrease chaperone activity.

SPTBN1 and cancer, which links?

SPTBN1 is a dynamic intracellular nonpleckstrin homology-domain protein, functioning as a transforming growth factor-β signal transducing adapter protein which is necessary to form Smad3/Smad4 complex. Recently SPTBN1 is considered to be associated with many kinds of cancers. SPTBN1 expression and function differ between different tumor states or types. This review summarizes the recent advances in the expression patterns of SPTBN1 in cancers, and in understanding the mechanisms by which SPTBN1 affects the occurrence, progression, and metastasis of cancer. Identifying SPTBN1 expression and function in cancers will contribute to the clinical diagnosis and treatment of cancer and the investigation of anticancer drugs.