Structural intermediates in folding of yeast iso-2 cytochrome c

Probing the cytochrome c' folding landscape

The folding kinetics of R. palustris cytochrome c' (cyt c') have been monitored by heme absorption and native Trp72 fluorescence at pH 5. The Trp72 fluorescence burst signal suggests early compaction of the polypeptide ensemble. Analysis of heme transient absorption spectra reveals deviations from two-state behavior, including a prominent slow phase that is accelerated by the prolyl isomerase cyclophilin. A nonnative proline configuration (Pro21) likely interferes with the formation of the helical bundle surrounding the heme.

Coupled kinetic traps in cytochrome c folding: His-heme misligation and proline isomerization

The effect of His-heme misligation on folding has been investigated for a triple mutant of yeast iso-2 cytochrome c (N26H,H33N,H39K iso-2). The variant contains a single misligating His residue at position 26, a location at which His residues are found in several cytochrome c homologues, including horse, tuna, and yeast iso-1. The amplitude for fast phase folding exhibits a strong initial pH dependence. For GdnHCl unfolded protein at an initial pH<5, the observed refolding at final pH 6 is dominated by a fast phase (tau(2f)=20 ms, alpha(2f)=90 %) that represents folding in the absence of misligation. For unfolded protein at initial pH 6, folding at final pH 6 occurs in a fast phase of reduced amplitude (alpha(2f) approximately 20 %) but the same rate (tau(2f)=20 ms), and in two slower phases (tau(m)=6-8 seconds, alpha(m) approximately 45 %; and tau(1b)=16-20 seconds, alpha(1b) approximately 35 %). Double jump experiments show that the initial pH dependence of the folding amplitudes results from a slow pH-dependent equilibrium between fast and slow folding species present in the unfolded protein. The slow equilibrium arises from coupling of the His protonation equilibrium to His-heme misligation and proline isomerization. Specifically, Pro25 is predominantly in trans in the unligated low-pH unfolded protein, but is constrained in a non-native cis isomerization state by His26-heme misligation near neutral pH. Refolding from the misligated unfolded form proceeds slowly due to the large energetic barrier required for proline isomerization and displacement of the misligated His26-heme ligand.

Cytochrome c folds through a smooth funnel

A dominant feature of folding of cytochrome c is the presence of nonnative His-heme kinetic traps, which either pre-exist in the unfolded protein or are formed soon after initiation of folding. The kinetically trapped species can constitute the majority of folding species, and their breakdown limits the rate of folding to the native state. A temperature jump (T-jump) relaxation technique has been used to compare the unfolding/folding kinetics of yeast iso-2 cytochrome c and a genetically engineered double mutant that lacks His-heme kinetic traps, H33N,H39K iso-2. The results show that the thermodynamic properties of the transition states are very similar. A single relaxation time tau(obs) is observed for both proteins by absorbance changes at 287 nm, a measure of solvent exclusion from aromatic residues. At temperatures near Tm, the midpoint of the thermal unfolding transitions, tau(obs) is four to eight times faster for H33N,H39K iso-2 (tau(obs) approximately 4-10 ms) than for iso-2 (tau(obs) approximately 20-30 ms). T-jumps show that there are no kinetically unresolved (tau < 1-3 micros T-jump dead time) "burst" phases for either protein. Using a two-state model, the folding (k(f)) and unfolding (k(u)) rate constants and the thermodynamic activation parameters standard deltaGf, standard deltaGu, standard deltaHf, standard deltaHu, standard deltaSf, standard deltaSu are evaluated by fitting the data to a function describing the temperature dependence of the apparent rate constant k(obs) (= tau(obs)(-1)) = k(f) + k(u). The results show that there is a small activation enthalpy for folding, suggesting that the barrier to folding is largely entropic. In the "new view," a purely entropic kinetic barrier to folding is consistent with a smooth funnel folding landscape.

Antibody-detected folding: kinetics of surface epitope formation are distinct from other folding phases

The rate of macromolecular surface formation in yeast iso-2 cytochrome c and its site-specific mutant, N52I iso-2, has been studied using a monoclonal antibody that recognizes a tertiary epitope including K58 and H39. The results indicate that epitope refolding occurs after fast folding but prior to slow folding, in contrast to horse cytochrome c where surface formation occurs early. The antibody-detected (ad) kinetic phase accompanying epitope formation has k(ad) = 0.2 s(-1) and is approximately 40-fold slower than the fastest detectable event in the folding of yeast iso-2 cytochrome c (k2f approximately 8 s(-1)), but occurs prior to the absorbance- and fluorescence-detected slow folding steps (k1a approximately 0.06 s(-1); k1b approximately 0.09 s(-1)). N5I iso-2 cytochrome c exhibits similar kinetic behavior with respect to epitope formation. A detailed dissection of the mechanistic differences between the folding pathways of horse and yeast cytochromes c identifies possible reasons for the slow surface formation in the latter. Our results suggest that non-native ligation involving H33 or H39 during refolding may slow down the formation of the tertiary epitope in iso-2 cytochrome c. This study illustrates that surface formation can be coupled to early events in protein folding. Thus, the rate of macromolecular surface formation is fine tuned by the residues that make up the surface and the interactions they entertain during refolding.

Thermal stability of hydrophobic heme pocket variants of oxidized cytochrome c

Microcalorimetry has been used to measure the stabilities of mutational variants of yeast iso-1 cytochrome c in which F82 and L85 have been replaced by other hydrophobic amino acids. Specifically, F82 has been replaced by Y and L85 by A. The double mutant F82Y,L85A iso-1 has also been studied, and the mutational perturbations are compared to those for the two single mutants, F82Y iso-1 and L85A iso-1. Results are interpreted in terms of known crystallographic structures. The data show that (1) the destabilization of the mutant proteins is similar in magnitude to that which is theoretically predicted by the more obvious mutation-induced structural effects; (2) the free energy of destabilization of the double mutant, F82Y,L85A iso-1, is less than the sum of those of the two single mutants, almost certainly because, in the double mutant, the -OH group of Y82 is able to protrude into the cavity formed by the L85A substitution. The more favorable structural accommodation of the new -OH group in the double mutant leads to additional stability through (1) further decreases in the volumes of internal cavities and (2) formation of an extra protein-protein hydrogen bond.

Refolding rate of stability-enhanced cytochrome c is independent of thermodynamic driving force

N52I iso-2 cytochrome c is a variant of yeast iso-2 cytochrome c in which asparagine substitutes for isoleucine 52 in an alpha helical segment composed of residues 49-56. The N52I substitution results in a significant increase in both stability and cooperativity of equilibrium unfolding, and acts as a "global suppressor" of destabilizing mutations. The equilibrium m-value for denaturant-induced unfolding of N52I iso-2 increases by 30%, a surprisingly large amount for a single residue substitution. The folding/unfolding kinetics for N52I iso-2 have been measured by stopped-flow mixing and by manual mixing, and are compared to the kinetics of folding/unfolding of wild-type protein, iso-2 cytochrome c. The results show that the observable folding rate and the guanidine hydrochloride dependence of the folding rate are the same for iso-2 and N52I iso-2, despite the greater thermodynamic stability of N52I iso-2. Thus, there is no linear free-energy relationship between mutation-induced changes in stability and observable refolding rates. However, for N52I iso-2 the unfolding rate is slower and the guanidine hydrochloride dependence of the unfolding rate is smaller than for iso-2. The differences in the denaturant dependence of the unfolding rates suggest that the N52I substitution decreases the change in the solvent accessible hydrophobic surface between the native state and the transition state. Two aspects of the results are inconsistent with a two-state folding/unfolding mechanism and imply the presence of folding intermediates: (1) observable refolding rate constants calculated from the two-state mechanism by combining equilibrium data and unfolding rate measurements deviate from the observed refolding rate constants; (2) kinetically unresolved signal changes ("burst phase") are observed for both N52I iso-2 and iso-2 refolding. The "burst phase" amplitude is larger for N52I iso-2 than for iso-2, suggesting that the intermediates formed during the "burst phase" are stabilized by the N52I substitution.

Fast folding of cytochrome c

Native iso-2 cytochrome c contains two residues (His 18, Met 80) coordinated to the covalently attached heme. On unfolding of iso-2, the His 18 ligand remains coordinated to the heme iron, whereas Met 80 is displaced by a non-native heme ligand, His 33 or His 39. To test whether non-native His-heme ligation slows folding, we have constructed a double mutant protein in which the non-native ligands are replaced by asparagine and lysine, respectively (H33N,H39K iso-2). The double mutant protein, which cannot form non-native histidine-heme coordinate bonds, folds significantly faster than normal iso-2 cytochrome c: gamma = 14-26 ms for H33N,H39K iso-2 versus gamma = 200-1,100 ms for iso-2. These results with iso-2 cytochrome c strongly support the hypothesis that non-native His-heme ligation results in a kinetic barrier to fast folding of cytochrome c. Assuming that the maximum rate of a conformational search is about 10(11) s-1, the results imply that the direct folding pathway of iso-2 involves passage through on the order of 10(9) or fewer partially folded conformers.

The barriers in protein folding

Elimination of an interaction which forms in denatured cytochrome c enables the majority of the molecules to fold to the native state on a 15 ms time scale, without populating observable intermediates. These results are contrary to the current view that particular steps in protein folding, including the supposedly rate-limiting molten globule to native transition, are intrinsically slow. Instead it appears that intermediates characterized so far may be kinetically trapped by barriers that are optional rather than integral to the folding process. Major barriers may result from misorganization of the chain in the initial condensation step.

Characterization of folding intermediates using prolyl isomerase

Structure-reactivity relationships of human peptidyl prolyl cis-trans isomerase (PPI) toward the two slow folding reactions of yeast iso-2 cytochrome c have been used to characterize the structure of folding intermediates in the vicinity of critical prolines. We propose that the relative catalytic efficiency of PPI for the protein substrate relative to a peptide substrate, (kcat/Km)rel, is a measure of structure in folding intermediates. The structural stability of slow-folding intermediates as detected by changes in (kcat/Km)rel was investigated using two structural perturbants: guanidine hydrochloride and site-directed mutagenesis. Neither of the two slow folding reactions for wild-type cytochrome c is catalyzed at low denaturant concentrations. However, both phases are catalyzed at moderate concentrations of guanidine hydrochloride. A mutation in cytochrome c enhances catalysis of the fluorescence-detected slow folding phase. For protein substrates destabilized by denaturants or mutation, we suggest that increases in (kcat/Km)rel result from a loosening of the substrate structure, providing better access of peptidyl prolyl isomerase to critical proline(s).

Structure determination and analysis of yeast iso-2-cytochrome c and a composite mutant protein

As part of a study of protein folding and stability, the three-dimensional structures of yeast iso-2-cytochrome c and a composite protein (B-2036) composed of primary sequences of both iso-1 and iso-2-cytochromes c have been solved to 1.9 A and 1.95 A resolutions, respectively, using X-ray diffraction techniques. The sequences of iso-1 and iso-2-cytochrome c share approximately 84% identity and the B-2036 composite protein has residues 15 to 63 from iso-2-cytochrome c with the rest being derived form the iso-1 protein. Comparison of these structures reveals that amino acid substitutions result in alterations in the details of intramolecular interactions. Specifically, the substitution Leu98Met results in the filling of an internal cavity present in iso-1-cytochrome c. Further substitutions of Val20Ile and Cys102Ala alter the packing of secondary structure elements in the iso-2 protein. Blending the isozymic amino acid sequences in this latter area results in the expansion of the volume of an internal cavity in the B-2036 structure to relieve a steric clash between Ile20 and Cys102. Modification of hydrogen bonding and protein packing without disrupting the protein fold is illustrated by the His26Asn and Asn63Ser substitutions between iso-1 and iso-2-cytochromes c. Alternatively, a change in main-chain fold is observed at Gly37 apparently due to a remote amino acid substitution. Further structural changes occur at Phe82 and the amino terminus where a four residue extension is present in yeast iso-2-cytochrome c. An additional comparison with all other eukaryotic cytochrome c structures determined to date is presented, along with an analysis of conserved water molecules. Also determined are the midpoint reduction potentials of iso-2 and B-2036 cytochromes c using direct electrochemistry. The values obtained are 286 and 288 mV, respectively, indicating that the amino acid substitutions present have had only a small impact on the heme reduction potential in comparison to iso-1-cytochrome c, which has a reduction potential of 290 mV.

Crystallization of yeast iso-2-cytochrome c using a novel hair seeding technique

A hair seeding technique has been developed to obtain diffraction quality crystals of yeast (Saccharomyces cerevisiae) iso-2-cytochrome c, a model for studies of protein folding and biological electron transfer reactions. Deep red crystals of this protein were obtained from 88 to 92% saturated solutions of ammonium sulfate containing 20 mg protein/ml, 0.1 M-sodium phoshate, 0.3 M-sodium chloride, 0.04 M-dithiothreitol and adjusted to phosphate, 0.3 M-sodium chloride, 0.04 M-dithiothreitol and adjusted to pH 6.0. Rapid crystal growth was observed, but only along the path of the seeding hair stroke. The space group is P4(3)2(1)2 (or P4(1)2(1)2) with a = b = 36.4 A, c = 137.8 A (1 A = 0.1 nm) and Z = 8. Crystals are stable in the X-ray beam for more than 10 days and diffract to at least 2.5 A resolution. The same hair seeding methodology has proven useful in obtaining crystals of specifically designed mutant iso-2 proteins and in other protein systems where consistent crystal growth had previously proven difficult to attain.

pH dependence of folding of iso-2-cytochrome c

Starting from a standard unfolded state (3.0 M guanidine hydrochloride, pH 7.2), the kinetics of refolding of iso-2-cytochrome c have been investigated as a function of final pH between pH 3 and pH 10. Absorbance in the ultraviolet and visible spectral regions and tryptophan fluorescence are used to monitor folding. Over most of the pH range, fast and slow folding phases are detected by both fluorescence and absorbance probes. Near neutral pH, the rate of fast folding appears to be the same when monitored by absorbance and fluorescence probes. At higher and lower pH, there are two fast folding reactions, with absorbance-detected fast folding occurring in a slightly faster time range than fluorescence-detected fast folding. The rates of both fast folding reactions pass through broad minima near neutral pH, indicating involvement of ionizable groups in rate-limiting steps. The rates of slow folding also depend on the final pH. At acid pH, there appears to be a single slow folding phase for both fluorescence and absorbance probes. At neutral pH, the absorbance-detected and fluorescence-detected slow folding phases separate into distinct kinetic processes which differ in rate and relative amplitude. At high pH, absorbance-detected slow folding is no longer observed, while fluorescence-detected slow folding is decreased in amplitude. In contrast, the equilibrium and kinetic properties of proline imide bond isomerization, believed to be involved in the slow folding reactions, are largely independent of pH. The results suggest that the pH dependence of slow folding involves coupling of pH-sensitive structure to proline imide bond isomerization.(ABSTRACT TRUNCATED AT 250 WORDS)

Guanidine hydrochloride induced equilibrium unfolding of mutant forms of iso-1-cytochrome c with replacement of proline-71

Proline-71, an evolutionally conserved residue that separates two short alpha-helical regions, is replaced by valine, threonine, or isoleucine in at least partially functional forms of iso-1-cytochrome c from Saccharomyces cerevisiae [Ernst, J. F., Hampsey, D. M., Stewart, J. W., Rackovsky, S., Goldstein, D., & Sherman, F. (1985) J. Biol. Chem. 260, 13225-13236]. Treatment of these proteins with a specific sulfhydryl blocking reagent (methyl methanethiosulfonate) to block Cys-102 has allowed investigation of the properties of monomeric forms of the proteins, denoted iso-1-MS. Comparison of the UV-visible absorbance properties (pH 6, 20 degrees C) shows minor differences between the normal Pro-71 iso-1-MS and two of the three mutant proteins. The Val-71 iso-1-MS protein has absorbance properties indistinguishable from those of the normal Pro-71 iso-1-MS protein, but the Ile-71 iso-1-MS and Thr-71 iso-1-MS proteins show reduced intensity of the 695-nm absorbance band and a small shift in the Soret maximum, from 408 nm for the Pro-71 iso-1-MS and Val-71 iso-1-MS proteins to 406 nm for the Thr-71 iso-1-MS and Ile-71 iso-1-MS proteins. Second derivative spectroscopy is used to assess differences in the polarity of the environment of tyrosine residues. The average degree of exposure of tyrosines to solvent is similar in all four proteins: 0.39 for the normal Pro-71 iso-1-MS and Val-71 iso-1-MS proteins; 0.40 for the Ile-71 iso-1-MS protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Folding/unfolding kinetics of mutant forms of iso-1-cytochrome c with replacement of proline-71

Proline-71, an evolutionally conserved residue that separates two short alpha-helical regions, is replaced by valine, threonine, or isoleucine in at least partially functional forms of iso-1-cytochrome c from Saccharomyces cerevisiae [Ernst, J. F., Hampsey, D. M., Stewart, J. W., Rackovsky, S., Goldstein, D., & Sherman, F. (1985) J. Biol. Chem. 260, 13225-13236]. To assign the effects of perturbations at position 71 to steps in the process of protein folding, the kinetic properties of the folding/unfolding reactions of normal protein and the three mutant forms are compared. At pH 6.0, 20 degrees C, fluorescence-detected folding/unfolding kinetics are monitored below, within, and above the equilibrium transition zone by using stopped-flow mixing to perform guanidine hydrochloride concentration jumps. Three kinetic phases are detected for each of the four proteins. The fastest of these phases (tau 3) differs in rate for the wild type and mutant proteins. The remaining kinetic phases (tau 1 and tau 2) have similar rates for all four proteins over the entire range of folding/unfolding conditions. The guanidine hydrochloride dependence of the relative amplitudes of the kinetic phases is complex and is sensitive to the nature of the substituent at position 71: each of the four proteins shows differences in the fraction of folding/unfolding associated with the two fastest rate processes. The results suggest that it is the location of the mutation in the primary structure rather than the nature of the substituent that determines which kinetic step (or steps) is changed in rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of a trypsin-pulse method to study the refolding pathway of ribonuclease

Trypsin pulses, applied after varying times of refolding, have been employed to probe the accessibility of the polypeptide chain of ribonuclease A during the process of refolding. The increase in resistance against proteolytic cleavage was measured by activity assays and by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The sites of cleavage which become inaccessible in the course of refolding are located in the 31 - 39 chain segment of the ribonuclease chain. Protection of this region against attack by trypsin is attained on the major slow refolding pathway in parallel with the formation of a native-like folded, active intermediate, when refolding is carried out under conditions which strongly stabilize the folded state. Under conditions of marginal stability intermediates are not observed during refolding and the formation of trypsin-resistant molecules occurs with the same kinetics as the generation of native ribonuclease. In the native protein the 31 - 39 region of the ribonuclease chain largely forms a loop structure, which is located at the surface of the molecule. Our results indicate that this part of the sequence is still accessible at early stages of refolding, when a hydrogen-bonded network is formed. It is structured and hence does not become inaccessible until the formation of the overall folded native or native-like structure. This suggests that the 31 - 39 region of the ribonuclease chain is not important for early steps which direct the pathway of refolding.

Native or nativelike species are transient intermediates in folding of alkaline iso-2 cytochrome c

Titration to high pH converts yeast iso-2 cytochrome c to an inactive but more stable alkaline form lacking a 695-nm absorbance band [Osterhout, J. J., Jr., Muthukrishnan, K., & Nall, B. T. (1985) Biochemistry 24, 6680-6684]. The kinetics of absorbance-detected refolding of the alkaline form have been measured by dilution of guanidine hydrochloride in a stopped-flow instrument. Fast-folding species (tau 2) are detected, as in refolding to the native state at neutral pH. An additional kinetic phase (tau a) is observed with an amplitude opposite in sign to the fast phase. The amplitude of this phase increases and the rate increases with increasing pH. Comparison to pH-jump measurements of the fully folded protein shows that phase tau a has the same sign, rate, and pH dependence as the alkaline isomerization reaction, suggesting that this new phase involves isomerization of native or nativelike species following fast folding. Absorbance difference spectra are taken at 5-s intervals during refolding at high pH. The spectra verify that nativelike species--with a 695-nm absorbance band--are formed transiently, before conversion of the protein to the alkaline form. Refolding in the presence of ascorbate shows that the transient, nativelike species are reducible, unlike alkaline iso-2. Thus, (1) refolding to the alkaline form of iso-2 cytochrome c proceeds through transient native or nativelike species, and (2) a folding pathway leading to native or nativelike forms is maintained at high pH, where native species are no longer the thermodynamically favored product.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of folding intermediates from the main toxin of Naja naja samarensis containing two blocked cysteines

The formation of folding intermediates blocked by iodoacetate from the main toxin of Naja naja samarensis was monitored by FPLC and the populations were identified by amino acid analyses. We have determined conditions allowing for the highest yield in the populations with two blocked cysteines. The free cysteines remaining under these conditions were labeled with iodo[14C]acetate and localized by peptide mapping in one of the products isolated by ion-exchange chromatography (NT3III). We have also investigated the effects on the native-like characteristics of such molecules of an incubation at equilibrium with a mixture of cysteine and cystine. We find that many different molecular populations are present during the folding process and that disulfide exchanges allow for the reconstitution of native-like products having open disulfides even under strongly denaturing conditions.

Role of proline peptide bond isomerization in unfolding and refolding of ribonuclease

The isomerization of the proline peptide bond between tyrosine-92 and proline-93 in bovine pancreatic ribonuclease A has been investigated in the unfolded protein as well as during the slow refolding process. This bond is in the cis state in the native protein. By comparison of various homologous ribonucleases we show that isomerization of proline-93 is associated with a change in fluorescence of tyrosine-92. This provides a spectroscopic probe to monitor this process in the disordered chain after unfolding as well as its reversal in the course of slow refolding. In unfolded ribonuclease incorrect trans isomers of proline-93 are found in both slow-folding species. trans----cis reversal of isomerization of this proline peptide bond during refolding shows kinetics that are identical with the time course of formation of native protein. Isomerization of proline-93 is slower than the formation of a native-like folded intermediate that accumulates on the major slow refolding pathway. Models to explain these results are discussed.