The charge and structural stability of apolipoprotein A-I in discoidal and spherical recombinant high density lipoprotein particles.

The details of how high density lipoprotein (HDL) microstructure affects the conformation and net charge of apolipoprotein (apo) A-I in various classes of HDL particles have been investigated in homogeneous recombinant HDL (rHDL) particles containing apoA-I, palmitoyl-oleoyl phosphatidylcholine (POPC) and cholesteryl oleate. Isothermal denaturation with guanidine HCl was used to monitor alpha-helix structural stability, whereas electrokinetic analyses and circular dichroism were used to determine particle charge and apoA-I secondary structure, respectively. Electrokinetic analyses show that at pH 8.6 apoA-I has a net negative charge on discoidal (POPC.apoA-I) particles (-5.2 electronic units/mol of apoA-I) which is significantly greater than that of apoA-I either free in solution or on spherical (POPC.cholesteryl oleate.apoA-I) rHDL (approximately -3.5 electronic units). Raising the POPC content (32-128 mol/ml of apoA-I) of discoidal particles 1) increases the particle major diameter from 9.3 to 12.1 nm, 2) increases the alpha-helix content from 62 to 77%, and 3) stabilizes the helical segments by increasing the free energy of unfolding (delta GD degree) from 1.4 to 3.0 kcal/mol of apoA-I. Raising the POPC content (28-58 mol/mol of apoA-I) of spherical particles 1) increases the particle diameter from 7.4 to 12.6 nm, 2) increases the percent alpha-helix from 62 to 69%, and 3) has no significant effect on delta GD degree (2.2 kcal/mol of apoA-I). This study shows that different HDL subspecies maintain particular apoA-I conformations that confer unique charge and structural characteristics on the particles. It is likely that the charge and conformation of apoA-I are critical molecular properties that modulate the metabolism of HDL particles and influence their role in cholesterol transport.     

Lipid-binding studies of human apolipoprotein A-I and its terminally truncated mutants

Apolipoprotein A-I (apoA-I, 243 amino acids) is the major protein of high-density lipoproteins (HDL) that plays an important structural and functional role in lipid transport and metabolism. The central region of apoA-I (residues 60-183) is predicted to contain exclusively amphipathic alpha-helices formed from tandem 22-mer sequence repeats. To analyze the lipid-binding properties of this core domain, four terminally truncated mutants of apoA-I, Delta(1-41), Delta(1-59), Delta(1-41,185-243), and Delta(1-59,185-243), were expressed in baculovirus infected Sf-9 cells. The effects of mutations on the ability of apoA-I to form bilayer disk complexes with dimyristoyl phosphatidylcholine (DMPC) that resemble nascent HDL were analyzed by density gradient ultracentrifugation and electron microscopy (EM). The N-terminal deletion mutants, Delta(1-41) and Delta(1-59), showed altered lipid-binding ability as compared to plasma and wild-type apoA-I, and in the double deletion mutants, Delta(1-41, 185-243) and Delta(1-59, 185-243), the lipid binding was abolished. Thermal unfolding of variant apoA-I/DMPC complexes monitored by circular dichroism (CD) showed hysteresis and a shift in the melting curves by about -12 degrees C upon reduction in the heating rate from 1.0 to 0.067 K/min. This indicates an irreversible kinetically controlled transition with a high activation energy E(a) = 60 +/- 5 kcal/mol. CD and EM studies of the apoA-I/DMPC complexes at different pH demonstrated that changes in the net charge or in the charge distribution on the apoA-I molecule have critical effects on the conformation and lipid-binding ability of the protein.     

Effects of the core lipid on the energetics of binding of ApoA-I to model lipoprotein particles of different sizes

Interaction of apolipoproteins (apo) with lipid surfaces plays crucial roles in lipoprotein metabolism and cholesterol homeostasis. To elucidate the thermodynamics of binding of apoA-I to lipid, we used lipid emulsions composed of triolein (TO) and egg phosphatidylcholine (PC) as lipoprotein models. Determination of the level of binding of wild-type (WT) apoA-I and some deletion mutants to large (120 nm diameter; LEM) and small (35 nm diameter; SEM) emulsions indicated that N-terminal (residues 44-65) and C-terminal (residues 190-243 and 223-243) deletions have large effects on lipid interaction, whereas deletion of the central region (residues 123-166) has little effect. Substitution of amino acids at either L230 or L230, L233, and Y236 with proline residues also decreases the level of binding, indicating that an alpha-helix conformation in this C-terminal region is required for efficient lipid binding. Calorimetry showed that binding of WT apoA-I to SEM generates endothermic heat (DeltaH approximately 30 kcal/mol) in contrast to the exothermic heat (ca. -85 kcal/mol) generated upon binding to LEM and egg PC small unilamellar vesicles (SUV). This exothermic heat arises from an approximately 25% increase in alpha-helix content, and it drives the binding of apoA-I to LEM and SUV. There is a similar increase in alpha-helix content of apoA-I upon binding to either SEM or SUV, but the binding of apoA-I to SEM is an entropy-driven process. These results suggest that the presence of a core triglyceride modifies the highly curved SEM surface packing and thereby the thermodynamics of apoA-I binding in a manner that compensates for the exothermic heat generated by alpha-helix formation.     

Alpha-helix formation is required for high affinity binding of human apolipoprotein A-I to lipids

Apolipoprotein (apo) A-I is thought to undergo a conformational change during lipid association that results in the transition of random coil to alpha-helix. Using a series of deletion mutants lacking different regions along the molecule, we examined the contribution of alpha-helix formation in apoA-I to the binding to egg phosphatidylcholine (PC) small unilamellar vesicles (SUV). Binding isotherms determined by gel filtration showed that apoA-I binds to SUV with high affinity and deletions in the C-terminal region markedly decrease the affinity. Circular dichroism measurements demonstrated that binding to SUV led to an increase in alpha-helix content, but the helix content was somewhat less than in reconstituted discoidal PC.apoA-I complexes for all apoA-I variants, suggesting that the helical structure of apoA-I on SUV is different from that in discs. Isothermal titration calorimetry showed that the binding of apoA-I to SUV is accompanied by a large exothermic heat and deletions in the C-terminal regions greatly decrease the heat. Analysis of the rate of release of heat on binding, as well as the kinetics of quenching of tryptophan fluorescence by brominated PC, indicated that the opening of the N-terminal helix bundle is a rate-limiting step in apoA-I binding to the SUV surface. Significantly, the correlation of thermodynamic parameters of binding with the increase in the number of helical residues revealed that the contribution of alpha-helix formation upon lipid binding to the enthalpy and the free energy of the binding of apoA-I is -1.1 and -0.04 kcal/mol per residue, respectively. These results indicate that alpha-helix formation, especially in the C-terminal regions, provides the energetic source for high affinity binding of apoA-I to lipids.     

Quantitative analysis of helix-coil transition of block copolypeptide, Glu12-Ala12, by combined use of CD and NMR spectroscopy

To investigate helix-coil transition mechanisms, conformations of Glu12-Ala12, EA, in aqueous solution have been studied in detail over the pH range from 2 to 8 and the temperature range from 20 to 60 degrees C using CD and NMR spectroscopy. The 750-MHz NMR spectra displayed excellent dispersion of the backbone amide proton signals, and permitted essentially complete sequence-specific resonance assignments. These assignments, together with short- and medium-range nuclear Overhauser effect (NOE) constraints and coupling constants, enable us to analyze conformational characteristics of all the residues in the EA peptide individually. A combined use of CD and NMR techniques reveals that the EA peptide assumes a stable alpha-helix from Glu12 to Ala19 in 0.1 M NaCl solution at 20 degrees C above pH 7. The alpha-helix is getting longer as decreasing pH. Below pH 4, the peptide assumes the longest alpha-helix from Glu3 to Ala23. The important observation of the present study is that the helix-coil transition occurs stepwise, residue by residue, from both the N- and C-termini of the alpha-helix. No conformational equilibrium between the helical and random-coil states is detected for the residues in the central region of the alpha-helix. Quantitative analysis of temperature-induced helix-to-coil transitions at various pHs provides a pH-independent residual enthalpy change delta H(r) = 0.95 kcal res(-1). Similar values have been reported for a 50-residue alanine-rich peptide (1.2 kcal res(-1)), poly-L-glutamate (1.1 kcal res(-1)), poly-L-lysine (1.1 kcal res(-1)), and poly-L-alanine (0.86 kcal res(-1)). Those investigations, along with our present result, suggest that delta H(r) is mainly determined by the transformation of the backbone associated with the disruption of the intramolecular hydrogen bond. These results should increase our understanding of the helix-coil transition.     

Thermodynamics of lipid protein associations. Thermodynamics of helix formation in the association of high density apolipoprotein A-I (apoA-I) to dimyristoyl phosphatidylcholine.

The structure and phospholipid-binding properties of human plasma high density apolipoprotein A-I (apoA-I) has been studied at pH 7.4 and 3.1 by microcalorimetry, circular dichroism and density gradient ultracentrifugation. At pH values of 7.4 and 3.1, apoA-I binds to dimyristoyl phosphatidylcholine (DMPC) to form complexes of similar composition (molar ratio of DMPC/apoA-I of 100) and helical content (67%). At pH 7.4, the lipid-protein association is accompanied by an increase in helical content from 58 to 67% and an exothermic enthalpy of binding (deltaHB) of -90 kcal/mol apoA-I. At pH 3.1, the helical content of apoA-I is increased from 48 to 67% on binding to DMPC and the enthalpy of binding was -170 kcal/mol. We suggest that the difference in the enthalpies of binding (-80 kcal/mol) at pH 3.1 compared to 7.4 is due to the greater coil leads to helix transition at the lower pH.     

The central helices of ApoA-I can promote ATP-binding cassette transporter A1 (ABCA1)-mediated lipid efflux. Amino acid residues 220-231 of the wild-type ApoA-I are required for lipid efflux in vitro and high density lipoprotein formation in vivo

We have mapped the domains of lipid-free apoA-I that promote cAMP-dependent and cAMP-independent cholesterol and phospholipid efflux. The cAMP-dependent lipid efflux in J774 mouse macrophages was decreased by approximately 80-92% by apoA-I[delta(185-243)], only by 15% by apoA-I[delta(1-41)] or apoA-I[delta(1-59)], and was restored to 75-80% of the wild-type apoA-I control value by double deletion mutants apoA-I[delta(1-41)delta(185-243)] and apoA-I[delta(1-59)delta(185-243)]. Similar results were obtained in HEK293 cells transfected with an ATP-binding cassette transporter A1 (ABCA1) expression plasmid. The double deletion mutant of apoA-I had reduced thermal and chemical stability compared with wild-type apoA-I. Sequential carboxyl-terminal deletions showed that cAMP-dependent cholesterol efflux was diminished in all the mutants tested, except the apoA-I[delta(232-243)] which had normal cholesterol efflux. In cAMP-untreated or in mock-transfected cells, cholesterol efflux was not affected by the amino-terminal deletions, but decreased by 30-40% and 50-65% by the carboxyl-terminal and double deletions, respectively. After adenovirus-mediated gene transfer in apoA-I-deficient mice, wild-type apoA-I and apoA-I[delta(1-41)] formed spherical high density lipoprotein (HDL) particles, whereas apoA-I[delta(1-41)delta(185-243)] formed discoidal HDL. The findings suggest that although the central helices of apoA-I alone can promote ABCA1-mediated lipid efflux, residues 220-231 are necessary to allow functional interactions between the full-length apoA-I and ABCA1 that are required for lipid efflux and HDL biogenesis.     

Conformational stability and mechanism of folding of ribonuclease T1

Urea and thermal unfolding curves for ribonuclease T1 (RNase T1) were determined by measuring several different physical properties. In all cases, steep, single-step unfolding curves were observed. When these results were analyzed by assuming a two-state folding mechanism, the plots of fraction unfolded protein versus denaturant were coincident. The dependence of the free energy of unfolding, delta G (in kcal/mol), on urea concentration is given by delta G = 5.6 - 1.21 (urea). The parameters characterizing the thermodynamics of unfolding are: midpoint of the thermal unfolding curve, Tm = 48.1 degrees C, enthalpy change at Tm, delta Hm = 97 kcal/mol, and heat capacity change, delta Cp = 1650 cal/mol deg. A single kinetic phase was observed for both the folding and unfolding of RNase T1 in the transition and post-transition regions. However, two slow kinetic phases were observed during folding in the pre-transition region. These two slow phases account for about 90% of the observed amplitude, indicating that a faster kinetic phase is also present. The slow phases probably result from cis-trans isomerization at the 2 proline residues that have a cis configuration in folded RNase T1. These results suggest that RNase T1 folds by a highly cooperative mechanism with no structural intermediates once the proline residues have assumed their correct isomeric configuration. At 25 degrees C, the folded conformation is more stable than the unfolded conformations by 5.6 kcal/mol at pH 7 and by 8.9 kcal/mol at pH 5, which is the pH of maximum stability. At pH 7, the thermodynamic data indicate that the maximum conformational stability of 8.3 kcal/mol will occur at -6 degrees C.     

Conformation and lipid binding of a C-terminal (198-243) peptide of human apolipoprotein A-I

Human apolipoprotein A-I (apoA-I) is the principle apolipoprotein of high-density lipoproteins that are critically involved in reverse cholesterol transport. The intrinsically flexibility of apoA-I has hindered studies of the structural and functional details of the protein. Our strategy is to study peptide models representing different regions of apoA-I. Our previous report on [1-44]apoA-I demonstrated that this N-terminal region is unstructured and folds into approximately 60% alpha-helix with a moderate lipid binding affinity. We now present details of the conformation and lipid interaction of a C-terminal 46-residue peptide, [198-243]apoA-I, encompassing putative helix repeats 10 and 9 and the second half of repeat 8 from the C-terminus of apoA-I. Far-ultraviolet circular dichroism spectra show that [198-243]apoA-I is also unfolded in aqueous solution. However, self-association induces approximately 50% alpha-helix in the peptide. The self-associated peptide exists mainly as a tetramer, as determined by native electrophoresis, cross-linking with glutaraldehyde, and unfolding data from circular dichroism (CD) and differential scanning calorimetry (DSC). In the presence of a number of lipid-mimicking detergents, above their CMC, approximately 60% alpha-helix was induced in the peptide. In contrast, SDS, an anionic lipid-mimicking detergent, induced helical folding in the peptide at a concentration of approximately 0.003% (approximately 100 microM), approximately 70-fold below its typical CMC (0.17-0.23% or 6-8 mM). Both monomeric and tetrameric peptide can solubilize dimyristoylphosphatidylcholine (DMPC) liposomes and fold into approximately 60% alpha-helix. Fractionation by density gradient ultracentrifugation and visualization by negative staining electromicroscopy demonstrated that the peptide binds to DMPC with a high affinity to form at least two sizes of relatively homogeneous discoidal HDL-like particles depending on the initial lipid:peptide ratio. The characteristics (lipid:peptide weight ratio, diameter, and density) of both complexes are similar to those of plasma A-I/DMPC complexes formed under similar conditions: small discoidal complexes (approximately 3:1 weight ratio, approximately 110 A, and approximately 1.10 g/cm3) formed at an initial 1:1 weight ratio and larger discoidal complexes (approximately 4.6:1 weight ratio, approximately 165 A, and approximately 1.085 g/cm3) formed at initial 4:1 weight ratio. The cross-linking data for the peptide on the complexes of two sizes is consistent with the calculated peptide numbers per particle. Compared to the approximately 100 A disk-like complex formed by the N-terminal peptide in which helical structure was insufficient to cover the disk edge by a single belt, the compositions of these two types of complexes formed by the C-terminal peptide are more consistent with a "double belt" model, similar to that proposed for full-length apoA-I. Thus, our data provide direct evidence that this C-terminal region of apoA-I is responsible for the self-association of apoA-I, and this C-terminal peptide model can mimic the interaction with the phospholipid of plasma apoA-I to form two sizes of homogeneous discoidal complexes and thus may be responsible for apoA-I function in the formation and maintenance of HDL subspecies in plasma.     

Site-directed mutagenesis and structure-function analysis of the human apolipoprotein A-I. Relation between lecithin-cholesterol acyltransferase activation and lipid binding.

We have mutagenized the human apoA-I gene and have generated cell lines which express normal and mutant apoA-I forms. Point mutations were introduced which changed Gln-1, Gln-2 to Arg,Arg, Pro99 to His, and Pro121 to His. In addition, the following amino acid deletions (delta) were generated: delta 113-124, delta 148-186, delta 212-233, and delta 213-243. The apoA-I form isolated from the culture medium of C127 cells was analyzed for its ability to activate lecithin-cholesterol acyltransferase (LCAT) and to bind to phospholipid vesicles and high density lipoprotein (HDL). Compared with the wild type (WT) apoA-I, the relative activation of LCAT achieved by the point mutations Gln-1, Gln-2----Arg,Arg, Pro99----His, and Pro121----His were 106 +/- 7, 92 +/- 6, and 77 +/- 9%, respectively. Kinetic analysis of one mutant apoA-I form showed similar Vmax but a 15-fold increase in the Km of the mutant apoA-I form. Furthermore, the activation achieved by the internal deletion mutants delta 113-124, delta 148-186, delta 212-233, and delta 213-243 was 47 +/- 3, 0.5 +/- 0.4, 28 +/- 4 and 13 +/- 5%, respectively. Mutants deficient in their ability to activate LCAT displayed alterations in liposome and HDL binding, compared with WT as determined by density gradient ultracentrifugation analysis of the culture medium. Thus, the peak recovery (approximately 50%) of apoA-I bound to HDL was at density 1.14 g/ml for the WT apoA-I, at 1.18 g/ml for the mutants delta 113-124 and delta 148-186, and at d greater than 1.21 g/ml for the delta 212-233 and delta 213-243. Electron microscopy of the proteoliposome LCAT substrate generated by WT and mutant apoA-I forms showed that the carboxyl-terminal deletion mutants which displayed aberrant binding to HDL also displayed reduced ability to convert the spherical lecithin-cholesterol vesicles into discs compared with WT. The findings suggest that (a) the importance of the carboxyl terminus of apoA-I for LCAT activation is related to its ability to bind to lipid and/or to form discoidal substrate for LCAT, and (b) the interaction of several domains of apoA-I are required for the activation of LCAT.     

Probing the lipid-free structure and stability of apolipoprotein A-I by mutation

To probe the secondary structure of the C-terminus (residues 165-243) of lipid-free human apolipoprotein A-I (apoA-I) and its role in protein stability, recombinant wild-type and seven site-specific mutants have been produced in C127 cells, purified, and studied by circular dichroism and fluorescence spectroscopy. A double substitution (G185P, G186P) increases the protein stability without altering the secondary structure, suggesting that G185 and G186 are located in a loop/disordered region. A triple substitution (L222K, F225K, F229K) leads to a small increase in the alpha-helical content and stability, indicating that L222, F225, and F229 are not involved in stabilizing hydrophobic core contacts. The C-terminal truncation Delta(209-243) does not change the alpha-helical content but reduces the protein stability. Truncation of a larger segment, Delta(185-243), does not affect the secondary structure or stability. In contrast, an intermediate truncation, Delta(198-243), leads to a significant reduction in the alpha-helical content, stability, and unfolding cooperativity. The internal 11-mer deletion Delta(187-197) has no significant effect on the conformation or stability, whereas another internal 11-mer deletion, Delta(165-175), dramatically disrupts and destabilizes the protein conformation, suggesting that the presence of residues 165-175 is crucial for proper apoA-I folding. Overall, the findings suggest the presence of stable helical structure in the C-terminal region 165-243 of lipid-free apoA-I and the involvement of segment 209-243 in stabilizing interactions in the molecule. The effect of the substitution (G185P, G186P) on the exposure of tryptophans located in the N-terminal half suggests an apoA-I tertiary conformation with the C-terminus located close to the N-terminus.     

The N-terminal (1-44) and C-terminal (198-243) peptides of apolipoprotein A-I behave differently at the triolein/water interface

Apolipoprotein A-I (apoA-I), the major protein of high-density lipoprotein (HDL), moves between HDL and triacylglycerol-rich lipoproteins during metabolism. We reported that apoA-I is conformationally flexible at the triolein/water (TO/W) interface, partially desorbing at low surface pressure (Pi) but totally desorbing at Pi > 19 mN/m. We now report the different behavior of the N- and C-terminal peptides of apoA-I ([1-44]apoA-I and [198-243]apoA-I) at the TO/W interface. While both peptides are surface active, [198-243]apoA-I is more stable at the TO/W interface. At equilibrium interfacial tension both peptides desorb from the interface when compressed, but [1-44]apoA-I is pushed off at 13 mN/m while [198-243]apoA-I can withstand Pi = 16 mN/m. Neither peptide is very elastic or flexible at the interface. Only at small changes of area (<8%), fast oscillations (4 and 8 s periods), and relatively low concentrations (2 x 10(-7) M) do these peptides show elastic behavior but with a relatively small modulus compared to that of apoA-I. When mixed together, they appear not to interact on the surface. [1-44]ApoA-I binds more rapidly but is replaced by [198-243]apoA-I within minutes. We suggest that when apoA-I partially desorbs from lipoprotein surfaces during lipid metabolism, the N-terminal is the first to detach while the C-terminal remains on the interface and only desorbs at higher pressures. Thus, the observations that different domains of apoA-I adsorb or desorb with small variations in surface pressure make apoA-I a very flexible protein with multiple functions, one of which is to stabilize surface pressure during lipoprotein metabolism as lipids move in and out of the lipoprotein surface.     

Influence of tertiary structure domain properties on the functionality of apolipoprotein A-I

The tertiary structure of apolipoprotein (apo) A-I and the contributions of structural domains to the properties of the protein molecule are not well defined. We used a series of engineered human and mouse apoA-I molecules in a range of physical-biochemical measurements to address this issue. Circular dichroism measurements of alpha-helix thermal unfolding and fluorescence spectroscopy measurements of 8-anilino-1-napthalenesulfonic acid binding indicate that removal of the C-terminal 54 amino acid residues from human and mouse apoA-I has similar effects; the molecules are only slightly destabilized, and there is a decrease in hydrophobic surface exposure. These results are consistent with both human and mouse apoA-I adopting a two-domain tertiary structure, comprising an N-terminal antiparallel helix bundle domain and a separate less ordered C-terminal domain. Mouse apoA-I is significantly less resistant than human apoA-I to thermal and chemical denaturation; the midpoint of thermal unfolding of mouse apoA-I at 45 degrees C is 15 degrees C lower and the midpoint of guanidine hydrochloride denaturation (D1/2) occurs at 0.5 M as compared to 1.0 M for human apoA-I. These differences reflect the overall greater stability of the helix bundle formed by residues 1-189 in human apoA-I. Measurements of the heats of binding to egg phosphatidylcholine (PC) small unilamellar vesicles and the kinetics of solubilization of dimyristoyl PC multilamellar vesicles indicate that the more stable human helix bundle interacts poorly with lipids as compared to the equivalent mouse N-terminal domain. The C-terminal domain of human apoA-I is much more hydrophobic than that of mouse apoA-I; in the lipid-free state the human C-terminal domain (residues 190-243) is partially alpha-helical and undergoes cooperative unfolding (D1/2 = 0.3 M) whereas the isolated mouse C-terminal domain (residues 187-240) is disordered in dilute solution. The human C-terminal domain binds to lipid surfaces much more avidly than the equivalent mouse domain. Human and mouse apoA-I have very different tertiary structure domain contributions for achieving functionality. It is clear that the stability of the N-terminal helix bundle, and the hydrophobicity and alpha-helix content of the C-terminal domain, are critical factors in determining the overall properties of the apoA-I molecule.     

Conformation and lipid binding of the N-terminal (1-44) domain of human apolipoprotein A-I

Because of its role in reverse cholesterol transport, human apolipoprotein A-I is the most widely studied exchangeable apolipoprotein. Residues 1-43 of human apoA-I, encoded by exon 3 of the gene, are highly conserved and less well understood than residues 44-243, encoded by exon 4. In contrast to residues 44-243, residues 1-43 do not contain the 22 amino acid tandem repeats thought to form lipid binding amphipathic helices. To understand the structural and functional roles of the N-terminal region, we studied a synthetic peptide representing the first 44 residues of human apoA-I ([1-44]apoA-I). Far-ultraviolet circular dichroism spectra showed that [1-44]apoA-I is unfolded in aqueous solution. However, in the presence of n-octyl beta-d-glucopyranoside, a nonionic lipid mimicking detergent, above its critical micelle concentration ( approximately 0.7% at 25 degrees C), sodium dodecyl sulfate, an ionic detergent, above its CMC ( approximately 0.2%), trimethylamine N-oxide, a folding inducing organic osmolyte, or trifluoroethanol, an alpha-helix inducer, alpha-helical structure was formed in [1-44]apoA-I up to approximately 45%. Characterization by density gradient ultracentrifugation and visualization by negative staining electron microscopy demonstrated that [1-44]apoA-I interacts with dimyristoylphosphatidylcholine (DMPC) over a wide range of lipid:peptide ratios from 1:1 to 12:1 (w/w). At 1:1 DMPC:[1-44]apoA-I (w/w) ratio, discoidal complexes with composition approximately 4:1 (w/w) and approximately 100 A diameter were formed in equilibrium with free peptide. At higher ratios, discoidal complexes were shown to exist together with a heterogeneous population of lipid vesicles with peptide bound also in equilibrium with free peptide. When bound to DMPC, [1-44]apoA-I has approximately 60% helical structure, independent of whether it forms discoidal or vesicular complexes. This helical content is consistent with that of the predicted G helix (residues 8-33). Our data provide the first strong and direct evidence that the N-terminal region of apoA-I binds lipid and can form discoidal structures and a heterogeneous population of vesicles. In doing so, approximately 60% of this region folds into alpha-helix from random coil. The composition of the 100 A discoidal complex is approximately 5 [1-44]apoA-I and approximately 150 DMPC molecules per disk. The helix length of 5 [1-44]apoA-I molecules in lipid-bound form is just long enough to wrap around the DMPC bilayer disk once.     

Free energy changes in ribonuclease A denaturation. Effect of urea, guanidine hydrochloride, and lithium salts

The unfolding of ribonuclease A by urea, guanidine hydrochloride, lithium perchlorate, lithium chloride, and lithium bromide has been followed by circular dichroic and difference spectral measurements. All three abnormal tyrosyl residues are normalized in urea and guanidine hydrochloride (delta epsilon 287 = -2700), only two are normalized in lithium bromide and lithium perchlorate (delta epsilon 287 = -1700), and only one is exposed in lithium chloride solutions (delta epsilon 287 = -700). The Gibbs energies are 4.7 +/- 0.1 kcal mol-1 for urea- and guanidine hydrochloride-denaturation, 3.8 +/- 0.2 kcal mol-1 for lithium perchlorate-denaturation, and 12.7 +/- 0.2 kcal mol-1 for lithium chloride- and lithium bromide-denaturation of ribonuclease A. The latter results suggest that the mechanism of the unfolding process in urea and guanidine hydrochloride is quite different from that in lithium salts.     

Cross-linking and lipid efflux properties of apoA-I mutants suggest direct association between apoA-I helices and ABCA1

To explore the functional interactions between apoA-I and ABCA1, we correlated the cross-linking properties of several apoA-I mutants with their ability to promote cholesterol efflux. In a competitive cross-linking assay, amino-terminal deletion and double amino- and carboxy-terminal deletion mutants of apoA-I competed effectively the cross-linking of WT (125)I-apoA-I to ABCA1, while the carboxy-terminal deletion mutant apoA-I[Delta(220-243)] competed poorly. Direct cross-linking of WT apoA-I, amino-terminal, and double deletion mutants of apoA-I to ABCA1 showed similar apparent K(d) values (49-74 nM), whereas the apparent K(d) values of the carboxy-terminal deletion mutants apoA-I[Delta(185-243)] and apoA-I[Delta(220-243)] were increased 3-fold. Analysis of several internal deletions and point mutants of apoA-I showed that apoA-I[Delta(61-78)], apoA-I[Delta(89-99)], apoA-I[Delta(136-143)], apoA-I[Delta(144-165)], apoA-I[D102A/D103A], apoA-I[E125K/E128K/K133E/E139K], apoA-I[L141R], apoA-I[R160V/H162A], and WT apoA-I had similar ABCA1-mediated lipid efflux, and all competed efficiently the cross-linking of WT (125)I-apoA-I to ABCA1. WT apoA-I and ABCA1 could be cross-linked with a 3 A cross-linker. The WT apoA-I, amino, carboxy and double deletion mutants of apoA-I showed differences in the cross-linking to WT ABCA1 and the mutant ABCA1[W590S]. The findings are consistent with a direct association of different combinations of apoA-I helices with a complementary ABCA1 domain. Mutations that alter ABCA1/apoA-I association affect cholesterol efflux and inhibit biogenesis of HDL.     

Preferential assembly of the tropomyosin heterodimer: equilibrium studies

Thermal unfolding/refolding studies of the three tropomyosin dimers, alpha alpha, alpha beta, and beta beta, from chicken gizzard muscle were performed to explain the preferential assembly of alpha- and beta-tropomyosin subunits into heterodimers, alpha beta [Lehrer, S. S., & Qian, Y. (1989) J. Biol. Chem. 265, 1134]. Circular dichroism measurements showed that all three dimers unfolded in cooperative reversible transitions with T1/2 = 40.0 degrees C and delta H degrees = 162 kcal/mol for alpha alpha and with T1/2 = 42.6 degrees C and delta H degree = 98 kcal/mol for beta beta at 0.4-0.5 microM concentrations. Fluorescence measurements on pyrenyliodoacetamide-labeled tropomyosin showed that (i) excimer fluorescence decreases in parallel with unfolding of homodimers, (ii) at physiological temperature, heterodimers are formed from micromolar mixtures of homodimers over a period of minutes, and (iii) heterodimers unfold/refold with temperature without appreciable formation of homodimers. To understand the preferential formation of alpha beta, we calculated the concentrations of all species present as a function of temperature for equal total amounts of alpha and beta, using the measured thermodynamic constants of the unfolding/dissociation equilibria for alpha alpha and beta beta. Values for delta H degrees = 225 kcal/mol and T1/2 = 43 degrees C for unfolding of alpha beta at 0.5 microM concentration were obtained from the best fit of the calculations to the measured helical content vs temperature of alpha beta.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of N-terminal helix bundle stability on the lipid-binding properties of human apolipoprotein A-I

As the principal component of high-density lipoprotein (HDL), apolipoprotein (apo) A-I plays essential roles in lipid transport and metabolism. Because of its intrinsic conformational plasticity and flexibility, the molecular details of the tertiary structure of lipid-free apoA-I have not been fully elucidated. Previously, we demonstrated that the stability of the N-terminal helix bundle structure is modulated by proline substitution at the most hydrophobic region (residues around Y18) in the N-terminal domain. Here we examine the effect of proline substitution at S55 located in another relatively hydrophobic region compared to most of the helix bundle domain to elucidate the influences on the helix bundle structure and lipid interaction. Fluorescence measurements revealed that the S55P mutation had a modest effect on the stability of the bundle structure, indicating that residues around S55 are not pivotally involved in the helix bundle formation, in contrast to the insertion of proline at position 18. Although truncation of the C-terminal domain (Δ190-243) diminishes the lipid binding of apoA-I molecule, the mutation S55P in addition to the C-terminal truncation (S55P/Δ190-243) restored the lipid binding, suggesting that the S55P mutation causes a partial unfolding of the helix bundle to facilitate lipid binding. Furthermore, additional proline substitution at Y18 (Y18P/S55P/Δ190-243), which leads to a drastic unfolding of the helix bundle structure, yielded a greater lipid binding ability. Thus, proline substitutions in the N-terminal domain of apoA-I that destabilized the helix bundle promoted lipid solubilization. These results suggest that not only the hydrophobic C-terminal helical domain but also the stability of the N-terminal helix bundle in apoA-I are important modulators of the spontaneous solubilization of membrane lipids by apoA-I, a process that leads to the generation of nascent HDL particles.     

Opposing effects of phosphoenolpyruvate and pyruvate with Mg(2+) on the conformational stability and dimerization of phosphotransferase enzyme I from Escherichia coli

The activity of enzyme I (EI), the first protein in the bacterial PEP:sugar phosphotransferase system, is regulated by a monomer-dimer equilibrium where a Mg(2+)-dependent autophosphorylation by PEP requires the homodimer. Using inactive EI(H189A), in which alanine is substituted for the active-site His189, substrate-binding effects can be separated from those of phosphorylation. Whereas 1 mM PEP (with 2 mM Mg(2+)) strongly promotes dimerization of EI(H189A) at pH 7.5 and 20 degrees C, 5 mM pyruvate (with 2 mM Mg(2+)) has the opposite effect. A correlation between the coupling of N- and C-terminal domain unfolding, measured by differential scanning calorimetry, and the dimerization constant for EI, determined by sedimentation equilibrium, is observed. That is, when the coupling between N- and C-terminal domain unfolding produced by 0.2 or 1.0 mM PEP and 2 mM Mg(2+) is inhibited by 5 mM pyruvate, the dimerization constant for EI(H189A) decreases from > 10(8) to < 5 x 10(5) or 3 x 10(7) M(-1), respectively. Incubation of the wild-type, dephospho-enzyme I with the transition-state analog phosphonopyruvate and 2 mM Mg(2+) also increases domain coupling and the dimerization constant approximately 42-fold. With 2 mM Mg(2+) at 15-25 degrees C and pH 7.5, PEP has been found to bind to one site/monomer of EI(H189A) with K(A)' approximately 10(6) M(-1) (deltaG' = -8.05 +/- 0.05 kcal/mole and deltaH = +3.9 kcal/mole at 20 degrees C); deltaC(p) = -0.33 kcal K(-1) mole(-1). The binding of PEP to EI(H189A) is synergistic with that of Mg(2+). Thus, physiological concentrations of PEP and Mg(2+) increase, whereas pyruvate and Mg(2+) decrease the amount of dimeric, active, dephospho-enzyme I.     

Probing the conformation of a human apolipoprotein C-1 by amino acid substitutions and trimethylamine-N-oxide.

To test, at the level of individual amino acids, the conformation of an exchangeable apolipoprotein in aqueous solution and in the presence of an osmolyte trimethylamine-N-oxide (TMAO), six synthetic peptide analogues of human apolipoprotein C-1 (apoC-1, 57 residues) containing point mutations in the predicted alpha-helical regions were analyzed by circular dichroism (CD). The CD spectra and the melting curves of the monomeric wild-type and plasma apoC-1 in neutral low-salt solutions superimpose, indicating 31 +/- 4% alpha-helical structure at 22 degrees C that melts reversibly with T(m,WT) = 50 +/- 2 degrees C and van't Hoff enthalpy deltaH(v,WT)(Tm) = 18 +/- 2 kcal/mol. G15A substitution leads to an increased alpha-helical content of 42 +/- 4% and an increased T(m,G15A) = 57 +/- 2 degrees C, which corresponds to stabilization by delta deltaG(app) = +0.4 +/- 1.5 kcal/mol. G15P mutant has approximately 20% alpha-helical content at 22 degrees C and unfolds with low cooperativity upon heating to 90 degrees C. R23P and T45P mutants are fully unfolded at 0-90 degrees C. In contrast, Q31P mutation leads to no destabilization or unfolding. Consequently, the R23 and T45 locations are essential for the stability of the cooperative alpha-helical unit in apoC-1 monomer, G15 is peripheral to it, and Q31 is located in a nonhelical linker region. Our results suggest that Pro mutagenesis coupled with CD provides a tool for assigning the secondary structure to protein groups, which should be useful for other self-associating proteins that are not amenable to NMR structural analysis in aqueous solution. TMAO induces a reversible cooperative coil-to-helix transition in apoC-1, with the maximal alpha-helical content reaching 74%. Comparison with the maximal alpha-helical content of 73% observed in lipid-bound apoC-1 suggests that the TMAO-stabilized secondary structure resembles the functional lipid-bound apolipoprotein conformation.