The effect of ions on the thermostability and unfolding of Na,K-ATPase from shark salt gland was studied and compared with that of Na,K-ATPase from pig kidney by using differential scanning calorimetry (DSC) and activity assays. In 1 mM histidine at pH 7, the shark enzyme inactivates rapidly at 20 degrees C, as does the kidney enzyme at 42 degrees C (but not at 20 degrees C). Increasing ionic strength by addition of 20 mM histidine, or of 1 mM NaCl or KCl, protects both enzymes against this rapid inactivation. As detected by DSC, the shark enzyme undergoes thermal unfolding at lower temperature (Tm approximately 45 degrees C) than does the kidney enzyme (Tm approximately 55 degrees C). Both calorimetric endotherms indicate multi-step unfolding, probably associated with different cooperative domains. Whereas the overall heat of unfolding is similar for the kidney enzyme in either 1 mM or 20 mM histidine, components with high mid-point temperatures are lost from the unfolding transition of the shark enzyme in 1 mM histidine, relative to that in 20 mM histidine. This is attributed to partial unfolding of the enzyme due to a high hydrostatic pressure during centrifugation of DSC samples at low ionic strength, which correlates with inactivation measurements. Addition of 10 mM NaCl to shark enzyme in 1 mM histidine protects against inactivation during centrifugation of the DSC sample, but incubation for 1 h at 20 degrees C prior to addition of NaCl results in loss of components with lower mid-point temperatures within the unfolding transition. Cations at millimolar concentration therefore afford at least two distinct modes of stabilization, likely affecting separate cooperative domains. The different thermal stabilities and denaturation temperatures of the two Na,K-ATPases correlate with the respective physiological temperatures, and may be attributed to the different lipid environments.
The rate of rotation of the rotor of the yeast vacuolar proton-ATPase (V-ATPase), relative to the stator or the steady parts of enzyme, is estimated in native vacuolar membrane vesicles of Saccharomyces cerevisiae under standardised conditions. Membrane vesicles are spontaneously formed after exposing purified yeast vacuoles to osmotic shock. The fraction of the total ATPase activity originating from V-ATPase is determined using the potent and specific inhibitor of the enzyme, concanamycin A. Inorganic phosphate liberated from ATP in the vacuolar membrane vesicle system, during 10 min of ATPase activity at 20 °C, is assayed spectrophotometrically for different concanamycin A concentrations. A fit to the quadratic binding equation, assuming a single concanamycin A binding site on a monomeric V-ATPase (our data is incompatible with models assuming more binding sites) to the inhibitor titration curve determines the concentration of the enzyme. Combining it with the known rotation:ATP stoichiometry of V-ATPase and the assayed concentration of inorganic phosphate liberated by V-ATPase leads to an average rate of ~9.53 Hz of the 360 degrees rotation, which, according to the time-dependence of the activity, extrapolates to ~14.14 Hz for the beginning of the reaction. These are low limit estimates. To our knowledge this is the first report of the rotation rate in a V-ATPase that is not subjected to genetic or chemical modification and it is not fixed on a solid support, instead it is functioning in its native membrane environment.Special Issue: Structure, function, folding and assembly of membrane proteins -Insight from Biophysics.
Organelles in eukaryotic cells often have complex shapes that deviate significantly from simple spheres. A prime example is the endoplasmic reticulum (ER) that forms an extensive network of membrane tubules in many mammalian cell types and in reconstitution assays in vitro. Despite the successful hunt for molecular determinants of ER shape we are still far from having a comprehensive understanding of ER network morphogenesis. Here, we have studied the hitherto neglected influence of the host substrate when reconstituting ER networks in vitro as compared to ER networks in vivo. In culture cells we observed cytoplasm-spanning ER networks with tubules being connected almost exclusively by three-way junctions and segment lengths being narrowly distributed around a mean length of about 1μm. In contrast, networks reconstituted from purified ER microsomes on flat glass or gel substrates of varying stiffness showed significantly broader length distributions with an up to fourfold larger mean length. Self-assembly of ER microsomes on small oil droplets, however, yielded networks that resembled more closely the native ER network of mammalian cells. We conclude from these observations that the ER microsomes' inherent self-assembly capacity is sufficient to support network formation with a native geometry if the influence of the host substrate's surface chemistry becomes negligible. We hypothesize that under these conditions the networks' preference for three-way junctions follows from creating 'starfish-shaped' vesicles when ER microsomes with a protein-induced spontaneous curvature undergo fusion.
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