Effect of alpha-crystallin on thermal aggregation of glycogen phosphorylase b from rabbit skeletal muscle

Anti-aggregation activity of small heat shock proteins under crowded conditions

It is becoming evident that small heat shock proteins (sHsps) are important players of protein homeostasis system. Their ability to bind misfolded proteins may play a crucial role in preventing protein aggregation in cells. The remarkable structural plasticity of sHsps is considered to underlie the mechanism of their activity. However, all our knowledge of the anti-aggregation functioning of sHsps is based on data obtained in vitro in media greatly different from the cellular highly crowded milieu. The present review highlights available data on the effect of crowding on the anti-aggregation activity of sHsps. There is some evidence that crowding affects conformation and dynamics of sHsps oligomers as well as their anti-aggregation properties. Crowding stimulates association of sHsp-client protein complexes into large-sized aggregates thus diminishing the apparent anti-aggregation activity of sHsps. Nevertheless, it is also shown that complexes between suboligomers (dissociated forms) of sHsps and client proteins may be stabilized and exist for longer period of time under crowded conditions. Moreover, crowding may retard the initial stages of aggregation which correspond to the formation of sHsp-containing nuclei and their clusters. Thus, dissociation of sHsps into suboligomers appears to be an important feature for the anti-aggregation activity of sHsps in crowded media.

A protein aggregation based test for screening of the agents affecting thermostability of proteins

To search for agents affecting thermal stability of proteins, a test based on the registration of protein aggregation in the regime of heating with a constant rate was used. The initial parts of the dependences of the light scattering intensity (I) on temperature (T) were analyzed using the following empiric equation: I = K_agg(T−T₀)², where K_agg is the parameter characterizing the initial rate of aggregation and T₀ is a temperature at which the initial increase in the light scattering intensity is registered. The aggregation data are interpreted in the frame of the model assuming the formation of the start aggregates at the initial stages of the aggregation process. Parameter T₀ corresponds to the moment of the origination of the start aggregates. The applicability of the proposed approach was demonstrated on the examples of thermal aggregation of glycogen phosphorylase b from rabbit skeletal muscles and bovine liver glutamate dehydrogenase studied in the presence of agents of different chemical nature. The elaborated approach to the study of protein aggregation may be used for rapid identification of small molecules that interact with protein targets.

Mechanism of aggregation of UV-irradiated β(L)-crystallin

Thermal denaturation and aggregation of UV-irradiated β(L)-crystallin from eye lenses of steers have been studied. The data on size-exclusion chromatography and SDS-PAGE indicated that UV irradiation of β(L)-crystallin at 10 °С resulted in fragmentation of the protein molecule and formation of cross-linked aggregates. Fluorescence data showed that tryptophan fluorescence in the irradiated protein decreased exponentially with the UV dose. Decrease in tryptophan fluorescence is a result of photochemical destruction, but not of conformational changes of protein, because there is no red shift in the fluorescence maximum. The differential scanning calorimetry (DSC) profiles of the samples of UV-irradiated and wild type β(L)-crystallin were registered. The area under curves, which is proportional to the amount of the native protein, decreased exponentially with increasing the irradiation dose. The shape of the DSC profiles for the samples of UV-irradiated β(L)-crystallin was identical to that for wild type β(L)-crystallin. The DSC data allowed estimating the portion of UV-denatured β(L)-crystallin, which is not registered by DSC, and the portion of the combined fraction consisting of native and UV-damaged molecules retaining the native structure. A conclusion has been made that UV-induced denaturation of β(L)-crystallin follows the one-hit model. The study of the kinetics of thermal aggregation of UV-irradiated β(L)-crystallin at 37 °С using dynamic light scattering showed that the initial stage of aggregation was that of formation of the start aggregates with the hydrodynamic radius of 20 nm. Further sticking of the start aggregates proceeded in the regime of reaction-limited cluster-cluster aggregation. Splitting of the aggregate population into two components occurred above a definite point in time.

Comparative analysis of the effects of alpha-crystallin and GroEL on the kinetics of thermal aggregation of rabbit muscle glyceraldehyde-3-phosphate dehydrogenase

Effects of alpha-crystallin and GroEL on the kinetics of thermal aggregation of rabbit muscle glyceraldehyde-3-phosphate dehydrogenase (GAPDH) have been studied using dynamic light scattering and analytical ultracentrifugation. The analysis of the initial parts of the dependences of the hydrodynamic radius of protein aggregates on time shows that in the presence of alpha-crystallin or GroEL the kinetic regime of GAPDH aggregation is changed from the regime of diffusion-limited cluster-cluster aggregation to the regime of reaction-limited cluster-cluster aggregation, wherein the sticking probability for the colliding particles becomes lower the unity. In contrast to alpha-crystallin, GroEL does not interfere with formation of the start aggregates which include denatured GAPDH molecules. On the basis of the analytical ultracentrifugation data the conclusion has been made that the products of dissociation of GAPDH and alpha-crystallin or GroEL play an important role in the interactions of GAPDH and chaperones at elevated temperatures.

Mechanism of suppression of dithiothreitol-induced aggregation of bovine alpha-lactalbumin by alpha-crystallin

The kinetics of dithiothreitol (DTT)-induced aggregation of alpha-lactalbumin from bovine milk has been studied using dynamic light-scattering technique. Analysis of the distribution of the particles formed in the solution of alpha-lactalbumin after the addition of DTT by size showed that the initial stage of the aggregation process was the stage of formation of the start aggregates with the hydrodynamic radius (R(h)) of 80-100nm. Further growth of the protein aggregates proceeds as a result of sticking of the start aggregates. Suppression of alpha-lactalbumin aggregation by alpha-crystallin is mainly due to the increase in the duration of the lag period on the kinetic curves of aggregation. It is assumed that the initially formed complexes of unfolded alpha-lactalbumin with alpha-crystallin were transformed to the primary clusters prone to aggregation as a result of the redistribution of the denatured protein molecules on the surface of the alpha-crystallin particles.

Glycogen phosphorylase b and phosphorylase kinase binding to glycogen under molecular crowding conditions. Inhibitory effect of FAD

Dynamic light scattering was used to study the interaction of phosphorylase kinase (PhK) and glycogen phosphorylase b (Phb) from rabbit skeletal muscle with glycogen under molecular crowding conditions arising from the presence of 1 M trimethylamine N-oxide and at physiological ionic strength. The mean value of hydrodynamic radius of the initial glycogen particles was 52 nm. Crowding stimulated Phb and PhK combined binding on glycogen particles. Two-stage character of PhK binding to glycogen particles containing adsorbed Phb was found in the presence of the crowding agent. At the initial stage, limited size particles with hydrodynamic radius of approximately 220 nm are formed, whereas the second stage is accompanied by linear growth of hydrodynamic radius. Flavin adenine dinucleotide (FAD) selectively inhibited PhK binding at the second stage. The data indicate that in the first stage Phb is involved in PhK binding by glycogen particles containing adsorbed Phb, whereas PhK binding in the second stage does not involve Phb.

Mechanism of suppression of protein aggregation by α-crystallin

This review summarizes experimental data illuminating the mechanism of suppression of heat-induced protein aggregation by alpha-crystallin, one of the small heat shock proteins. The dynamic light scattering data show that the initial stage of thermal aggregation of proteins is the formation of the initial aggregates involving hundreds of molecules of the denatured protein. Further sticking of the starting aggregates proceeds in a regime of diffusion-limited cluster-cluster aggregation. The protective effect of alpha-crystallin is due to transition of the aggregation process to the regime of reaction-limited cluster-cluster aggregation, wherein the sticking probability for the colliding particles becomes lower than unity.

Study of kinetics of thermal aggregation of mitochondrial aspartate aminotransferase by dynamic light scattering: protective effect of alpha-crystallin

Thermal aggregation of aspartate aminotransferase from pig heart mitochondria (mAAT) has been studied at various temperatures and various protein concentrations by dynamic light scattering. The character of the dependence of protein aggregate size on time indicates that aggregation of mAAT proceeds in the regime of diffusion-limited cluster-cluster aggregation. Suppression of mAAT aggregation by alpha-crystallin is due to transition of the aggregation process into the regime of reaction-limited cluster-cluster aggregation. Realization of this regime of aggregation means that the sticking probability for the colliding particles is less than unity.

Kinetics of thermal aggregation of glycogen phosphorylase b from rabbit skeletal muscle: mechanism of protective action of alpha-crystallin

The kinetics of thermal aggregation of glycogen phosphorylase b (Phb) from rabbit skeletal muscle have been studied by dynamic light scattering (0.08M Hepes, pH 6.8, containing 0.1M NaCl; 48 degrees C). The hydrodynamic radius of the start aggregates determined from the initial linear parts of the dependences of the hydrodynamic radius (R(h)) on time was found to be 16.7 +/- 1.0 nm. At rather high values of time, the R(h) value for the protein aggregates becomes proportional to t(1/1.8) = t(0.56) suggesting that the aggregation process proceeds in the regime of diffusion-limited cluster-cluster aggregation. In the presence of alpha-crystallin, a protein possessing the chaperone-like activity, the process of protein aggregation switches to the regime of reaction-limited cluster-cluster aggregation as indicated by the exponential dependence of the R(h) value on time. It was shown that the addition of alpha-crystallin raises the rate of thermal inactivation of Phb. These data in combination with the results of the study of interaction of Phb with alpha-crystallin by analytical ultracentrifugation suggest that alpha-crystallin interacts with the intermediates of unfolding of the Phb molecule.

Small heat shock protein Hsp27 prevents heat-induced aggregation of F-actin by forming soluble complexes with denatured actin

Previously, we have shown that the small heat shock protein with apparent molecular mass 27 kDa (Hsp27) does not affect the thermal unfolding of F-actin, but effectively prevents aggregation of thermally denatured F-actin [Pivovarova AV, Mikhailova VV, Chernik IS, Chebotareva NA, Levitsky DI & Gusev NB (2005) Biochem Biophys Res Commun331, 1548-1553], and supposed that Hsp27 prevents heat-induced aggregation of F-actin by forming soluble complexes with denatured actin. In the present work, we applied dynamic light scattering, analytical ultracentrifugation and size exclusion chromatography to examine the properties of complexes formed by denatured actin with a recombinant human Hsp27 mutant (Hsp27-3D) mimicking the naturally occurring phosphorylation of this protein at Ser15, Ser78, and Ser82. Our results show that formation of these complexes occurs upon heating and accompanies the F-actin thermal denaturation. All the methods show that the size of actin-Hsp27-3D complexes decreases with increasing Hsp27-3D concentration in the incubation mixture and that saturation occurs at approximately equimolar concentrations of Hsp27-3D and actin. Under these conditions, the complexes exhibit a hydrodynamic radius of approximately 16 nm, a sedimentation coefficient of 17-20 S, and a molecular mass of about 2 MDa. It is supposed that Hsp27-3D binds to denatured actin monomers or short oligomers dissociated from actin filaments upon heating and protects them from aggregation by forming relatively small and highly soluble complexes. This mechanism might explain how small heat shock proteins prevent aggregation of denatured actin and by this means protect the cytoskeleton and the whole cell from damage caused by accumulation of large insoluble aggregates under heat shock conditions.

Evidence for the formation of start aggregates as an initial stage of protein aggregation

The kinetics of thermal aggregation of glycogen phosphorylase b and glyceraldehyde 3-phosphate dehydrogenase from rabbit skeletal muscles were studied using dynamic light scattering. Use of high concentrations of the enzymes (1-3 mg/ml) provided a simultaneous registration of the native enzyme forms and protein aggregates. It was shown that initially registered aggregates (start aggregates) were large-sized particles. The hydrodynamic radius of the start aggregates was about 100 nm. The intermediate states between the native enzyme forms and start aggregates were not detected. The initial increase in the light scattering intensity is connected with accumulation of the start aggregates, the size of the latter remaining unchanged. From a certain moment in time aggregates of higher order, formed as a result of sticking of the start aggregates, make a major contribution to the enhancement of the light scattering intensity.