Effects of cavity-creating mutations on conformational stability and structure of the dimeric 4-alpha-helical protein ROP: thermal unfolding studies.

The structural and energetic perturbations caused by cavity-creating mutations (Leu-41-->Val and Leu-41-->Ala) in the dimeric 4-alpha-helical-bundle protein ROP have been characterized by CD spectroscopy and differential scanning calorimetry (DSC). Deconvolution of the CD spectra showed a decrease in alpha-helicity as a result of the amino acid exchanges that follows qualitatively the overall decrease in conformational stability. Transition enthalpies are sensitive probes of the energetic change associated with point mutations. delta H zero values at the respective transition temperatures, T 1/2 (71.0, 65.3, and 52.9 degrees C at 0.5 mg/ml) decrease from 580 +/- 20 to 461 +/- 20 kJ/(mol of dimer) and 335 +/- 20 kJ/(mol of dimer) for wild-type ROP (Steif, C., Weber, P., Hinz, H.-J., Flossdorf, J., Cesareni, G., Kokkinidis, M. Biochemistry 32:3867-3876, 1993), L41V, and L41A, respectively. The conformational stabilities at 25 degrees C expressed by the standard Gibbs energies of denaturation, delta GzeroD, are 71.7, 61.1, and 46.1 kJ/(mol of dimer). The corresponding transition enthalpies have been obtained from extrapolation using the cDp(T) and cNp(T) functions. Their values at 25 degrees C are 176.3, 101.9, and 141.7 kJ/(mol of dimer) for wild-type ROP, L41V, and L41A, respectively. When the stability perturbation resulting from the cavity creating mutations is referred to the exchange of 1 mol of CH2 group, the average delta delta GzeroD value is -5.0 +/- 1 kJ/(mol of CH2 group). This decrease in conformation stability suggests that dimeric ROP exhibits the same susceptibility to Leu-->Val and Leu-->Ala exchanges as small monomeric proteins. Careful determinations of the partial specific heat capacities of wild-type and mutated protein solutions suggest that the mutational effects are predominantly manifested in the native rather than the unfolded state.

Restored heptad pattern continuity does not alter the folding of a four-alpha-helix bundle.

The sequences of alpha-helical coiled-coils and bundles are characterized by a specific pattern of hydrophobic and hydrophilic residues which is repeated every seven residues. Highly conserved breaks in this pattern frequently occur in segments of otherwise continuous heptad substructures. The hairpin bend of the ROP protein coincides with such a break and provides a model system for the study of the structural effects induced by heptad discontinuities. The structure of a ROP mutant which re-establishes a continuous heptad pattern, shows insignificant changes relative to the wild-type protein, as is also reflected in its conformational stability, spectroscopic properties and unfolding behaviour. Thus, formation of alpha-alpha-hairpin bends may occur both in the presence and absence of heptad breaks.

Structure-activity studies of human sterol carrier protein 2.

Recombinant human sterol carrier protein 2 (SCP2) variants were generated by site-directed mutagenesis and expression in Escherichia coli. The ability of the variants to stimulate microsomal conversion of 7-dehydrocholesterol to cholesterol (sterol carrier activity) and to transfer cholesterol and phosphatidylcholine from donor small unilamellar vesicles to acceptor membranes (cholesterol and phosphatidylcholine transfer activities) was compared with wild-type recombinant SCP2. Our results indicate that all measured activities of recombinant human pre-SCP2 (including the 20-amino acid leader sequence) and mature SCP2 were similar. Expressed glutathione S-transferase fusion proteins (GST-SCP2 and GST-pre-SCP2) possessed considerable activity, suggesting that steric obstruction at the amino terminus causes only minor inactivation. The effect of progressive removal of peptides from the carboxyl terminus showed that amino acids between Lys100 and Asn104 are essential for SCP2 activity. This conclusion was substantiated by the observation that replacing Asn104 with Asp or Ile caused considerable inactivation, whereas replacing Met105 with Leu had almost no effect. Since N-ethylmaleimide is known to inhibit SCP2 activity, substitutions were also introduced in the vicinity of Cys71. Whereas Val71 and Ser71 variants possessed wild-type activity, replacing Asp70 with Asn almost completely abolished SCP2 activity. Further, the importance of residues located close to the amino terminus was indicated by complete inactivation of a 10-amino-terminal amino acid deletion mutant and by replacing Leu20 with Glu. Circular dichroism results showed that Leu20 and Asp70 may serve to stabilize the overall fold, whereas residue 104 appears to play a role in the specific lipid binding and/or transfer activity of SCP2.

Subunit interactions provide a significant contribution to the stability of the dimeric four-alpha-helical-bundle protein ROP.

Detailed thermodynamic and spectroscopic studies were carried out on the ColE1-ROP protein in order to establish a quantitative basis for the contribution of noncovalent interactions to the stability of four-helix-bundle proteins. The energetics of both heat- and GdnHCl-induced denaturation were measured by differential scanning microcalorimetry (DSC) and/or by following the change in circular dichroism in the far-UV range. Sedimentation equilibrium analyses were performed to characterize the state of aggregation of the protein. No intermediate species could be detected during thermal unfolding of the dimer in the absence of GdnHCl. Under these conditions ROP unfolding exhibits a strict two-state behavior. The thermodynamic parameters for the reaction N2<->2D are delta HD = 580 +/- 20 kJ.(mol of dimer)-1, delta Cp = 10.3 +/- 1.3 kJ.(mol of dimer)-1.K-1, and Tm = 71.0 +/- 0.5 degrees C. The corresponding Gibbs energy change of unfolding is delta GD degree = 71.7 kJ.(mol of dimer)-1 at 25 degrees C and pH 6. In the presence of 2.5 M GdnHCl, however, ROP dissociates into monomers at elevated temperatures, as the loss of the concentration dependence of Tm and the decreased molecular weight demonstrate. The corresponding transition parameters are delta HD (2.5 M GdnHCl) = 130 +/- 10 kJ.(mol of monomer)-1 and Tm = 51.6 +/- 0.3 degrees C. Isothermal unfolding studies at 19 degrees C using GdnHCl as denaturant yielded a Gibbs energy change of unfolding of 22.4 kJ.(mol of monomer)-1. This extrapolated value is 38% lower than the corresponding delta GD degree value of 35.85 kJ.(mol of monomer)-1 calculated from thermal unfolding for the monomer in the absence of GdnHCl, where the protein is known to be a dimer. These results suggest that subunit interactions are an important source of stabilization of the native four-helix-bundle structure of ROP.