Isolation of four novel tachykinins from frog (Rana catesbeiana) brain and intestine

In a survey for unknown bioactive peptides in frog (Rana catesbeiana) brain and intestine, we isolated four novel peptides that exhibit potent stimulant effects on smooth muscle preparation of guinea pig ileum. By microsequencing and synthesis, these peptides were identified as Lys- Pro- Ser- Pro- Asp- Arg- Phe- Tyr- Gly- Leu- Met- NH2 (ranatachykinin A), Tyr- Lys- Ser- Asp- Ser- Phe- Tyr- Gly- Leu- Met- NH2 (ranatachykinin B), His- Asn- Pro- Ala- Ser- Phe- Ile- Gly- Leu- Met- NH2 (ranatachykinin C) and Lys- Pro- Ans- Pro- Glu- Arg- Phe- Tyr- Ala- Pro- Met- NH2 (ranatachykinin D). Ranatachykinin (RTK) A, B and C conserve the C- terminal sequence, Phe- X- Gly- Leu- Met- NH2, which is common to known members of the tachykinin family. On the other hand, RTK-D has a striking feature in its C-terminal sequence, Phe- Tyr- Ala- Pro- Met- NH2, which has never been found in other known tachykinins, and may constitute a new subclass in the tachykinin family.     

Replacements of lysine 32 in yeast cytochrome c. Effects on the binding and reactivity with physiological partners

Lysine 32 has been previously implicated by chemical modification and modeling studies as a key component of the domain which controls recognition and binding of cytochrome c to its physiological partners, e.g. cytochrome b2, cytochrome c peroxidase, and cytochrome oxidase. In order to quantitate the importance of this residue, we have investigated the role of Lys-32 in the reactivity of cytochrome c in redox reactions in vitro and in vivo with protein partners by using a series of altered forms of iso-1-cytochrome c from the yeast Saccharomyces cerevisiae in which Lys-32 is replaced by Leu-32, Gln-32, Trp-32, and Tyr-32. Leu-32 and Gln-32 represent substitutions which change charge without seriously affecting the steric bulk of the side chain or the stability of the protein. For the Leu-32- and Gln-32-altered proteins, steady state kinetic studies with cytochrome c peroxidase, cytochrome b2, and cytochrome oxidase showed that neither of the steady state kinetic parameters, Km nor Vmax, were substantially modified by mutation. Studies of single turnover kinetics with a small molecule (ascorbate) or within bound complexes with either cytochrome b5 or cytochrome c peroxidase demonstrated that redox kinetics are only slightly affected by these substitutions. NMR experiments demonstrated that the Gln-32-altered protein can still bind strongly to a physiological partner, cytochrome c peroxidase. Growth in lactate medium demonstrated that the activity in vivo compared with the normal value was reduced to only 85% with the Gln-32- and Leu-32-altered proteins and to 65% with the Trp-32- and Tyr-32-altered proteins. These findings suggest that the evolutionary invariance of Lys-32 reflects only small quantitative changes in the binding and reactivity of cytochrome c.     

Sequential1H-NMR assignments of neurotoxin III from the sea anemoneHeteractis macrodactylus and structural comparison with related toxins

The complete sequence-specific assignment of resonances in the¹H-NMR spectrum of the polypeptide neurotoxin III (Hm III) from the sea anemoneHeteractis macrodactylus is described. Comparison of the chemical shifts and pattern of NOEs for Hm III with those for the related toxin Hp III fromHeteractis paumotensis, which differs only in the substitution of Asn for Tyr at position 11, shows that the overall secondary and tertiary structures are conserved. The largest differences in chemical shift caused by the substitution at position 11 are observed for the NH resonances of Arg-13, Thr-14, Ala-15, Leu-17, and Cys-26. The C^αH resonances influenced most are those of ASP-6, Gly-9, Leu-17, and Glu-42, while the most affected C^βH resonances are from Leu-17, Glu-28, and Lys-32. The absence of long-range NOEs to the aromatic ring of Tyr-11 as well as the lack of significant chemical shift effects on residues outside the loop comprising residues 7–16 confirm that this part of the loop makes no long-lived contacts with the rest of the molecule. The deviations from random coil shifts of Hm III are compared with those of the related anemone toxins Hp II, Hp III, and toxin I fromStichodactyla helianthus (Sh I). The similarity in deviations in chemical shift as a function of sequence position for these four toxins emphasizes the overall structural homology among these polypeptides.     

Patterns of codon recognition by isoacceptor aminoacyl-tRNAs from wheat germ.

Isoacceptors of Ala-, Arg-, Glu-, Gln-, Ile-, Leu-, Lys-, Ser-, Thr- and Val-tRNAs from wheat germ have been resolved by reverse phast chromatography. Codon recognition properties have been determined on isolated fractions of each of these aa-tRNAs and codon assignments have been made to a number of isoacceptors. Evolutionary changes which have occurred in patterns of codon recognition by isoacceptor aa-tRNAs in wheat germ and other organisms are discussed.     

Primary structure of a 21-kD protein from potato tubers

A 21-kD protein isolated earlier from potato tubers (Solanum tuberosum L.) has two isoforms, with pI 6.3 and 5.2, which were separated by fast protein ion-exchange chromatography on a Mono Q column. The primary structures of the two forms consisted of 187 and 186 amino acid residues. Both isoforms are composed of two polypeptide chains, designated A and B, linked by a single disulfide bond between Cys-146 of the A chain and Cys-7 of the B chain. The amino acid sequences of the A chains of the two forms, consisting of 150 residues each, differ in a single amino acid residue at position 52 (Val --> Ile), while the B chains, containing 37 and 36 residues, respectively, have substitutions at nine positions (Leu-8 --> Ser-8, Lys-25--Asp-26 --> Asn-25--Glu-26, Ile-31--Ser-32 --> Val-31--Leu-32, Lys-34--Gln-35--Val-36--Gln-37 --> Gln-34--Glu-35--Val-36). Both isoforms form stable inhibiting complexes with human leukocyte elastase and are less effective against chymotrypsin and trypsin.     

Site-specific mutations in the N-terminal region of human C5a that affect interactions of C5a with the neutrophil C5a receptor.

C5a is an inflammatory mediator that evokes a variety of immune effector functions including chemotaxis, cell activation, spasmogenesis, and immune modulation. It is well established that the effector site in C5a is located in the C-terminal region, although other regions in C5a also contribute to receptor interaction. We have examined the N-terminal region (NTR) of human C5a by replacing selected residues in the NTR with glycine via site-directed mutagenesis. Mutants of rC5a were expressed as fusion proteins, and rC5a was isolated after factor Xa cleavage. The potency of the mutants was evaluated by measuring both neutrophil chemotaxis and degranulation (beta-glucuronidase release). Mutants that contained the single residue substitutions Ile-6-->Gly or Tyr-13-->Gly were reduced in potency to 4-30% compared with wild-type rC5a. Other single-site glycine substitutions at positions Leu-2, Ala-10, Lys-4, Lys-5, Glu-7, Glu-8, and Lys-14 showed little effect on C5a potency. The double mutant, Ile-6-->Gly/Tyr-13-->Gly, was reduced in potency to < 0.2%, which correlated with a correspondingly low binding affinity for neutrophil C5a receptors. Circular dichroism studies revealed a 40% reduction in alpha-helical content for the double mutant, suggesting that the NTR contributes stabilizing interactions that maintain local secondary or tertiary structure of C5a important for receptor interaction. We conclude that the N-terminal region in C5a is involved in receptor binding either through direct interaction with the receptor or by stabilizing a binding site elsewhere in the intact C5a molecule.     

Mapping of the amino acids in membrane-embedded helices that interact with the retinal chromophore in bovine rhodopsin

By using a photoactivatable analog of 11-cis-retinal in rhodopsin, we have previously identified the amino acids Phe-115, Ala-117, Glu-122, Trp-126, Ser-127, and Trp-265 as major sites of cross-linking to the chromophore. To further investigate the amino acids that interact with retinal, we have now used site-directed mutagenesis to replace a variety of amino acids in the membrane-embedded helices in bovine rhodopsin, including those that were indicated by cross-linking studies. The mutant rhodopsin genes were expressed in monkey kidney cells (COS-1) and purified. The mutant proteins were studied for their spectroscopic properties and their ability to activate transducin. Substitution of the two amino acids, Trp-265 and Glu-122 by Tyr, Phe, and Ala and by Gln, Asp and Ala, respectively, resulted in blue-shifted (20-30 nm) chromophore, and substitution of Trp-265 by Ala resulted in marked reduction in the extent of chromophore regeneration. Light-dependent bleaching behavior was significantly altered in Ala-117----Phe, Trp-265----Phe, Ala, and Ala-292----Asp mutants. Transducin activation was reduced in these mutants, in particular Trp-265 mutants, as well as in Glu-122----Gln, Trp-126----Leu (Ala), Pro-267----Ala (Asn, Ser), and Tyr-268----Phe mutants. These findings indicate that Trp-265 is located close to retinal and Glu-122, Trp-126, and probably Tyr-268 are also likely to be near retinal.     

Specificity of an extracellular proteinase from Brevibacterium linens ATCC 9174 on bovine beta-casein.

The specificity of the extracellular proteinase from Brevibacterium linens ATCC 9174 on bovine beta-casein was studied. Hydrolysis was monitored over time by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (PAGE) and urea-PAGE. The major pH 4.6-soluble peptides were isolated by high-performance liquid chromatography and identified by N-terminal amino acid sequencing and mass spectrometry. The major sites of hydrolysis were Ser-18-Ser-19, Glu-20-Glu-21, Gln-56-Ser-57, Gln-72-Asn-73, Leu-77-Thr-78, Ala-101-Met-102, Phe-119-Thr-120, Leu-139-Leu-140, Ser-142-Trp-143, His-145-Gln-146, Gln-167-Ser-168, Gln-175-Lys-176, Tyr-180-Pro-181, and Phe-190-Leu-191. The proteinase had a broad specificity for the amino acid residues present at the P1 and P'1 positions but showed a preference for hydrophobic residues at the P2, P3, P4, P'2, P'3, and P'4 positions.     

The yeast mitochondrial citrate transport protein. Probing the secondary structure of transmembrane domain iv and identification of residues that likely comprise a portion of the citrate translocation pathway

The mitochondrial citrate transport protein (CTP) has been investigated by replacing 22 consecutive residues within transmembrane domain IV, one at a time, with cysteine. A cysteine-less CTP retaining wild-type functional properties served as the starting template. The single Cys CTP variants were overexpressed in Escherichia coli, isolated, and functionally reconstituted in a liposomal system. The accessibility of each single Cys mutant to three methanethiosulfonate reagents was evaluated by determining the pseudo first order rate constants for inhibition of CTP function. These rate constants varied by seven orders of magnitude. With three independent data sets we observed peaks and troughs in the rate constant data at identical amino acid positions and a periodicity of four was observed from residues 177-193. Based on the pattern of accessibility we conclude that residues 177-193 exist as an alpha-helix. Furthermore, a water-accessible face of the helix has been defined consisting of Pro-177, Val-178, Arg-181, Gln-182, Asn-185, Gln-186, Arg-189, Leu-190, and Tyr-193, and a water-inaccessible face has been delineated consisting of Ser-179, Met-180, Ala-183, Ala-184, Ala-187, Val-188, Gly-191, and Ser-192. We infer that the water-accessible face comprises a portion of the substrate translocation pathway through the CTP, whereas the water-inaccessible surface faces the lipid bilayer.     

Proton transfer roles of lysine 64 and glutamic acid 64 replacing histidine 64 in the active site of human carbonic anhydrase II.

The CO2 hydration activities of cloned human carbonic anhydrase II (carbonate hydro-lyase, EC 4.2.1.1) and variants with Lys, Glu, Gln or Ala replacing His at sequence position 64 have been measured in a variety of different buffers in the pH range 6-9. The variants with Lys-64, Gln-64 and Ala-64 showed non-Michaelis-Menten behavior under some conditions, apparent substrate inhibition being prominent near pH 9. However, asymptotic Michaelis-Menten parameters could be estimated for the limit of low substrate concentrations. All variants show distinct buffer specificities, and imidazole derivatives, Ches and phosphate buffers yield higher kcat values that Bicine, Taps and Mops buffers under otherwise similar conditions. These results are interpreted in terms of different pathways for a rate-limiting proton transfer. In unmodified enzyme, the very high catalytic activity depends on His-64 functioning as an efficient proton transfer group, but this pathway is not available in the variants with Gln-64 and Ala-64. Imidazoles, Ches and phosphate are thought to participate in a metal center-to-buffer proton transfer pathway, whereas Bicine, Taps, Mops and Mes appear to lack this capacity, so that the rate-limiting proton transfer occurs in a metal center-to-bulk water pathway for these variants. The Lys-64 and Glu-64 variants give significantly higher kcat values in Taps, Mops and Mes buffers than the Ala-64 and Gln-64 variants. The pH dependencies of these kcat values are compatible with the hypothesis that Lys-64 and Glu-64 can function as proton transfer groups. Thus, at pH near 9, Lys-64 appears to be only 5-times less efficient than His-64, while Glu-64 is inefficient. At pH 6, Lys-64 is an inefficient proton transfer group, but Glu-64 is only 2-3-times less efficient than His-64. The data indicate that Lys-64 and Glu-64 have pKa values near 8 and below 6, respectively.     

Stability-increasing mutants of glucose dehydrogenase from Bacillus megaterium IWG3

A glucose dehydrogenase gene was isolated from Bacillus megaterium IWG3, and its nucleotide sequence was identified. The amino acid sequence of the enzyme deduced from the nucleotide sequence is very similar to the protein sequence of the enzyme from B. megaterium M1286 reported by Jany et al. (Jany, K.-D., Ulmer, W., Froschle, M., and Pfleiderer, G. (1984) FEBS Lett. 165, 6-10). The isolated gene was mutagenized with hydrazine, formic acid, or sodium nitrite, and 12 clones (H35, H39, F18, F20, F191, F192, N1, N13, N14, N28, N71, and N72) containing mutant genes for thermostable glucose dehydrogenase were obtained. The nucleotide sequences of the 12 genes show that they include 8 kinds of mutants having the following amino acid substitutions: H35 and H39, Glu-96 to Gly; F18 and F191, Glu-96 to Ala; F20, Gln-252 to Leu; F192, Gln-252 to Leu and Ala-258 to Gly; N1, Glu-96 to Lys and Val-183 to Ile; N13 and N14, Glu-96 to Lys, Val-112 to Ala, Glu-133 to Lys, and Tyr-217 to His; N28, Glu-96 to Lys, Asp-108 to Asn, Pro-194 to Gln, and Glu-210 to Lys; and N71 and N72, Tyr-253 to Cys. These mutant enzymes have higher stability at 60 degrees C than the wild-type enzyme. The results of this study indicate that the tetrameric structure of glucose dehydrogenase is stabilized by several kinds of mutation, and at least one of the following amino acid substitutions stabilizes the enzyme: Glu-96 to Gly, Glu-96 to Ala, Gln-252 to Leu, and Tyr-253 to Cys.     

Aminoacyl-tRNAs from Physarum polycephalum: patterns of codon recognition.

Isoacceptors of Physarum polycephalum Ala-, Arg-, Glu-, Gln-, Gly-, Ile-, Leu-, Lys-, Ser-, Thr-, and Val-tRNAs were resolved by reverse-phase chromatography and isolated, and their codon recognition properties were determined in a ribosomal binding assay. Codon assignments were made to most isoacceptors, and they are summarized along with those determined in other studies from Escherichia coli, yeasts, wheat germ, hymenoptera, Xenopus, and mammals. The patterns of codon recognition by isoacceptors from P. polycephalum are more similar to those of animals than to those of plants or lower fungi.     

Factor XI homodimer structure is essential for normal proteolytic activation by factor XIIa, thrombin, and factor XIa

Coagulation factor XI (FXI) is a covalent homodimer consisting of two identical subunits of 80 kDa linked by a disulfide bond formed by Cys-321 within the Apple 4 domain of each subunit. Because FXI(C321S) is a noncovalent dimer, residues within the interface between the two subunits must mediate its homodimeric structure. The crystal structure of FXI demonstrates formation of salt bridges between Lys-331 of one subunit and Glu-287 of the other subunit and hydrophobic interactions at the interface of the Apple 4 domains involving Ile-290, Leu-284, and Tyr-329. FXI(C321S), FXI(C321S,K331A), FXI(C321S,E287A), FXI(C321S,I290A), FXI(C321S,Y329A), FXI(C321S,L284A), FXI(C321S,K331R), and FXI(C321S,H343A) were expressed in HEK293 cells and characterized using size exclusion chromatography, analytical ultracentrifugation, electron microscopy, and functional assays. Whereas FXI(C321S) and FXI(C321S,H343A) existed in monomer/dimer equilibrium (K(d) approximately 40 nm), all other mutants were predominantly monomers with impaired dimer formation by analytical ultracentrifugation (K(d)=3-38 microm). When converted to the active enzyme, FXIa, all the monomeric mutants activated FIX similarly to wild-type dimeric FXIa. In contrast, these monomeric mutants could not be activated efficiently by FXIIa, thrombin, or autoactivation in the presence of dextran sulfate. We conclude that salt bridges formed between Lys-331 of one subunit and Glu-287 of the other together with hydrophobic interactions at the interface, involving residues Ile-290, Leu-284, and Tyr-329, are essential for homodimer formation. The dimeric structure of FXI is essential for normal proteolytic activation of FXI by FXIIa, thrombin, or FXIa either in solution or on an anionic surface but not for FIX activation by FXIa in solution.     

Secondary structure and side-chain 1H and 13C resonance assignments of calmodulin in solution by heteronuclear multidimensional NMR spectroscopy

Heteronuclear 2D and 3D NMR experiments were carried out on recombinant Drosophila calmodulin (CaM), a protein of 148 residues and with molecular mass of 16.7 kDa, that is uniformly labeled with 15N and 13C to a level of greater than 95%. Nearly complete 1H and 13C side-chain assignments for all amino acid residues are obtained by using the 3D HCCH-COSY and HCCH-TOCSY experiments that rely on large heteronuclear one-bond scalar couplings to transfer magnetization and establish through-bond connectivities. The secondary structure of this protein in solution has been elucidated by a qualitative interpretation of nuclear Overhauser effects, hydrogen exchange data, and 3JHNH alpha coupling constants. A clear correlation between the 13C alpha chemical shift and secondary structure is found. The secondary structure in the two globular domains of Drosophila CaM in solution is essentially identical with that of the X-ray crystal structure of mammalian CaM [Babu, Y., Bugg, C. E., & Cook, W.J. (1988) J. Mol. Biol. 204, 191-204], which consists of two pairs of a "helix-loop-helix" motif in each globular domain. The existence of a short antiparallel beta-sheet between the two loops in each domain has been confirmed. The eight alpha-helix segments identified from the NMR data are located at Glu-6 to Phe-19, Thr-29 to Ser-38, Glu-45 to Glu-54, Phe-65 to Lys-77, Glu-82 to Asp-93, Ala-102 to Asn-111, Asp-118 to Glu-127, and Tyr-138 to Thr-146. Although the crystal structure has a long "central helix" from Phe-65 to Phe-92 that connects the two globular domains, NMR data indicate that residues Asp-78 to Ser-81 of this central helix adopt a nonhelical conformation with considerable flexibility.     

Sequential1H-NMR assignments of neurotoxin III from the sea anemoneHeteractis macrodactylus and structural comparison with related toxins

The complete sequence-specific assignment of resonances in the¹H-NMR spectrum of the polypeptide neurotoxin III (Hm III) from the sea anemoneHeteractis macrodactylus is described. Comparison of the chemical shifts and pattern of NOEs for Hm III with those for the related toxin Hp III fromHeteractis paumotensis, which differs only in the substitution of Asn for Tyr at position 11, shows that the overall secondary and tertiary structures are conserved. The largest differences in chemical shift caused by the substitution at position 11 are observed for the NH resonances of Arg-13, Thr-14, Ala-15, Leu-17, and Cys-26. The CH resonances influenced most are those of ASP-6, Gly-9, Leu-17, and Glu-42, while the most affected CH resonances are from Leu-17, Glu-28, and Lys-32. The absence of long-range NOEs to the aromatic ring of Tyr-11 as well as the lack of significant chemical shift effects on residues outside the loop comprising residues 7–16 confirm that this part of the loop makes no long-lived contacts with the rest of the molecule. The deviations from random coil shifts of Hm III are compared with those of the related anemone toxins Hp II, Hp III, and toxin I fromStichodactyla helianthus (Sh I). The similarity in deviations in chemical shift as a function of sequence position for these four toxins emphasizes the overall structural homology among these polypeptides.     

Secondary structure determination of human beta-endorphin by 1H NMR spectroscopy

The 1H NMR spectra of human beta-endorphin indicate that the peptide exists in random-coil form in aqueous solution but becomes helical in mixed solvent. Thermal denaturation NMR experiments show that in water there is no transition between 24 and 75 degrees C, while a slow noncooperative thermal unfolding is observed in a 60% methanol-40% water mixed solvent in the same temperature range. These findings are consistent with circular dichroism studies by other workers concluding that beta-endorphin is a random coil in water but that it forms 50% alpha-helix or more in mixed solvents. The peptide in the mixed water-methanol solvent was further studied by correlated spectroscopy (COSY) and nuclear Overhauser effect spectroscopy (NOESY) experiments. These allow a complete set of assignments to be made and establish two distinct stretches over which the solvent induces formation of alpha-helices: the first occurs between Tyr-1 and Thr-12 and the second between Leu-14 and extending to Lys-28. There is evidence that the latter is capped by a turn occurring between Lys-28 and Glu-31. These helices form at the enkephalin receptor binding site, which is at the amino terminus, and at the morphine receptor binding site, located at the carboxyl terminus [Li, C. H. (1982) Cell (Cambridge, Mass.) 31, 504-505]. Our findings suggest that these two receptors may specifically recognize alpha-helices.     

Kinetic consequences of site-specific mutation of Glu-239----Gln in E. coli aspartate transcarbamylase: comparison with catalytic subunits and Phe-240 mutant enzyme

The kinetic characteristics of E. coli aspartate transcarbamylase, altered by site-specific mutagenesis of Glu-239----Gln, have been determined by equilibrium isotope-exchange kinetics and compared to the wild-type system. In wild-type enzyme, residue Glu-239 helps to stabilize the T-state structure by multiple bonding interactions with Tyr-165 and Lys-164 across the c1-c4 subunit interface; upon conversion to the R-state, these bonds are re-formed within c-chains. Catalysis of both the [14C]Asp in equilibrium C-Asp and [32P]ATP in equilibrium Pi exchanges by mutant enzyme occurs at rates comparable to those for wild-type enzyme. Saturation with different reactant/product pairs produced kinetic patterns consistent with strongly preferred order binding of carbamyl-P prior to Asp and carbamyl-Asp release before Pi. The kinetics for the Gln-239 mutant enzyme resemble those observed for catalytic subunits (c3), namely a R-state enzyme (Hill coefficient nH = 1.0) and Km (Asp) approximately equal to 6 mM. The Glu-239----Gln mutation appears to destablize both the T- and R-states, whereas the Tyr-240----Phe mutation destablizes only the T-state.     

Structural analysis of the zinc hydroxide–Thr-199–Glu-106 hydrogen-bond network in human carbonic anhydrase II

The singnificance of the zinc hydroxide–Thr-199–Glu-106 hydrogen-bond network in the active site of human carbonic anhydrase II has been examined by X-ray crystallographic analyses of site-specific mutants. Mutants with Ala-199 and Ala-106 or Gln-106 have low catalytic activities, while a mutant with Asp-106 has almost full CO₂ hydration activity. The structures of these four mutants, as well as that of the bicarbonate complex of the mutant with Ala-199, have been determined at 1.7 to 2.2 Å resolution. Removal of the γ atoms of residue 199 leads to distorted tetrahedral geometry at the zine ion, and a catalytically important zinc-bound water molecule has moved towards Glu-106. In the bicarbonate complex of the mutant with Ala-199 one oxygen atom from bicarbonate binds to zinc without displacing this water molecule. Tetrahedral coordination geometries are retained in the mutants at position 106. The mutants with Ala-106 and Gln-106 have a zinc-bound sulfate ion, whereas this sulfate site is only partially occupied in the mutant with Asp-106. The hydrogen-bond network seems to be “reversed” in the mutants with Ala-106 and Gln-106. The network is preserved as in native enzyme in the mutant with Asp-106 but the side chain of Asp-106 is more extended than that of Glu-106 in the native enzyme. These results illustrate the importance of Glu-106 and Thr-199 for controlling the precise coordination geometry of the zinc ion and its ligand preferences with results in an optimal orientation of a zine-bound hydroxide ion for an attack on the CO₂ substrate. © 1993 Wiley-Liss, Inc.     

Thermodynamic stabilities of yeast iso-1-cytochromes c having amino acid substitutions for lysine 32

Iso-1-cytochromes c having lysine 32 replaced by leucine, glutamine, tyrosine, and tryptophan were prepared from strains of bakers' yeast, Saccharomyces cerevisiae, and chemically blocked at cysteine 107 with methyl methanethiolsulfonate to prevent dimerization. These modified ferricytochromes c were guanidine denatured, and the unfolding thermodynamics were determined by circular dichroism and fluorescence measurements. Thermal unfolding was also monitored by absorbance measurements. The guanidine denaturation midpoints for the altered proteins are smaller than the wild type, while the orders of stability from unfolding free energy changes are: Lys-32 (wild type) approximately Leu-32 approximately Gln-32 (circular dichroism), greater than Gln-32 (fluorescence) greater than Tyr-32 approximately Trp-32. Midpoints and differences in free energy changes for thermal unfolding parallel the fluorescence free energy changes for guanidine-induced unfolding. Thus, the blocked Leu-32 and Lys-32 proteins are equally stable with respect to both chemical and thermal denaturation. The reported data indicate that single replacements may significantly modify protein stability, and that substitution for an evolutionarily retained residue in normal cytochrome c structures does not always destabilize the protein. In addition, in vitro thermal stabilities approximately correlate with in vivo specific activities.     

Selective proteolytic cleavage of recombinant human interleukin 4. Evidence for a critical role of the C-terminus

Human interleukin 4 is a 129 amino acid lymphokine secreted by activated T cells that exerts pleiotropic biological effects on B and T lymphocytes and other hematopoietic cells. Structure-function relations were studied by employing selective proteolytic cleavage of purified recombinant human interleukin 4 (rhuIL-4). Limited proteolysis with endoprotease Glu-C from Staphylococcus aureus (V8) produced two digestion products that were observed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with apparent molecular weight values of 19K (I) and 15K (II), respectively. These species were isolated by reversed-phase HPLC. Amino acid sequencing indicated that species II was an 84 amino acid core fragment extending from Gln-20 to Glu-103 and containing a hydrolyzed peptide bond at Glu-26. On the basis of known disulfide bond assignments, it was concluded that species II was stabilized by two disulfide bonds (Cys-24/Cys-65 and Cys-46/Cys-99). Analysis of its secondary structure by circular dichroism revealed a high content of alpha helix. Species I was the full-length rhuIL-4 with selective cleavage at Glu-26 and Glu-103. Both species I and II were inactive in an in vitro assay based on proliferation of peripheral blood lymphocyte blasts and lacked the ability to bind to teh rhuIL-4 receptor on Daudi cells. In order to elucidate further the role of the residues removed by S. aureus V8 protease, rabbit antisera were raised to synthetic peptides corresponding to residues 1-26 at the N-terminus and 104-129 at the C-terminus. Only antisera directed to the C-terminal peptide inhibited binding of 125I-rhuIL-4 to Daudi cells.(ABSTRACT TRUNCATED AT 250 WORDS)