Glycyl betaine is effective in slowing down the irreversible denaturation of a detergent-solubilized membrane protein, sarcoplasmic reticulum Ca2+-ATPase (SERCA1a)

Isolation of the Sarcoplasmic Reticulum Ca2+-ATPase from Rabbit Fast-Twitch Muscle

The sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) is a membrane protein that is destabilized during purification in the absence of calcium ions. The disaccharide trehalose is a protein stabilizer that accumulates in the yeast cytoplasm when under stress. In the present work, SERCA was purified by including trehalose in the purification protocol. The purified SERCA showed high protein purity (~95%) and ATPase activity. ATP hydrolysis was dependent on the presence of Ca2+ and the enzyme kinetics showed a hyperbolic dependence on ATP (Km = 12.16 ± 2.25 μM ATP). FITC labeling showed the integrity of the ATP-binding site and the identity of the isolated enzyme as a P-type ATPase. Circular dichroism (CD) spectral changes at a wavelength of 225 nm were observed upon titration with ATP, indicating α-helical rearrangements in the nucleotide-binding domain (N-domain), which correlated with ATP affinity (Km). The presence of Ca2+ did not affect FITC labeling or the ATP-mediated structural changes at the N-domain. The use of trehalose in the SERCA purification protocol stabilized the enzyme. The isolated SERCA appears to be suitable for structural and ligand binding studies, e.g., for testing newly designed or natural inhibitors. The use of trehalose is recommended for the isolation of unstable enzymes.

Screening of Detergents for Stabilization of Functional Membrane Proteins

Membrane protein studies usually require use of detergents to extract and isolate proteins from membranes and manipulate them in a soluble context for their functional or structural characterization. However, solubilization with detergent may interfere with MP stability and may directly affect MP function or structure. Moreover, detergent properties can be affected such as critical micellar concentration (CMC) can be affected by the experimental conditions. Consequently, the experimenter must pay attention to both the protein and the behavior of the detergent. This article provides a convenient protocol for estimating the CMC of detergents in given experimental conditions. Then, it presents two protocols aimed at monitoring the function of a membrane protein in the presence of detergent. Such experiments may help to test various detergents for their inactivating or stabilizing effects on long incubation times, ranging from few hours to some days. © 2018 by John Wiley & Sons, Inc.

Quality assessment of recombinant proteins by infrared spectroscopy. Characterisation of a protein aggregation related band of the Ca2+-ATPase

FTIR spectroscopy detects aggregates of recombinantly produced protein and can therefore be used for quality control.

The structural basis for phospholamban inhibition of the calcium pump in sarcoplasmic reticulum

P-type ATPases are a large family of enzymes that actively transport ions across biological membranes by interconverting between high (E1) and low (E2) ion-affinity states; these transmembrane transporters carry out critical processes in nearly all forms of life. In striated muscle, the archetype P-type ATPase, SERCA (sarco(endo)plasmic reticulum Ca(2+)-ATPase), pumps contractile-dependent Ca(2+) ions into the lumen of sarcoplasmic reticulum, which initiates myocyte relaxation and refills the sarcoplasmic reticulum in preparation for the next contraction. In cardiac muscle, SERCA is regulated by phospholamban (PLB), a small inhibitory phosphoprotein that decreases the Ca(2+) affinity of SERCA and attenuates contractile strength. cAMP-dependent phosphorylation of PLB reverses Ca(2+)-ATPase inhibition with powerful contractile effects. Here we present the long sought crystal structure of the PLB-SERCA complex at 2.8-Å resolution. The structure was solved in the absence of Ca(2+) in a novel detergent system employing alkyl mannosides. The structure shows PLB bound to a previously undescribed conformation of SERCA in which the Ca(2+) binding sites are collapsed and devoid of divalent cations (E2-PLB). This new structure represents one of the key unsolved conformational states of SERCA and provides a structural explanation for how dephosphorylated PLB decreases Ca(2+) affinity and depresses cardiac contractility.

Structure, thermodynamics and dynamics of the isotropic phase of spherical non-ionic surfactant micelles

We investigate a non-ionic surfactant (C(12)E(8))/water binary mixture, over a wide range of concentrations and temperatures (i.e. 1-35 wt.% of C(12)E(8) and 10-60 °C in temperature) by means of different experimental techniques: Small-Angle Neutron Scattering (SANS), Quasi Elastic Light Scattering (QELS) and High Frequency Rheology. The aims of this work are to provide information on structure, thermodynamics and dynamics of the isotropic phase of such a micellar system and, by combining these different types of information, to obtain a comprehensive image of the behaviour of this phase. Our results demonstrate that structural, thermodynamic and dynamic properties of these solutions are fully monitored by the temperature-induced changes in the ethylene-glycol chain hydration. They confirm that C(12)E(8) micelles are spherical and do not grow in the investigated range of concentrations and temperatures. They demonstrate that the interaction potential between C(12)E(8) micelles is more complicated than what was previously described, with an additional repulsive interaction. They allow us to put forward explanations for the Isotropic-Ordered phase transition as well as for the temperature behaviour of the viscosity of C(12)E(8) micellar solutions. Our investigation provides new and valuable information on the dynamics of these mixtures that reflect the complexity of the interaction potential between the C(12)E(8) micelles. It shows that concentrated solutions exhibit a viscoelastic behaviour that can be described by a simple Maxwell model.