Quail cystatin: isolation and characterisation of a new member of the cystatin family and its hypothetical interaction with cathepsin B

Quail cystatin, a new cysteine proteinase inhibitor protein of the cystatin superfamily, was purified from egg albumen of Japanese quail Coturnix coturnix japonica. Amino acid sequencing and mass spectrometry revealed the complete 116 amino acid residue primary structure of a phosphorylated form (13,173 Da). The inhibitor has a 90% sequence identity with chicken cystatin. Its interaction with papain is rapid and tight (Ki = 4.4 pM; k(on) = 1.8x10(7) M(-1) s(-1); k(off) = 0.8x10(-4) s(-1)) and very similar to that of chicken cystatin. Surprisingly, however, cathepsin B was inhibited 15-fold more strongly by quail cystatin (Ki = 47 pM; k(on) = 19x10(7) M(-1) s(-1); k(off) = 9x10(-4) s(-1)) than by chicken cystatin (Ki = 784 pM; k(on) = 2.9x10(7) M(-1) s(-1); k(off) = 24x10(-4) s(-1)). Intuitive comparative conformational inspection of related inhibitors and of cognate enzymes suggest that: (i) the 3D structure of quail cystatin is nearly identical to that of chicken cystatin, (ii) quail cystatin can interact with cathepsin B analogous to the stefin B-papain interaction, if the 'occluding loop' of cathepsin B possesses an 'open' conformation, (iii) the greater inhibition of cathepsin B by quail cystatin compared to chicken cystatins probably arises from two additional ionic interactions between residues Arg15 and Lys112 of the inhibitor and Glu194 and Asp124 of the enzyme, respectively. The two potential salt bridges are located outside of the known contact regions between cystatins and peptidases of the papain family.

Analysis of proteins by direct-scanning infrared-MALDI mass spectrometry after 2D-PAGE separation and electroblotting

A novel approach is reported for the analysis and identification of proteins separated by 2D-PAGE with scanning infrared matrix-assisted laser desorption/ionization mass spectrometry (scanning IR-MALDI-MS). The proteins of human blood plasma were separated by 2D-PAGE, electroblotted onto PVDF membranes, incubated in matrix solution, and then scanned by IR-MALDI-MS. Mass contour plots of selected spots were obtained. Protein separation is shown to be conserved by comparison with silver-stained gels. The sensitivity for the protein detection is comparable if not better than that of silver-stained gels. Posttranslational modifications were identified by comparing the measured mass to the one calculated from the known DNA sequence. Adduct formation to unprotected cysteine residues during gel separation is demonstrated for selected proteins.

Differential translational initiation of lbp mRNA is caused by a 5' upstream open reading frame

Expression of the luciferin-binding protein (LBP) from Gonyaulax polyedra is regulated by the circadian clock at the translational level. Here we report that in vitro translation of lbp mRNA results in the synthesis of two LBP variants of different sizes, which is shown to be due to translational initiation at different in-frame AUG codons on lbp mRNA. Differential initiation is caused by a small open reading frame (ORF, situated in the 5' untranslated region of lbp mRNA), which gives rise to a leaky scanning mechanism. In Gonyaulax, only one of these variants, which is produced by initiation from the first AUG of the lbp ORF, exhibits a circadian rhythm and is far more abundant during night phase.

Tom5 functionally links mitochondrial preprotein receptors to the general import pore

Most mitochondrial proteins are synthesized as preproteins on cytosolic polysomes and are subsequently imported into the organelle. The mitochondrial outer membrane contains a multisubunit preprotein translocase (Tom) which has receptors on the cytosolic side and a general import pore (GIP) in the membrane. Tom20-Tom22 and Tom70-Tom37 function as import receptors with a preference for preproteins that have amino-terminal presequences or internal targeting information, respectively. Tom40 is an essential constituent of the GIP, whereas Tom6 and Tom7 modulate the assembly and dissociation of the Tom machinery. Here we report the identification of Tom5, a small subunit that has a crucial role importing preproteins destined for all four mitochondrial subcompartments. Tom5 has a single membrane anchor and a cytosolic segment with a negative net charge, and accepts preproteins from the receptors and mediates their insertion into the GIP. We conclude that Tom5 represents a functional link between surface receptors and GIP, and is part of an 'acid chain' that guides the stepwise transport of positively charged mitochondrial targeting sequences.

High-sensitivity peptide mapping by micro-LC with on-line membrane blotting and subsequent detection by scanning-IR-MALDI mass spectrometry

A novel approach to the on-line mass determination of peptides from digested proteins by scanning infrared matrix-assisted laser desorption/ionization (scanning-IR-MALDI) is described. The peptides were continuously collected directly onto a PVDF (polyvinylidene fluoride) strip during a HPLC run. Individual peptides were detected by lining up the PVDF strip with the UV trace from the HPLC run, using visible dye markers as reference points. The local resolution of the peptides on the PVDF membrane is preserved during matrix incubation for MALDI-MS as shown by comparing the UV chromatogram and the total ion current (TIC) from an on-line coupled electrospray ionization (ESI) mass spectrometer with the scanning-IR-MALDI data from the corresponding areas on the PVDF strip. The intensities of the mass profiles obtained by scanning-IR-MALDI reflect the amount of peptides present on the PVDF strip. The higher sensitivity of IR-MALDI-MS yielded mass information not detectable by ESI-MS. After the scanning-IR-MALDI experiment, the same membrane strip can be used directly for automated Edman degradation. Comparable initial and repetitive yields were obtained for blotted peptides with and without matrix incubation.

A recombinant polypeptide model of the second nucleotide-binding fold of the cystic fibrosis transmembrane conductance regulator functions as an active ATPase, GTPase and adenylate kinase

CFTR-NBF-2 expressed and purified in fusion with the maltose-binding protein was shown to catalyse the reaction ATP-->ADP+Pi by three different assays, monitoring ATP turnover, formation of ADP and release of Pi (Km 86 microM, rate constant 0.37 min(-1)). The reaction product ADP inhibits this ATPase activity. In a similar manner the hydrolysis of GTP to GDP and Pi was demonstrated (Km 40 microM, rate constant 0.29 min(-1)). In the presence of AMP the ATPase reaction was superseded by the formation of two ADP from ATP and AMP. As typical for adenylate kinases a distinct AMP-binding site could be verified for CFTR-NBF-2 by the inability of TNP-ATP and AMP to compete for binding. All three enzymatic activities were inhibited by the symmetric double-substrate-mimicking inhibitor Ap5A. As NBF-2 plays a central role in CFTR channel opening and closing the results reported here are fundamental in understanding mechanisms of CFTR channel activity regulation.

Molecular cloning of SKAP55, a novel protein that associates with the protein tyrosine kinase p59fyn in human T-lymphocytes

In human T-lymphocytes the Src family protein tyrosine kinase p59(fyn) associates with three phosphoproteins of 43, 55, and 85 kDa (pp43, pp55, and pp85). Employing a GST-Fyn-Src homology 2 (SH2) domain fusion protein pp55 was purified from lysates of Jurkat T-cells. Molecular cloning of the pp55 cDNA reveals that the pp55 gene codes for a so far nondescribed polypeptide of 359 amino acids that comprises a pleckstrin homology domain, a C-terminal SH3 domain, as well as several potential tyrosine phosphorylation sites, among which one fulfills the criteria to bind Src-like SH2 domains with high affinity. Consistent with this observation, pp55 selectively binds to isolated SH2 domains of Lck, Lyn, Src, and Fyn but not to the SH2 domains of ZAP70, Syk, Shc, SLP-76, Grb2, phosphatidylinositol 3-kinase, and c-abl in vitro. Based on these properties the protein was termed SKAP55 (src kinase-associated phosphoprotein of 55 kDa). Northern blot analysis shows that SKAP55 mRNA is preferentially expressed in lymphatic tissues. SKAP55 is detected in resting human T-lymphocytes as a constitutively tyrosine phosphorylated protein that selectively interacts with p59(fyn). These data suggest that SKAP55 represents a novel adaptor protein likely involved in Fyn-mediated signaling in human T-lymphocytes.

Postimport methylation of the small subunit of ribulose-1,5-bisphosphate carboxylase in chloroplasts

Electron impact mass spectronomy analysis of the amino-terminal amino acid of the small subunit (SSU) of ribulose-1,5-bisphosphate carboxylase (Rubisco) showed that the amino-terminal methionine residue is post-translationally modified to N-methyl-methionine. Modification of the amino-terminal methionine residue was found in mature SSU proteins from the dicotyledonous plants pea and spinach as well as the monocotyledonous plants barley and corn. SSU methyltransferase is a soluble protein in the chloroplast stroma and accepts heterologously expressed non-methylated SSU as a substrate using S-adenosylmethionine as methyl-group donor. We show that this modification occurs after post-translational uptake of the precursor form of SSU into chloroplasts and processing to its mature size. This reaction represents a new step in the import and assembly pathway of Rubisco holoenzyme.

Electrospray ionization mass spectrometry of genetically and chemically modified bacteriorhodopsins

A simple method for the preparation of bacterio-rhodopsin for mass spectrometry is described, consisting of precipitation of the sample, removal of lipids by washing the sample, and dissolving it in an acidic mixture of organic solvents. Examples demonstrate the method's suitability for the analysis of genetically and chemically modified bacteriorhodopsins. The observed molecular masses are within 0.01% in accordance with the theoretical masses of the wild-type protein and modification products. This allows the detection of modifications accompanied by a mass change greater than 2 Da. Thus, only 9 of 380 possible binary amino acid exchanges are not amenable to analysis. Also chemical modifications of bacteriorhodopsin can be followed since it is possible to characterize reaction products and to approximate yields. However, secondary reactions like hydrolysis in the reaction medium or in the acid solvent used for sample preparation must be considered.

Cloning of a cDNA encoding a plasma membrane-associated, uronide binding phosphoprotein with physical properties similar to viral movement proteins

Oligogalacturonides are structural and regulatory homopolymers from the extracellular pectic matrix of plants. In vitro micromolar concentrations of oligogalacturonates and polygalacturonates were shown previously to stimulate the phosphorylation of a small plasma membrane-associated protein in potato. Immunologically cross-reactive proteins were detected in plasma membrane-enriched fractions from all angiosperm subclasses in the Cronquist system. Polygalacturonate-enhanced phosphorylation of the protein was observed in four of the six dicotyledon subclasses but not in any of the five monocotyledon subclasses. A cDNA for the protein was cloned from potato. The deduced protein is extremely hydrophilic and has a proline-rich N terminus. The C-terminal half of the protein was predicted to be a coiled coil, suggesting that the protein interacts with other macromolecules. The recombinant protein was found to bind both simple and complex galacturonides. The behavior of the protein suggests several parallels with viral proteins involved in intercellular communication.

The amino acid sequence of the spinach basic cupredoxin plantacyanin

We describe the amino acid sequence of plantacyanin isolated from spinach leaves. This small basic copper-binding plant cupredoxin contains 91 amino acids (accession no. S70746) and its theoretical average mass of 9788.2 Da was confirmed by electrospray ionization mass spectrometry. When compared to the sequences of other plant cupredoxins, the most similar homologs were cucumber basic blue protein and stellacyanin from Laquer tree with 44-58% identical residues. Other known plant cupredoxins were significantly less similar which may allow to divide the plant cupredoxins into two subgroups, the single-domain cupredoxins (plantacyanin, cucumber basic blue protein and stellacyanin) and the two-domain, hydroxyproline-containing cupredoxins (cucumber peeling cupredoxin, basic blue protein from Arabidopsis thaliana and horseradish umecyanin).

Isolation of highly purified, functional carboxy-terminally truncated hepatitis B virus middle surface protein activators from eucaryotic expression systems

Carboxy-terminally truncated hepatitis B virus (HBV) middle surface proteins (MHBst) show a transcriptional activator function. Two different subtypes of MHBst activators can be distinguished: an ER-localized type, represented here by MHBst76 (truncated at amino acid 76), and a cytosol-localized type, represented here by MHBst63. To characterize the MHBst activator on the protein level and to analyze posttranslational modifications, we established recombinant baculoviruses encoding for fusion proteins of MHBst76 or MHBst63 and of an amino terminal hexa-his tag. Both proteins could be obtained in high purity by affinity chromatography using Ni-nitrilo-tri-acetate agarose. In addition, 6H-MHBst76 was also isolated from transiently transfected HepG2 cells. Both the Spodoptera frugiperda (Sf9) cell-derived and the HepG2 cell-derived MHBst proteins were found to be unglycosylated. A detailed analysis of Sf9 cell-derived 6H-MHFBst76 by electrospray-ionization mass spectrometry showed that a fraction of this protein is N-terminally acetylated and phosphorylated or sulfated. Electric-field-mediated transfer of the highly purified proteins into reporter cells demonstrated that the isolated proteins are functional transcriptional activators. These experiments further showed that Sf9 cell-derived and HepG2 cell-derived 6H-MHBst do not differ in their functionality. This system allowed production and purification of functional 6H-MHBst in amounts sufficient enough to allow a further detailed analysis of MHBst activators on the protein level.

Glycosylated threonine but not 4-hydroxyproline dominates the triple helix stabilizing positions in the sequence of a hydrothermal vent worm cuticle collagen

The cuticle collagen of the vestimentiferan Riftia pachyptila, an organism which is endemic to deep-sea hydrothermal vents, has several unusual properties including an extraordinary length (1.5 microns), a high thermal stability (37 degrees C) in spite of a low 4-hydroxyproline content and an atypically high threonine content (20 mol%). We have now purified the constituent chain of cuticle collagen and show that it contains about 40% carbohydrate, which is mainly galactose, indicating that the chain has a molecular mass of approximately 750 kDa. Several large (30 to 150 kDa) fragments, which all contained carbohydrate, could be produced by cleavage with endoproteinase Lys-C, bacterial collagenase and cyanogen bromide (CNBr). Edman degradation of these and several smaller fragments was used to determine about 3000 sequence positions comprising 60% of the total triple-helical sequence. This demonstrated mainly typical Gly-X-Y triplet repeats with a few imperfections and a longer N-terminal non-triplet sequence. Most of the 4-hydroxyproline was found in triplet position X, where it decreases the stability of the triple helix. About 40% of the Y positions could not be identified, which correlated with a low abundance of threonine in the sequence and the demonstration of threonine in these positions after deglycosylation of several peptides by treatment with hydrofluoric acid. Matrix-assisted laser desorption ionisation mass spectrometry of selected peptides indicated that the blocked threonine residues are occupied by chains of one, two or three hexoses (presumably galactose). These glycosylated threonine residues in Y positions are therefore likely to replace 4-hydroxyproline as the major contributor to triple helix stabilization. Studies with a synthetic (Gly-Pro-Thr)10 oligopeptide demonstrated a low thermal stability of its triple helix which emphasizes a crucial role of glycosylation for stabilization.

A novel class of small amphipathic peptides affect aerial hyphal growth and surface hydrophobicity in Ustilago maydis

Ustilago maydis, a fungal pathogen of corn, can alternate between yeast-like and filamentous growth. This dimorphic switch is governed by the mating-type loci. We have identified an abundant class of small SDS-insoluble cell wall proteins, designated repellents, specifically present in the filamentous form. Genetic analysis revealed that these peptides are processed from a single precursor protein, Rep1. Rep1 comprises 652 amino acids with a leader sequence for secretion. A characteristic feature of Rep1 is 12 repeats of a 37 amino acid consensus sequence; 10 of these repeats are separated by Kex2 protease cleavage sites. In (delta)rep1 mutants formation of aerial hyphae and surface hydrophobicity were reduced dramatically. This and the fact that expression of rep1 is regulated by the mating-type loci indicates that repellents play a structural role in the formation of aerial hyphae.

Structure/function studies on the pH-dependent actin-binding protein hisactophilin in Dictyostelium mutants

Our previous studies have shown that the actin-binding protein hisactophilin from Dictyostelium discoideum is a candidate for organizing the actin cytoskeleton at the plasma membrane in a pH-dependent manner. To further characterize this interaction we isolated hisactophilin overexpression (hisII+) and hisactophilin minus (his-) mutants. D. discoideum contains two hisactophilin isoforms; both genes are independently transcribed and carry a short intron at the same position of the coding region. The deduced amino acid sequence of hisactophilin II showed a characteristic high content of 35 histidine residues out of a total 118 amino acids. After transformation of Dictyostelium AX2 wild-type cells with a genomic fragment designed to inactivate the hisactophilin I gene we obtained hisactophilin II overexpressing mutants (hisII+). Multiple integration of the vector led to strong overexpression of hisactophilin II which even outnumbered the actin concentration by a factor of two. Hisactophilin II protein showed the same biochemical properties as hisactophilin I during purification and in its pH-dependent binding to F-actin; as shown by mass spectrometry the hisactophilin II fraction was almost completely myristoylated despite of this high overexpression. The inactivation of both hisactophilin genes was achieved by gene replacement with a vector construct encompassing parts of gene I and gene II connected by a geneticin cassette. The properties of the hisII+ and his- cells with regard to growth in shaking culture and on Klebsiella plates, development, chemotaxis and morphology were not affected under normal conditions. However, the hisII+ transformants revealed a significant difference to wild-type cells and his- cells when the cytoplasmic pH was lowered by diethylstilbestrol (DES), a proton pump inhibitor. HisII+ cells were more resistant to the acidification; in contrast to AX2 wild-type cells and his- cells they did not form plasma membrane protrusions, showed an increase in F-actin content, and contained large clusters of F-actin. Lowering the internal pH caused an accumulation of hisactophilin below the plasma membrane. The fact that cells deficient in hisactophilin again lose resistance to acidification is in good agreement with the hypothesis that hisactophilin functions as a pH sensor at the plasma membrane by reversibly connecting the membrane with the actin cortical network upon local changes of the proton concentration.

Anaerobic O-demethylations of methoxynaphthols, methoxyfuran, and fluoroanisols by Sporomusa ovata

In vitro experiments with 3,4-dimethoxybenzoate-induced Sporomusa enzymes a broad O-methyl ether cleavage capacity. The O-demethylase activity hydrolized the methyl-oxygen linkages of methoxynaphtholes of the heterocycles 2-methoxyfuran or 2-methoxythiophene as well as of several dimethoxy and monomethoxy aryls under anaerobic conditions. Also, fluoro and chloro substituents of anisoles enhanced the O-demethylation rate, indicating that an electron delocalized aromatic structure supported the methyl ether activation mechanism. Monomethoxy aromatics with additional chargeable groups, however, were less effectively transformed by the O-demethylase activity. No transformations into hydroxylated products occurred with 4-(trifluoromethoxy)benzyl alcohol, 4-(trifluoromethoxy)fluorobenzene, 2,5-dimethoxytetrahydrofuran, or alkyl-O-methyl ethers. The inert ethers did not affect the 3,4-dimethoxybenzoate metabolism. Ether activation or the following methyl transfer to the methyl acceptor tetrahydrofolate involved a prominent 31 kDa peptide from the cytoplasmic cell fraction, because this particular peptide was lacking in cells grown with methanol, betaine or fructose.

Myristoylated and non-myristoylated forms of the pH sensor protein hisactophilin II: intracellular shuttling to plasma membrane and nucleus monitored in real time by a fusion with green fluorescent protein

Hisactophilins are myristoylated proteins that are rich in histidine residues and known to exist in Dictyostelium cells in a plasma membrane-bound and a soluble cytoplasmic state. Intracellular translocation of these proteins in response to pH changes was monitored using hisactophilin fusions with green fluorescent protein (GFP) and confocal laser scanning microscopy. Both the normal and a mutated non-myristoylated fusion protein shuffled within the cells in a pH-dependent manner. After lowering the pH, these proteins translocated within minutes between the cytoplasm, the plasma membrane and the nucleus. The role of histidine clusters on the surface of hisactophilin molecules in binding of the proteins to the plasma membrane and in their transfer to the nucleus is discussed on the basis of a pH switch mechanism.

A hyperthermostable protease of the subtilisin family bound to the surface layer of the archaeon Staphylothermus marinus

BACKGROUND

Staphylothermus marinus, an archaeon isolated from a geothermally heated marine environment, is a peptide-fermenting, sulphur-dependent organism with an optimum growth temperature of 92 degrees C. It forms grapes of cells, which adhere to each other and to sulphur granules via their surface layer. This glycoprotein layer forms a canopy which is held at a distance of about 70 nm from the cell membrane by membrane-anchored stalks, thereby enclosing a 'quasi-periplasmic space'. Two copies of a globular protease, which probably serves an exodigestive function related to the organism's energy metabolism, are attached near the middle of each stalk.

RESULTS

We have purified and characterized this protease with regard to its enzymatic properties and thermostability, and have sequenced its gene using an approach based entirely on the polymerase chain reaction. The precursor form is 1345 amino acids long; between residues 64-741, it contains a domain with clear homology to subtilisins, which is interrupted by two large insertions. The enzyme has a broad substrate specificity and a pH optimum of 9.0. It is fully stable from pH 3.2 to 12.7 and is resistant to heat-inactivation to 95 degrees C in the free form and to 125 degrees C in the stalk-bound form.

CONCLUSIONS

This protease is one of the most stable proteases known. Its high resistance towards denaturing agents makes it an interesting target for practical applications. Despite its large size, it is clearly a member of the subtilisin family and represents the only known enzyme that is a stoichiometric S-layer component.

Enhanced in situ gel digestion of electrophoretically separated proteins with automated peptide elution onto mini reversed-phase columns

An improved method for the generation and automated isolation of internal peptides by in situ gel digestion of electrophoretically separated proteins is described. To enhance the sensitivity of the method, and to reduce the amount of sample handling steps, we have automated the extraction procedure of peptides after protein cleavage in a sodium dodecyl sulfate (SDS) gel matrix. The excised protein-containing polyacrylamide bands or spots are first minced to defined particles of about 30 microns. After in situ gel digestion, the gel slurry is transferred into a mini reversed-phase column-funnel assembly in the sample loading station of the Hewlett-Packard protein sequencer. Applying nitrogen pressure elutes peptides from the gel slurry onto the reversed-phase material. The mini reversed-phase column is then placed in an in-line column adapter and connected to a micropreparative high performance liquid chromatography (HPLC) column, where separation of the peptides under standard conditions is achieved. In the work described here complete digestions and excellent peptide recoveries allowed the generation of extensive internal sequence information from low picomole amounts of proteins. The method has been routinely applied in both laboratories for two years.

Tom7 modulates the dynamics of the mitochondrial outer membrane translocase and plays a pathway-related role in protein import

The preprotein translocase of the outer mitochondrial membrane is a multi-subunit complex with receptors and a general import pore. We report the molecular identification of Tom7, a small subunit of the translocase that behaves as an integral membrane protein. The deletion of TOM7 inhibited the mitochondrial import of the outer membrane protein porin, whereas the import of preproteins destined for the mitochondrial interior was impaired only slightly. However, protein import into the mitochondrial interior was strongly inhibited when it occurred in two steps: preprotein accumulation at the outer membrane in the absence of a membrane potential and subsequent further import after the re-establishment of a membrane potential. The delay of protein import into tom7delta mitochondria seemed to occur after the binding of preproteins to the outer membrane receptor sites. A lack of Tom7 stabilized the interaction between the receptors Tom20 and Tom22 and the import pore component Tom40. This indicated that Tom7 exerts a destabilizing effect on part of the outer membrane translocase, whereas Tom6 stabilizes the interaction between the receptors and the import pore. Synthetic growth defects of the double mutants tom7delta tom20delta and tom7delta tom6delta provided genetic evidence for the functional relationship of Tom7 with Tom20 and Tom6. These results suggest that (i) Tom7 plays a role in sorting and accumulation of the preproteins at the outer membrane, and (ii) Tom7 and Tom6 perform complementary functions in modulating the dynamics of the outer membrane translocase.

Ultraviolet matrix assisted laser desorption ionization-mass spectrometry of electroblotted proteins

Direct mass spectrometric analysis of proteins electroblotted onto polyvinylidene fluoride membranes after sodium dodecyl sulfate-polyacrylamide gel electrophoresis is demonstrated by matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS) with a linear time-of-flight instrument, equipped with a nitrogen laser (337 nm). The blotted proteins were desorbed directly from the blotting membrane after incubation with sinapinic acid as matrix. Different commercially available membranes resulted in high quality protein signals for hydrophobic membranes exhibiting high specific surface areas (Immobilon PSQ or Trans-Blot) or for charged membranes (Immobilon CD). Systematic investigations with standard proteins were performed to compare standard preparation procedures for ultraviolet (UV) MALDI-MS on stainless steel sample stages and preparation of proteins immobilized onto membranes either by direct application from protein solutions (spotting) or by electrotransfer from gels (electroblotting). Aspects such as mass resolution reproducibility from shot to shot and spot to spot, mass accuracy, and preservation of protein localization are addressed in this paper.

Evaluation of the metal ion requirement of the human deoxyhypusine hydroxylase from HeLa cells using a novel enzyme assay

Hypusine synthesis in the eukaryotic initiation factor 5A is a unique two-step posttranslational modification. After deoxyhypusine is generated by the deoxyhypusine synthase, the deoxyhypusine hydroxylase (EC 1.14.99.29) catalyzes the formation of mature hypusine. A rapid assay for monitoring the deoxyhypusine hydroxylase activity was established, employing the oxidative cleavage of the hypusyl residue and subsequent extraction of the generated aldehydes. As metal ion chelators have been reported to inhibit the deoxyhypusine hydroxylase, the mechanism of this inhibition and the effect of transition metal ions on enzyme activity were investigated. A ferric ion appears to be essential for enzymatic activity, the inhibition of which is entirely attributed to the metal ion binding capacity of the chelators.

Hybrids of chicken cystatin with human kininogen domain 2 sequences exhibit novel inhibition of calpain, improved inhibition of actinidin and impaired inhibition of papain, cathepsin L and cathepsin B

Chicken cystatin and human kininogen domain 2 are members of the cystatin superfamily of protein-type cysteine proteinase inhibitors. They show structural and functional similarities, but only human kininogen domain 2 inhibits calpain. Using recombinant chicken cystatin as a scaffold for hybrid cassette analysis, the known reactive-site regions (N-terminus, first hairpin loop and second hairpin loop) were substituted by the corresponding sequences of human kininogen domain 2 in a single and combined manner. Seven hybrids were expressed, purified to homogeneity, characterized protein-chemically, and their inhibition of papain, actinidin, human cathepsin B, human cathepsin L and calpain (80-kDa subunit of rabbit skeletal muscle calpain II and porcine erthrocyte calpain 1) was determined. Strong but temporary inhibition of calpain by chicken cystatin hybrids carrying the N-terminus alone (variant sc1-KD2) or the N-terminus together with the first hairpin loop (variant sc1/2-KD2) was observed; hybrids of the second hairpin loop (sc3-KD2, sc1/3-KD2, sc2/3-KD2, sc1/2/3-KD2) were less strong calpain inhibitors. These data indicate that the inhibiton of calpain by human kininogen domain 2 requires the correct conformation and combination of several contact sites, and suggest that the N-terminus and the first hairpin loop play a major role in this ensemble. Remarkably, hybrid sc2-KD2 exhibited 5 or 150 times stronger inhibition of actinidin compared to native chicken cystatin or to proteolytically isolated human kininogen domain 2, respectively. This indicates an important role of the first hairpin loop of cystatins in the interaction with actinidin. Along with the impaired inhibition of cathepsin L, papain, actinidin, cathepsin B and calpain by the hybrids sc1/3-KD2, sc2/3-KD2 and sc1/2/3-KD2, these results support our hypothesis that all three predicted contact regions of kininogen domain 2 contribute to binding in the active-site clefts of papain-like enzymes in a finely balanced manner.