Nucleotide-dependent bisANS binding to tubulin

Application of BisANS fluorescent dye for developing a novel protein assay

In many biology- and chemistry-related research fields and experiments the quantification of the peptide and/or protein concentration in samples are essential. Every research environment has unique requirements, e.g. metal ions, incubation times, photostability, pH, protease inhibitors, chelators, detergents, etc. A new protein assay may be adequate in different experiments beyond or instead of the well-known standard protocols (e.g. Qubit, Bradford or bicinchoninic acid) in related conceptions. Based on our previous studies, we developed a novel protein assay applying the 4,4′-Dianilino-1,1′-binaphthyl-5,5′-disulfonic acid dipotassium salt (BisANS) fluorescent dye. This molecule has several advantageous properties related to protein detection: good solubility in water, high photostability at adequate pH, quick interaction kinetics (within seconds) with proteins and no exclusionary sensitivity to the chelator, detergent and inhibitor ingredients. The protocol described in this work is highly sensitive in a large spectrum to detect protein (100-fold diluted samples) concentrations (from 0.28 up to more than 100 μg/mL). The BisANS protein assay is valid and applicable for quantification of the amount of protein in different biological and/or chemical samples.

A novel application of the fluorescent dye bis-ANS for labeling neurons in acute brain slices

The cell-impermeant oligomer-(e.g. beta-amyloid-, or tubulin-) specific fluorescent dye, bis-ANS (4,4'-bis-1-anilinonaphtalene-8-sulfonate), was successfully used for labeling mechanically damaged but still viable neuron bodies, neurites and neurite cross sections in acute brain slices. Acute hippocampal brain slices of rats were co-stained with bis-ANS and the cell-impermeant, DNA-specific dye propidium iodide (PI) and were then analyzed using fluorescence and confocal microscopes. Both the neuron bodies and the neurites were found to exhibit increased fluorescence intensities, suggesting that using this method they can be detected more easily. In addition, bis-ANS showed good region - but not cell specific co-localization with the neuron-specific fluorescent dye Dil (1,1'-Dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate). These two dyes label different neuronal structures: Dil binds specifically to intact cell membranes while bis-ANS can enter cells with compromised cell membranes and then stain the microtubules in the cytoplasm. For a quick (10min) staining of acute brain slices with bis-ANS both HEPES and NaHCO(3) were needed in order to achieve high signal intensity. Labeling with bis-ANS fluorescent dye is an easy method for imaging the neuronal structures on the surface of acute brain slices.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Cytoskeleton localization in the sperm head prior to fertilization

Three major cytoskeletal proteins, actin, tubulin and spectrin, are present in the head of mammalian spermatozoa. Although cytoskeletal proteins are implicated in the regulation of capacitation and the acrosome reaction (AR), their exact role remains poorly understood. The aim of this study was to compare the distribution of the sperm head cytoskeleton before and after the AR in spermatozoa representing a range of acrosome size and shape. Spermatozoa from the human and three rodents (rat, hamster and grey squirrel) were fixed before and after the AR in appropriate medium in vitro. Indirect immunofluorescent localization of cytoskeletal proteins was undertaken with antibodies recognizing actin, spectrin and alpha-tubulin. Preparations were counterstained with propidium iodide and examined by epifluorescent and confocal microscopy. Our results clearly demonstrated changes in localization of cytoskeleton during the AR, mainly in the apical acrosome with further changes to the equatorial segment and post-acrosomal regions. The pattern of cytoskeletal proteins in the sperm head of all the species was similar in respect to various sub-compartments. These observations indicated that the sperm head cortical cytoskeleton exhibits significant changes during the AR and, therefore, support the image of cytoskeletal proteins as highly dynamic structures participating actively in processes prior to fertilization.

MAP2 prevents protein aggregation and facilitates reactivation of unfolded enzymes. Implications for the chaperone-like activity of MAP2

It is well established that in addition to its functional role in cell motility, cell division and intracellular transport, cytoskeletal protein tubulin also possesses significant chaperone-like activity. In vitro studies from our laboratory showed that dimeric tubulin can prevent stress induced aggregation of substrate proteins, can resist thermal deactivation of enzymes and can also refold enzymes from their fully denatured state [Manna, T., Sarkar, T., Poddar, A., Roychowdhury, M., Das, K.P. & Bhattacharyya, B. (2001) J. Biol. Chem.276, 39742-39747]. Negative charges of the C-termini of both subunits of tubulin are essential for this chaperone-like property as the deletion of only beta-C-terminus or the binding of a 14-residue basic peptide P2 to the alpha-C-terminus completely abolishes this property [Sarkar, T., Manna, T., Bhattacharyya, S., Mahapatra, P., Poddar, A., Roy, S., Pena, J., Solana, R., Tarazona, R. & Bhattacharyya, B. (2001) Proteins Struct. Funct. Genet.44, 262-269]. Based on these results, one would expect that the microtubular proteins (MTP, tubulin with microtubular-associated proteins, i.e. MAPs bound to the C-terminus) should not possess any chaperone-like activity. To our surprise we noticed excellent chaperone-like activity of MTP. MTP prevents chemical and thermal aggregation of other proteins and can enhance the extent of refolding of fully unfolded substrate enzymes. Because MTP contains tubulin as well as several MAPs bound to the C-termini of tubulin, we fractionated and purified microtubular associated protein 2 (MAP2) and tau using phosphocellulose chromatography. Experiments with purified proteins demonstrated that it is the MAP2 of MTP that exhibits significant chaperone-like activity. This has been shown by the prevention of dithiothreitol-induced aggregation of insulin, thermal aggregation of alcohol dehydrogenase and regain of enzymatic activity during refolding of unfolded substrates. Tau, which shares a homologous C-terminal domain with MAP2, possesses no such activity.

1,1'-bis(anilino)-4-,4'-bis(naphtalene)-8,8'-disulfonate acts as an inhibitor of lipoprotein lipase and competes for binding with apolipoprotein CII

Lipoprotein lipase (LPL) is dependent on apolipoprotein CII (apoCII), a component of plasma lipoproteins, for function in vivo. The hydrophobic fluorescent probe 1,1'-bis(anilino)-4,4'-bis(naphthalene)-8,8'-disulfonate (bis-ANS) was found to be a potent inhibitor of LPL. ApoCII prevented the inhibition by bis-ANS, and was also able to restore the activity of inhibited LPL in a competitive manner, but only with triacylglycerols with acyl chains longer than three carbons. Studies of fluorescence and surface plasmon resonance indicated that LPL has an exposed hydrophobic site for binding of bis-ANS. The high affinity interaction was characterized by an equilibrium constant Kd of 0.10-0.26 microm and by a relatively high on rate constant kass = 2.0 x 10(4) m(-1) s(-1) and a slow off-rate with a dissociation rate constant kdiss = 1.2 x 10(-4) s(-1). The high affinity binding of bis-ANS did not influence interaction of LPL with heparin or with lipid/water interfaces and did not dissociate the active LPL dimer into monomers. Analysis of fragments of LPL after photoincorporation of bis-ANS indicated that the high affinity binding site was located in the middle part of the N-terminal folding domain. We propose that bis-ANS binds to an exposed hydrophobic area that is located close to the active site. This area may be the binding site for individual substrate molecules and also for apoCII.

BisANS binding to tubulin: isothermal titration calorimetry and the site-specific proteolysis reveal the GTP-induced structural stability of tubulin

Interactions of bisANS and ANS to tubulin in the presence and absence of GTP were investigated, and the binding and thermodynamic parameters were determined using isothermal titration calorimetry. Like bisANS binding to tubulin, we observed a large number of lower affinity ANS binding sites (N1 = 1.3, K1 = 3.7 x 10(5) M(-1), N2 = 10.5, K2 = 7 x 10(4)/M(-1)) in addition to 1-2 higher affinity sites. Although the presence of GTP lowers the bisANS binding to both higher and lower affinity sites (N1 = 4.3, N2 = 11.7 in absence and N1 = 1.8, N2 = 3.6 in presence of GTP), the stoichiometries of both higher and lower affinity sites of ANS remain unaffected in the presence of GTP. BisANS-induced structural changes on tubulin were studied using site-specific proteolysis with trypsin and chymotrypsin. Digestion of both alpha and beta tubulin with trypsin and chymotrypsin, respectively, has been found to be very specific in presence of GTP. GTP has dramatic effects on lowering the extent of nonspecific digestion of beta tubulin with trypsin and stabilizing the intermediate bands produced from both alpha and beta. BisANS-treated tubulin is more susceptible to both trypsin and chymotrypsin digestion. At higher bisANS concentration (>20 microM) both alpha and beta tubulins are almost totally digested with enzymes, indicating bisANS-induced unfolding or destabilization of tubulin structure. Again, the addition of GTP has remarkable effect on lowering the bisANS-induced enhanced digestion of tubulin as well as stabilizing effect on intermediate bands. These results of isothermal titration calorimetry, proteolysis and the DTNB-kinetics data clearly established that the addition of GTP makes tubulin compact and rigid and hence the GTP-induced stabilization of tubulin structure. No such destabilization of tubulin structure has been noticed with ANS, although, like bisANS, ANS possesses a large number of lower affinity binding sites. On the basis of these results, we propose that the unique structure of bisANS, which in absence of GTP can bind tubulin as a bifunctional ligand (through its two ANS moieties), is responsible for the structural changes of tubulin.

Mildly acidic pH activates the extracellular molecular chaperone clusterin

Many features of the chaperone action of clusterin are similar to those of the intracellular small heat shock proteins (sHSPs) that, like clusterin, exist in solution as heterogeneous aggregates. Increased temperature induces dissociation of some sHSP aggregates and an enhanced chaperone action, suggesting that a dissociated form is the active chaperone species. We recently reported that clusterin aggregates dissociate at mildly acidic pH. To further explore the similarities between clusterin and the sHSPs, we tested the effects of temperature and pH on the structure of clusterin and its chaperone action. Our results demonstrate that increased temperature does not induce dissociation of clusterin aggregates, or other major structural changes, and has little effect on its chaperone action. However, we show that the chaperone action of clusterin is enhanced at mildly acidic pH. Clusterin is the first chaperone shown to be activated by reduced pH. This unique mode of activation appears to result from an increase in regions of solvent-exposed hydrophobicity, which is independent of any major changes in secondary or tertiary structure. We propose a model in which low pH-induced dissociation of clusterin aggregates increases the abundance of the heterodimeric chaperone-active species, which has greater hydrophobicity exposed to solution.