Outer membrane protein A of Escherichia coli inserts and folds into lipid bilayers by a concerted mechanism

Unfolded outer membrane protein A (OmpA) of Escherichia coli spontaneously inserts and refolds into lipid bilayers upon dilution of denaturing urea. In the accompanying paper, we have developed a new technique, time-resolved distance determination by fluorescence quenching (TDFQ), which is capable of monitoring the translocation across lipid bilayers of fluorescence reporter groups such as tryptophan in real time [Kleinschmidt, J. H., and Tamm, L. K. (1999) Biochemistry 38, 4996-5005]. Specifically, we have shown that wild-type OmpA, which contains five tryptophans, inserts into lipid bilayers via three structurally distinct membrane-bound folding intermediates. To take full advantage of the TDFQ technique and to further dissect the folding pathway, we have made five different mutants of OmpA, each containing a single tryptophan and four phenylalanines in the five tryptophan positions of the wild-type protein. All mutants refolded in vivo and in vitro and, as judged by SDS-PAGE, trypsin fragmentation, and Trp fluorescence, their refolded state was indistinguishable from the native state of OmpA. TDFQ analysis of the translocation across the lipid bilayer of the individual Trps of OmpA yielded the following results: Below 30 degrees C, all Trps started from a far distance from the bilayer center and then gradually approached a distance of approximately 10 A from the bilayer center. In a narrow temperature range between 30 and 35 degrees C, Trp-15, Trp-57, Trp-102, and Trp-143 were detected very close to the center of the lipid bilayer in the first few minutes and then moved to greater distances from the center. When monitored at 40 degrees C, which resolved the last steps of OmpA refolding, these four tryptophans crossed the center of the bilayer and approached distances of approximately 10 A from the center after refolding was complete. In contrast Trp-7 approached the 10 A distance from a far distance at all temperatures and was never detected to cross the center of the lipid bilayer. The translocation rates of Trp-15, Trp-57, Trp-102, and Trp-143 which are each located in different outer loop regions of the four beta-hairpins of the eight-stranded beta-barrel of OmpA were very similar to one another. This result and the common distances of these Trps from the membrane center observed in the third membrane-bound folding intermediate provide strong evidence for a synchronous translocation of all four beta-hairpins of OmpA across the lipid bilayer and suggest that OmpA inserts and folds into lipid bilayers by a concerted mechanism.     

Folding and insertion of the outer membrane protein OmpA is assisted by the chaperone Skp and by lipopolysaccharide

We have studied the folding pathway of a beta-barrel membrane protein using outer membrane protein A (OmpA) of Escherichia coli as an example. The deletion of the gene of periplasmic Skp impairs the assembly of outer membrane proteins of bacteria. We investigated how Skp facilitates the insertion and folding of completely unfolded OmpA into phospholipid membranes and which are the biochemical and biophysical requirements of a possible Skp-assisted folding pathway. In refolding experiments, Skp alone was not sufficient to facilitate membrane insertion and folding of OmpA. In addition, lipopolysaccharide (LPS) was required. OmpA remained unfolded when bound to Skp and LPS in solution. From this complex, OmpA folded spontaneously into lipid bilayers as determined by electrophoretic mobility measurements, fluorescence spectroscopy, and circular dichroism spectroscopy. The folding of OmpA into lipid bilayers was inhibited when one of the periplasmic components, either Skp or LPS, was absent. Membrane insertion and folding of OmpA was most efficient at specific molar ratios of OmpA, Skp, and LPS. Unfolded OmpA in complex with Skp and LPS folded faster into phospholipid bilayers than urea-unfolded OmpA. Together, these results describe a first assisted folding pathway of an integral membrane protein on the example of OmpA.     

Thermodynamics of the folding of D-glyceraldehyde-3-phosphate dehydrogenase assisted by protein disulfide isomerase studied by microcalorimetry.

Thermodynamics of the refolding of denatured D-glyceraldehyde 3-phosphate dehydrogenase (GAPDH) assisted by protein disulfide isomerase (PDI), a molecular chaperone, has been studied by isothermal microcalorimetry at different molar ratios of PDI/GAPDH and temperatures using two thermodynamic models proposed for chaperone-substrate binding and chaperone-assisted substrate folding, respectively. The binding of GAPDH folding intermediates to PDI is driven by a large favorable enthalpy decrease with a large unfavorable entropy reduction, and shows strong enthalpy-entropy compensation and weak temperature dependence of Gibbs free energy change. A large negative heat-capacity change of the binding, -156 kJ.mol(-1).K(-1), at all temperatures examined indicates that hydrophobic interaction is a major force for the binding. The binding stoichiometry shows one dimeric GAPDH intermediate per PDI monomer. The refolding of GAPDH assisted by PDI is a largely exothermic reaction at 15.0-25.0 degrees C. With increasing temperature from 15.0 to 37.0 degrees C, the PDI-assisted reactivation yield of denatured GAPDH upon dilution decreases. At 37.0 degrees C, the spontaneous reactivation, PDI-assisted reactivation and intrinsic molar enthalpy change during the PDI-assisted refolding of GAPDH are not detected.     

Förster resonance energy transfer as a probe of membrane protein folding

The folding reaction of a β-barrel membrane protein, outer membrane protein A (OmpA), is probed with F?rster resonance energy transfer (FRET) experiments. Four mutants of OmpA were generated in which the donor fluorophore, tryptophan, and acceptor molecule, a naphthalene derivative, are placed in various locations on the protein to report the evolution of distances across the bilayer and across the protein pore during a folding event. Analysis of the FRET efficiencies reveals three timescales for tertiary structure changes associated with insertion and folding into a synthetic bilayer. A narrow pore forms during the initial stage of insertion, followed by bilayer traversal. Finally, a long-time component is attributed to equilibration and relaxation, and may involve global changes such as pore expansion and strand extension. These results augment the existing models that describe concerted insertion and folding events, and highlight the ability of FRET to provide insight into the complex mechanisms of membrane protein folding. This article is part of a Special Issue entitled: Membrane protein structure and function.     

Secondary and tertiary structure formation of the beta-barrel membrane protein OmpA is synchronized and depends on membrane thickness

The mechanism of membrane insertion and folding of a beta-barrel membrane protein has been studied using the outer membrane protein A (OmpA) as an example. OmpA forms an eight-stranded beta-barrel that functions as a structural protein and perhaps as an ion channel in the outer membrane of Escherichia coli. OmpA folds spontaneously from a urea-denatured state into lipid bilayers of small unilamellar vesicles. We have used fluorescence spectroscopy, circular dichroism spectroscopy, and gel electrophoresis to investigate basic mechanistic principles of structure formation in OmpA. Folding kinetics followed a second-order rate law and is strongly depended on the hydrophobic thickness of the lipid bilayer. When OmpA was refolded into model membranes of dilaurylphosphatidylcholine, fluorescence kinetics were characterized by a rate constant that was about fivefold higher than the rate constants of formation of secondary and tertiary structure, which were determined by circular dichroism spectroscopy and gel electrophoresis, respectively. The formation of beta-sheet secondary structure and closure of the beta-barrel of OmpA were correlated with the same rate constant and coupled to the insertion of the protein into the lipid bilayer. OmpA, and presumably other beta-barrel membrane proteins therefore do not follow a mechanism according to the two-stage model that has been proposed for the folding of alpha-helical bundle membrane proteins. These different folding mechanisms are likely a consequence of the very different intramolecular hydrogen bonding and hydrophobicity patterns in these two classes of membrane proteins.     

In vitro folding of KvAP, a voltage-gated K+ channel

In this contribution, we report in vitro folding of the archaebacterial voltage-gated K(+) channel, K(v)AP. We show that in vitro folding of the K(v)AP channel from the extensively unfolded state requires lipid vesicles and that the refolded channel is biochemically and functionally similar to the native channel. The in vitro folding process is slow at room temperature, and the folding yield depends on the composition of the lipid bilayer. The major factor influencing refolding is temperature, and almost quantitative refolding of the K(v)AP channel is observed at 80 °C. To differentiate between insertion into the bilayer and folding within the bilayer, we developed a cysteine protection assay. Using this assay, we demonstrate that insertion of the unfolded protein into the bilayer is relatively fast at room temperature and independent of lipid composition, suggesting that temperature and bilayer composition influence folding within the bilayer. Further, we demonstrate that in vitro folding provides an effective method for obtaining high yields of the native channel. Our studies suggest that the K(v)AP channel provides a good model system for investigating the folding of a multidomain integral membrane protein.     

Mechanism of phage P22 tailspike protein folding mutations.

Temperature-sensitive folding (tsf) and global-tsf-suppressor (su) point mutations affect the folding yields of the trimeric, thermostable phage P22 tailspike endorhamnosidase at elevated temperature, both in vivo and in vitro, but they have little effect on function and stability of the native folded protein. To delineate the mechanism by which these mutations modify the partitioning between productive folding and off-pathway aggregation, the kinetics of refolding after dilution from acid-urea solutions and the thermal stability of folding intermediates were analyzed. The study included five tsf mutations of varying severity, the two known su mutations, and four tsf/su double mutants. At low temperature (10 degrees C), subunit-folding rates, measured as an increase in fluorescence, were similar for wild-type and mutants. At 25 degrees C, however, tsf mutations reduced the rate of subunit folding. The su mutations increased this rate, when present in the tsf-mutant background, but had no effect in the wild-type background. Conversely, tsf mutations accelerated, and su mutations retarded the irreversible off-pathway reaction, as revealed by temperature down-shifts after varied times during refolding at high temperature (40 degrees C). The kinetic results are consistent with tsf mutations destabilizing and su mutations stabilizing an essential subunit folding intermediate. In accordance with this interpretation, tsf mutations decreased, and su mutations increased the temperature resistance of folding intermediates, as disclosed by temperature up-shifts during refolding at 25 degrees C. The stabilizing and destabilizing effects were most pronounced early during refolding. However, they were not limited to subunit-folding intermediates and were also observable during thermal unfolding of the native protein.     

Thermal unfolding of an intermediate is associated with non-Arrhenius kinetics in the folding of hen lysozyme

A variety of techniques, including quenched-flow hydrogen exchange labelling monitored by electrospray ionization mass spectrometry, and stopped-flow absorbance, fluorescence and circular dichroism spectroscopy, has been used to investigate the refolding kinetics of hen lysozyme over a temperature range from 2 degrees C to 50 degrees C. Simple Arrhenius behaviour is not observed, and although the overall rate of folding increases from 2 to 40 degrees C, it decreases above 40 degrees C. In addition, the transient intermediate on the major folding pathway at 20 degrees C, in which the alpha-domain is persistently structured in the absence of a stable beta-domain, is thermally unfolded in a sigmoidal transition (T(m) approximately 40 degrees C) indicative of a cooperatively folded state. At all temperatures, however, there is evidence for fast ( approximately 25 %) and slow ( approximately 75 %) populations of refolding molecules. By using transition state theory, the kinetic data from various experiments were jointly fitted to a sequential three-state model for the slow folding pathway. Together with previous findings, these results indicate that the alpha-domain intermediate is a productive species on the folding route between the denatured and native states, and which accumulates as a consequence of its intrinsic stability. Our analysis suggests that the temperature dependence of the rate constant for lysozyme folding depends on both the total change in the heat capacity between the ground and transition states (the dominant factor at low temperatures) and the heat-induced destabilization of the alpha-domain intermediate (the dominant factor at high temperatures). Destabilization of such kinetically competent intermediate species is likely to be a determining factor in the non-Arrhenius temperature dependence of the folding rate of those proteins for which one or more intermediates are populated.     

Effect of crowding by Ficolls on OmpA and OmpT refolding and membrane insertion

Folding of outer membrane proteins (OMPs) has been studied extensively in vitro. However, most of these studies have been conducted in dilute buffer solution, which is different from the crowded environment in the cell periplasm, where the folding and membrane insertion of OMPs actually occur. Using OmpA and OmpT as model proteins and Ficoll 70 as the crowding agent, here we investigated the effect of the macromolecular crowding condition on OMP membrane insertion. We found that the presence of Ficoll 70 significantly slowed down the rate of membrane insertion of OmpA while had little effect on those of OmpT. To investigate if the soluble domain of OmpA slowed down membrane insertion in the presence of the crowding agent, we created a truncated OmpA construct that contains only the transmembrane domain (OmpA171). In the absence of crowding agent, OmpA171 refolded at a similar rate as OmpA, although with decreased efficiency. However, under the crowding condition, OmpA171 refolded significantly faster than OmpA. Our results suggest that the periplasmic domain slows down the rate, while improves the efficiency, of OmpA folding and membrane insertion under the crowding condition. Such an effect was not obvious when refolding was studied in buffer solution in the absence of crowding.     

耐冷希瓦氏菌外膜蛋白的体外折叠研究

目的:旨在建立耐低温革兰氏阴性菌外膜蛋白体外折叠体系,为膜蛋白合成耐低温机制提供理论基础。方法:以包涵体的形式在大肠杆菌中过量表达了来源于耐低温希瓦氏菌的OmpA同源外膜蛋白Omp74的全蛋白质和N端跨膜结构域,纯化包涵体后,用高浓度尿素或强阴离子表面活性剂溶液溶解包涵体,以非离子表面活性剂为折叠介质,建立该外膜蛋白的体外折叠体系,同时以大肠杆菌的OmpA作为对照进行了比较研究。结果:与OmpA相比,Omp74体外折叠受温度影响较小,低浓度的阴离子表面活性剂能促Omp74的折叠,但对OmpA的折叠没有影响;C端结构域抑制Omp74在表面活性剂中的折叠;Omp74在0.5%的月桂酰基麦芽糖苷(DDM)和0.4%的十二烷基肌氨酸钠的混合溶液中能达到接近100%的折叠效率。     

Refolded outer membrane protein A of Escherichia coli forms ion channels with two conductance states in planar lipid bilayers

Outer membrane protein A (OmpA), a major structural protein of the outer membrane of Escherichia coli, consists of an N-terminal 8-stranded beta-barrel transmembrane domain and a C-terminal periplasmic domain. OmpA has served as an excellent model for studying the mechanism of insertion, folding, and assembly of constitutive integral membrane proteins in vivo and in vitro. The function of OmpA is currently not well understood. Particularly, the question whether or not OmpA forms an ion channel and/or nonspecific pore for uncharged larger solutes, as some other porins do, has been controversial. We have incorporated detergent-purified OmpA into planar lipid bilayers and studied its permeability to ions by single channel conductance measurements. In 1 M KCl, OmpA formed small (50-80 pS) and large (260-320 pS) channels. These two conductance states were interconvertible, presumably corresponding to two different conformations of OmpA in the membrane. The smaller channels are associated with the N-terminal transmembrane domain, whereas both domains are required to form the larger channels. The two channel activities provide a new functional assay for the refolding in vitro of the two respective domains of OmpA. Wild-type and five single tryptophan mutants of urea-denatured OmpA are shown to refold into functional channels in lipid bilayers.     

Outer membrane protein A of Escherichia coli forms temperature-sensitive channels in planar lipid bilayers

The temperature dependence of single-channel conductance and open probability for outer membrane protein A (OmpA) of Escherichia coli were examined in planar lipid bilayers. OmpA formed two interconvertible conductance states, small channels, 36-140 pS, between 15 and 37 degrees C, and large channels, 115-373 pS, between 21 and 39 degrees C. Increasing temperatures had strong effects on open probabilities and on the ratio of large to small channels, particularly between 22 and 34 degrees C, which effected sharp increases in average conductance. The data infer that OmpA is a flexible temperature-sensitive protein that exists as a small pore structure at lower temperatures, but refolds into a large pore at higher temperatures.     

Structure of a rapidly formed intermediate in ribonuclease T1 folding.

Kinetic intermediates in protein folding are short-lived and therefore difficult to detect and to characterize. In the folding of polypeptide chains with incorrect isomers of Xaa-Pro peptide bonds the final rate-limiting transition to the native state is slow, since it is coupled to prolyl isomerization. Incorrect prolyl isomers thus act as effective traps for folding intermediates and allow their properties to be studied more easily. We employed this strategy to investigate the mechanism of slow folding of ribonuclease T1. In our experiments we use a mutant form of this protein with a single cis peptide bond at proline 39. During refolding, protein chains with an incorrect trans proline 39 can rapidly form extensive secondary structure. The CD signal in the amide region is regained within the dead-time of stopped-flow mixing (15 ms), indicating a fast formation of the single alpha-helix of ribonuclease T1. This step is correlated with partial formation of a hydrophobic core, because the fluorescence emission maximum of tryptophan 59 is shifted from 349 nm to 325 nm within less than a second. After about 20 s of refolding an intermediate is present that shows about 40% enzymatic activity compared to the completely refolded protein. In addition, the solvent accessibility of tryptophan 59 is drastically reduced in this intermediate and comparable to that of the native state as determined by acrylamide quenching of the tryptophan fluorescence. Activity and quenching measurements have long dead-times and therefore we do not know whether enzymatic activity and solvent accessibility also change in the time range of milliseconds. At this stage of folding at least part of the beta-sheet structure is already present, since it hosts the active site of the enzyme. The trans to cis isomerization of the tyrosine 38-proline 39 peptide bond in the intermediate and consequently the formation of native protein is very slow (tau = 6,500 s at pH 5.0 and 10 degrees C). It is accompanied by an additional increase in tryptophan fluorescence, by the development of the fine structure of the tryptophan emission spectrum, and by the regain of the full enzymatic activity. This indicates that the packing of the hydrophobic core, which involves both tryptophan 59 and proline 39, is optimized in this step. Apparently, refolding polypeptide chains with an incorrect prolyl isomer can very rapidly form partially folded intermediates with native-like properties.     

Skp, a molecular chaperone of gram-negative bacteria, is required for the formation of soluble periplasmic intermediates of outer membrane proteins.

Using a cross-linking approach, we have analyzed the function of Skp, a presumed molecular chaperone of the periplasmic space of Escherichia coli, during the biogenesis of an outer membrane protein (OmpA). Following its transmembrane translocation, OmpA interacts with Skp in close vicinity to the plasma membrane. In vitro, Skp was also found to bind strongly and specifically to pOmpA nascent chains after their release from the ribosome suggesting the ability of Skp to recognize early folding intermediates of outer membrane proteins. Pulse labeling of OmpA in spheroplasts prepared from an skp null mutant revealed a specific requirement of Skp for the release of newly translocated outer membrane proteins from the plasma membrane. Deltaskp mutant cells are viable and show only slight changes in the physiology of their outer membranes. In contrast, double mutants deficient both in Skp and the periplasmic protease DegP (HtrA) do not grow at 37 degrees C in rich medium. We show that in the absence of an active DegP, a lack of Skp leads to the accumulation of protein aggregates in the periplasm. Collectively, our data demonstrate that Skp is a molecular chaperone involved in generating and maintaining the solubility of early folding intermediates of outer membrane proteins in the periplasmic space of Gram-negative bacteria.     

Chaperonins facilitate the in vitro folding of monomeric mitochondrial rhodanese.

In vitro refolding of the monomeric mitochondrial enzyme, rhodanese (thiosulfate sulfurtransferase; EC 2.8.1.1) is facilitated by molecular chaperonins. The four components: two proteins from Escherichia coli, chaperonin 60 (groEL) and chaperonin 10 (groES), MgATP, and K+, are necessary for the in vitro folding of rhodanese. These were previously shown to be necessary for the in vitro folding of ribulose-1,5-bisphosphate carboxylase at temperatures in excess of 25 degrees C (Viitanen, P. V., Lubben, T. H., Reed, J., Goloubinoff, P., O'Keefe, D. P., and Lorimer, G. H. (1990) Biochemistry 29, 5665-5671). The labile folding intermediate, rhodanese-I, which rapidly aggregates at 37 degrees C in the absence of the chaperonins, can be stabilized by forming a binary complex with chaperonin 60. The discharge of the binary chaperonin 60-rhodanese-I complex, results in the formation of active rhodanese, and requires the presence of chaperonin 10. Optimal refolding is associated with a K(+)-dependent hydrolysis of ATP. At lower protein concentrations and 25 degrees C, where aggregation is reduced, a fraction of the rhodanese refolds to an active form in the absence of the chaperonins. This spontaneous refolding can be arrested by chaperonin 60. There is some refolding (approximately equal to 20%) when ATP is replaced by nonhydrolyzable analogs, but there is no refolding in the presence of ADP or AMP. ATP analogs may interfere with the interaction of rhodanese-I with the chaperonins. Nondenaturing detergents facilitate rhodanese refolding by interacting with exposed hydrophobic surfaces of folding intermediates and thereby prevent aggregation (Tandon, S., and Horowitz, P. (1986) J. Biol. Chem. 261, 15615-15618). The chaperonin proteins appear to play a similar role in as much as they can replace the detergents. Consistent with this view, chaperonin 60, but not chaperonin 10, binds 2-3 molecules of the hydrophobic fluorescent reporter, 1,1'-bi(4-anilino)naphthalene-S,5'-disulfonic acid, indicating the presence of hydrophobic surfaces on chaperonin 60. The number of bound probe molecules is reduced to 1-2 molecules when chaperonin 10 and MgATP are added. The results support a model in which chaperonins facilitate folding, at least in part, by interacting with partly folded intermediates, thus preventing the interactions of hydrophobic surfaces that lead to aggregation.     

A carboxypeptidase Y pulse method to study the accessibility of the C-terminal end during the refolding of ribonuclease A.

Carboxypeptidase Y pulses, applied after various times of refolding, were employed to probe the accessibility of the C-terminus of RNAase A during the refolding process. The increase in resistance against proteolytic cleavage was measured by determination of the amount of liberated C-terminal amino acids and by activity assays. The results indicate that the C-terminus of RNAase becomes inaccessible early in the course of refolding, if folding is carried out at low temperatures under conditions that effectively stabilize the native state. At higher temperatures (25 degrees C) or under conditions of marginal stability, intermediates are not populated and protection against proteolytic cleavage is not detectable before the formation of the native state. The method described may be used to monitor the accessibility of the C-terminus of various proteins during refolding. However, intermediates on the folding pathway can only be observed if the native state is stable against carboxypeptidase attack.     

Cleaved serpin refolds into the relaxed state via a stressed conformer

Serine proteinase inhibitors (serpins) are believed to fold in vivo into a metastable "stressed" state with cleavage of their P1-P1' bond resulting in reactive center loop insertion and a thermostable "relaxed" state. To understand this unique folding mechanism, we investigated the refolding processes of the P1-P1'-cleaved forms of wild type ovalbumin (cl-OVA) and the R339T mutant (cl-R339T). In the native conditions, cl-OVA is trapped as the stressed conformer, whereas cl-R339T attains the relaxed structure. Under urea denaturing conditions, these cleaved proteins completely dissociated into the heavy (Gly(1)-Ala(352)) and light (Ser(353)-Pro(385)) chains. Upon refolding, the heavy chains of both proteins formed essentially the same initial burst refolding intermediates and then reassociated with the light chain counterparts. The reassociated intermediates both refolded into the native states with indistinguishable kinetics. The two refolded proteins, however, had a notable difference in thermostability. cl-OVA refolded into the stressed form with T(m) = 68.4 degrees C, whereas cl-R339T refolded into the relaxed form with T(m) = 85.5 degrees C. To determine whether cl-R339T refolds directly to the relaxed state or through the stressed state, conformational analyses by anion-exchange chromatography and fluorescence measurements were executed. The results showed that cl-R339T refolds first to the stressed conformation and then undergoes the loop insertion. This is the first demonstration that the P1-P1'-cleaved serpin peptide capable of loop insertion refolds to the stressed conformation. This highlights that the stressed conformation of serpins is an inevitable intermediate state on the folding pathway to the relaxed structure.     

Unassisted refolding of urea unfolded rhodanese

In vitro refolding after urea unfolding of the enzyme rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) normally requires the assistance of detergents or chaperonin proteins. No efficient, unassisted, reversible unfolding/folding transition has been demonstrated to date. The detergents or the chaperonin proteins have been proposed to stabilize folding intermediates that kinetically limit folding by aggregating. Based on this hypothesis, we have investigated a number of experimental conditions and have developed a protocol for refolding, without assistants, that gives evidence of a reversible unfolding transition and leads to greater than 80% recovery of native enzyme. In addition to low protein concentration (10 micrograms/ml), low temperatures are required to maximize refolding. Otherwise optimal conditions give less than 10% refolding at 37 degrees C, whereas at 10 degrees C the recovery approaches 80%. The unfolding/refolding phases of the transition curves are most similar in the region of the transition, and refolding yields are significantly reduced when unfolded rhodanese is diluted to low urea concentrations, rather than to concentrations near the transition region. This is consistent with the formation of "sticky" intermediates that can remain soluble close to the transition region. Apparently, nonnative structures, e.g. aggregates, can form rapidly at low denaturant concentrations, and their subsequent conversion to the native structure is slow.     

Modulation of folding and assembly of the membrane protein bacteriorhodopsin by intermolecular forces within the lipid bilayer.

Three different lipid systems have been developed to investigate the effect of physicochemical forces within the lipid bilayer on the folding of the integral membrane protein bacteriorhodopsin. Each system consists of lipid vesicles containing two lipid species, one with phosphatidylcholine and the other with phosphatidylethanolamine headgroups, but the same hydrocarbon chains: either L-alpha-1, 2-dioleoyl, L-alpha-1,2-dipalmitoleoyl, or L-alpha-1,2-dimyristoyl. Increasing the mole fraction of the phosphatidylethanolamine lipid increases the desire of each monolayer leaflet in the bilayer to curve toward water. This increases the torque tension of such monolayers, when they are constrained to remain flat in the vesicle bilayer. Consequently, the lateral pressure in the hydrocarbon chain region increases, and we have used excimer fluorescence from pyrene-labeled phosphatidylcholine lipids to probe these pressure changes. We show that bacteriorhodopsin regenerates to about 95% yield in vesicles of 100% phosphatidylcholine. The regeneration yield decreases as the mole fraction of the corresponding phosphatidylethanolamine component is increased. The decrease in yield correlates with the increase in lateral pressure which the lipid chains exert on the refolding protein. We suggest that the increase in lipid chain pressure either hinders insertion of the denatured state of bacterioopsin into the bilayer or slows a folding step within the bilayer, to the extent that an intermediate involved in bacteriorhodopsin regeneration is effectively trapped.     

Intermediates in the refolding of ribonuclease at subzero temperatures. 3. Multiple folding pathways

The kinetics of refolding of ribonuclease A have been measured at -15 degrees C by monitoring the intrinsic fluorescence and absorbance signals from the six tyrosine residues. For each probe multiphasic kinetics were observed. The burial of tyrosine residues, as determined by the change in absorbance at 286 nm, revealed four phases, whereas the kinetics of refolding monitored by fluorescence revealed only two phases. The rates of the transients detected by fluorescence were independent of pH. One of the faster transients detected by delta A286 involved a decrease in absorbance, which is consistent with solvent exposure, rather than burial, and suggests the possibility of an abortive partially folded intermediate in the earlier stages of folding. Double-jump unfolding assays were used to follow the buildup and decay of an intermediate in the refolding reaction at -15 degrees C. At both pH* 3.0 and pH* 6.0 the maximum concentration of the intermediate was 25-30% of the total protein. The existence of a second pathway of slow folding was inferred from the difference in rate of formation of native enzyme and breakdown of the observed intermediate, and by computer simulations. In addition, the unfolding assay demonstrated that 20% of the unfolded protein was converted to native at a much faster rate, consistent with observations in aqueous solution that 80% of unfolded ribonuclease A consists of slow-folding species. Kinetics and amplitude data from these and other refolding experiments with different probes were used to develop possible models for the pathway of refolding. The simplest system consistent with the results for the slow-refolding species involves two parallel pathways with multiple intermediates on each of them. Several independent lines of evidence indicate that about 30% of the unfolded state refolds by the minor pathway, in which the slowest observed phase is attributed to the isomerization of Pro-93. The major pathway involves 50% of the unfolded state; the reason why it refolds slowly is not apparent. A native-like intermediate is formed considerably more rapidly in the major slow-refolding pathway, compared to the minor pathway.