Binding of polarity-sensitive hydrophobic ligands to erythroid and nonerythroid spectrin: fluorescence and molecular modeling studies

Spectrin: an alternate target for cytoskeletal drugs

Cytoskeletal drugs having enormous therapeutic potential act on the cytoskeletal components like actin, tubulin either by promoting polymerization or destabilizing the same. Here we present the interaction of the popular cytoskeletal drugs such as taxol, latrunculin and cytochalasin with spectrin, a huge protein with multi domains that forms the cytoskeletal network. Particularly, the actin binding domain of spectrin regulates the dynamics of the actin cytoskeleton. We followed the binding of these drugs to its actin binding domain and intact spectrin as well. These drugs bind with moderate affinity (K_b ∼ 10⁴ M^-1) and the interaction with actin binding domain is entropy driven and hydrophobic in nature as determined by Van't Hoff plot. The docking studies and molecular dynamics simulations further corroborate the experimental findings. Particularly the higher binding constants in the case of latrunculin and cytochalasin to the actin binding domain of spectrin suggest the binding sites are presumably located in its actin binding domain.Communicated by Ramaswamy H. Sarma.

Polarity-based fluorescence probes: properties and applications

Local polarity can affect the physical or chemical behaviors of surrounding molecules, especially in organisms. Cell polarity is the ultimate feedback of cellular status and regulation mechanisms. Hence, the abnormal alteration of polarity in organisms is closely linked with functional disorders and many diseases. It is incredibly significant to monitor and detect local polarity to explain the biological processes and diagnoses of some diseases. Because of their in vivo safe and real-time monitoring, several polarity-sensitive fluorophores and fluorescent probes have gradually emerged and been used in modern research. This review summarizes the fluorescence properties and applications of several representative polarity-sensitive fluorescent probes.

Effect of organo-selenium anticancer drugs on nitrite induced methemoglobinemia: A spectroscopic study

Selenium containing drugs like selenomethionine, selenocystine, selenourea and methylseleninic acid are reported to exhibit potential anticancer effect. However, these anticancer drugs may exert adverse effects when used over a prolonged period. Little is known about the interaction of these selenium containing drugs with the vital erythroid protein hemoglobin. In this work a comparative study of the interaction of organo-selenium drugs with hemoglobin and heme moiety has been performed using different spectroscopic techniques to find out their role on drug induced methemoglobinemia. We found that though these selenium containing drugs have similar binding affinity towards hemoglobin, they have differential interactions with the heme group. Isothermal calorimetric titration study showed that selenourea has the lowest binding affinity (K_d 19.28 μM) towards HbA as compared to other drugs, selenomethionine, selenocystine and methylseleninic acid (K_d 7.69 μM, 4.88 μM and 10.5 μM at 37 °C respectively). This result is also supported by the molecular docking study. Methylseleninic acid was found to have detrimental effects on nitrite induced methemoglobinemia, a hematological disorder caused due to excessive conversion of Fe²⁺ to Fe³⁺ in hemoglobin. Hence the results of the study would help to develop a better insight on the mechanism of action and anticipate the toxicity of these drugs which require further optimization before their actual use in the treatment of cancer.

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.

Differential interactions of imatinib mesylate with the membrane skeletal protein spectrin and hemoglobin

The anti-leukaemia drug imatinib has been shown to bind to spectrin, and to hemoglobin in its oxy-form with binding dissociation constants of 48 μM and 63 μM at 25 °C respectively.

Binding of hemin, hematoporphyrin, and protoporphyrin with erythroid spectrin: fluorescence and molecular docking studies

Free heme has toxic effects, for example lipid peroxidation, DNA damage, and protein aggregation. In severe hemolysis, which occurs during pathological states, for example sickle cell disease, ischemia reperfusion, and malaria, levels of free heme increase inside erythrocytes. The purpose of this study was to investigate whether spectrin, the major erythroid cytoskeleton protein, is involved as an acceptor of free heme. We compared the interactions of three heme derivatives, hemin chloride, hematoporphyrin, and protoporphyrin-IX, with dimeric and tetrameric spectrin. The dissociation constants (K d) for binding to spectrin dimer and tetramer were 0.57 and 1.16 µM respectively. Thermodynamic data associated with this binding revealed the binding to be favored by a positive change in entropy. Although molecular docking studies identified the SH3 domain as the unique binding site of these heme derivatives to erythroid spectrin, experimental results indicated a binding stoichiometry of 1 heme attached to both dimeric and tetrameric spectrin, indicating the common self-associating domain to be the unique binding site. We also noticed heme-induced structural changes in the membrane skeletal protein. Erythroid spectrin could thus act as a potential acceptor of heme, particularly relevant under disease conditions.

Probing conformational stability and dynamics of erythroid and nonerythroid spectrin: effects of urea and guanidine hydrochloride

We have studied the conformational stability of the two homologous membrane skeletal proteins, the erythroid and non-erythroid spectrins, in their dimeric and tetrameric forms respectively during unfolding in the presence of urea and guanidine hydrochloride (GuHCl). Fluorescence and circular dichroism (CD) spectroscopy have been used to study the changes of intrinsic tryptophan fluorescence, anisotropy, far UV-CD and extrinsic fluorescence of bound 1-anilinonapthalene-8-sulfonic acid (ANS). Chemical unfolding of both proteins were reversible and could be described as a two state transition. The folded erythroid spectrin and non-erythroid spectrin were directly converted to unfolded monomer without formation of any intermediate. Fluorescence quenching, anisotropy, ANS binding and dynamic light scattering data suggest that in presence of low concentrations of the denaturants (up-to 1M) hydrogen bonding network and van der Waals interaction play a role inducing changes in quaternary as well as tertiary structures without complete dissociation of the subunits. This is the first report of two large worm like, multi-domain proteins obeying twofold rule which is commonly found in small globular proteins. The free energy of stabilization (ΔGuH20) for the dimeric spectrin has been 20 kcal/mol lesser than the tetrameric from.

Malachite green interacts with the membrane skeletal protein, spectrin

Energy minimized complex of MG with the self association domain of spectrin.