A study of human furin specificity using synthetic peptides derived from natural substrates, and effects of potassium ions

We explored furin substrate requirements in addition to the motif R-X-K/R-R using synthetic fluorescent resonance energy transfer (FRET) decapeptides. These decapeptides were derived from furin cleavage sites in viral coat glycoproteins and human and bacterial protein precursors. The hydrolysis by furin of most substrate was activated by K⁺ ion, whereas kosmotropic anions of the Hofmeister series were inhibitors. The analysis of furin hydrolytic activity showed that its efficiency is highly dependent on the particular combinations of amino acids at different substrate positions. There is a clear interdependence of furin subsites that must be taken in account in determining its specificity and also for the design of inhibitors. However, clear preferences were detected for substrates with S at P₁′, and V at P₂′, at P₃′ the amino acids D, S, L and A are almost equally frequent. In the non-prime subsites the best substrates presented S and H at P₆; basic amino acids at P₅; and no clear tendency at P₃. Interestingly, two amino acid substitutions on the prime side of the peptide derived from H5N1 influenza hemagglutinin furin processing site highly improved its hydrolysis. These modifications are possible by single point mutations, suggesting a potential yield of a more infectious virus.     

Neutron and X-ray crystallographic analysis of Achromobacter protease I at pD 8.0: Protonation states and hydration structure in the free-form

The structure of the free-form of Achromobacter protease I (API) at pD 8.0 was refined by simultaneous use of single crystal X-ray and neutron diffraction data sets to investigate the protonation states of key catalytic residues of the serine protease. Occupancy refinement of the catalytic triad in the active site of API free-form showed that ca. 30% of the imidazole ring of H57 and ca. 70% of the hydroxyl group of S194 were deuterated. This observation indicates that a major fraction of S194 is protonated in the absence of a substrate. The protonation state of the catalytic triad in API was compared with the bovine β-trypsin–BPTI complex. The comparison led to the hypothesis that close contact of a substrate with S194 could lower the acidity of its hydroxyl group, thereby allowing H57 to extract the hydrogen from the hydroxyl group of S194. H210, which is a residue specific to API, does not form a hydrogen bond with the catalytic triad residue D113. Instead, H210 forms a hydrogen bond network with S176, H177 and a water molecule. The close proximity of the bulky, hydrophobic residue W169 may protect this hydrogen bond network, and this protection may stabilize the function of API over a wide pH range.     

A Single Nucleotide Polymorphism and Sequence Analysis of CSN1S1 Gene Promoter Region in Chinese BOS Grunniens (YAK)

The aim of this study was to investigate the polymorphism of the CSN1S1 gene promoter region in 4 Chinese yak breeds, and compare the yak CSN1S1 gene promoter region sequences with other ruminants. A Polymerase Chain Reaction-Single Strand Conformation Polymorphism protocol was developed for rapid genotyping of the yak CSN1S1 gene. One hundred fifty-eight animals from 4 Chinese yak breeds were genotyped at the CSN1S1 locus using the protocol developed. A single nucleotide polymorphism of the CSN1S1 gene promoter region has been identified in all yak breeds investigated. The polymorphism consists of a single nucleotide substitution G→A at position 386 of the CSN1S1 gene promoter region, resulting in two alleles named, respectively, G₃₈₆ and A₃₈₆, based on the nucleotide at position 386. The allele G₃₈₆ was found to be more common in the animals investigated. The corresponding nucleotide sequences in GenBank of yak (having the same nucleotides as allele G₃₈₆ in this study), bovine, water buffalo, sheep, and goat had similarity of 99.68%, 99.35%, 97.42%, 95.14%, and 94.19%, respectively, with the yak allele A_386.     

Regulation of Endosome Sorting by a Specific PP2A Isoform

The regulated sorting of proteins within the trans-Golgi network (TGN)/endosomal system is a key determinant of their biological activity in vivo. For example, the endoprotease furin activates of a wide range of proproteins in multiple compartments within the TGN/endosomal system. Phosphorylation of its cytosolic domain by casein kinase II (CKII) promotes the localization of furin to the TGN and early endosomes whereas dephosphorylation is required for efficient transport between these compartments (Jones, B.G., L. Thomas, S.S. Molloy, C.D. Thulin, M.D. Fry, K.A. Walsh, and G. Thomas. 1995. EMBO [Eur. Mol. Biol. Organ.] J. 14:5869–5883). Here we show that phosphorylated furin molecules internalized from the cell surface are retained in a local cycling loop between early endosomes and the plasma membrane. This cycling loop requires the phosphorylation state-dependent furin-sorting protein PACS-1, and mirrors the trafficking pathway described recently for the TGN localization of furin (Wan, L., S.S. Molloy, L. Thomas, G. Liu, Y. Xiang, S.L. Ryback, and G. Thomas. 1998. Cell. 94:205–216). We also demonstrate a novel role for protein phosphatase 2A (PP2A) in regulating protein localization in the TGN/endosomal system. Using baculovirus recombinants expressing individual PP2A subunits, we show that the dephosphorylation of furin in vitro requires heterotrimeric phosphatase containing B family regulatory subunits. The importance of this PP2A isoform in directing the routing of furin from early endosomes to the TGN was established using SV-40 small t antigen as a diagnostic tool in vivo. The role of both CKII and PP2A in controlling multiple sorting steps in the TGN/endosomal system indicates that the distribution of itinerant membrane proteins may be acutely regulated via signal transduction pathways.     

Functional Genetic Variants of the Catecholamine-Release-Inhibitory Peptide Catestatin in an Indian Population: ALLELE-SPECIFIC EFFECTS ON METABOLIC TRAITS*

Catestatin (CST), a chromogranin A (CHGA)-derived peptide, is a potent inhibitor of catecholamine release from adrenal chromaffin cells and postganglionic sympathetic axons. We re-sequenced the CST region of CHGA in an Indian population (n = 1010) and detected two amino acid substitution variants: G364S and G367V. Synthesized CST variant peptides (viz. CST-Ser-364 and CST-Val-367) were significantly less potent than the wild type peptide (CST-WT) to inhibit nicotine-stimulated catecholamine secretion from PC12 cells. Consistently, the rank-order of blockade of nicotinic acetylcholine receptor (nAChR)-stimulated inward current and intracellular Ca²⁺ rise by these peptides in PC12 cells was: CST-WT > CST-Ser-364 > CST-Val-367. Structural analysis by CD spectroscopy coupled with molecular dynamics simulations revealed the following order of α-helical content: CST-WT > CST-Ser-364 > CST-Val-367; docking of CST peptides onto a major human nAChR subtype and molecular dynamics simulations also predicted the above rank order for their binding affinity with nAChR and the extent of occlusion of the receptor pore, providing a mechanistic basis for differential potencies. The G364S polymorphism was in strong linkage disequilibrium with several common CHGA genetic variations. Interestingly, the Ser-364 allele (detected in ∼15% subjects) was strongly associated with profound reduction (up to ∼2.1-fold) in plasma norepinephrine/epinephrine levels consistent with the diminished nAChR desensitization-blocking effect of CST-Ser-364 as compared with CST-WT. Additionally, the Ser-364 allele showed strong associations with elevated levels of plasma triglyceride and glucose levels. In conclusion, a common CHGA variant in an Indian population influences several biochemical parameters relevant to cardiovascular/metabolic disorders.     

Human Nucleotide Excision Repair Protein XPA: 1H NMR and CD Solution Studies of a Synthetic Peptide Fragment Corresponding to the Zinc-Binding Domain (101–141)

Abstract

A peptide corresponding to residues 101–141 of the human nucleotide excision repair protein XPA was synthesized with an isoleucine substituted for L138 and its solution structure studied by circular dichroism and homonuclear ¹H NMR spectroscopy. The peptide, (XPA-41), contains a C₄?type zinc-binding motif, C105-(X)₂C108-(X)_l7?C126-(X)₂ C129, which XPA requires for damaged-DNA binding activity. The proton resonances of XPA-41without zinc (apoXPA-41) were assigned using homonuclear TOCSY, NOESY and DQF-COSY data and show the apo-zinc peptide is a random coil. The peptide was folded with the addition of 1.2 equivalents of ZnCl₂ in dilute solution at pH 4.0. Electrospray ionization mass spectroscopy illustrated an increase in the molecular weight of XPA-41 by 65 amu. Circular dichroism spectra of the zinc-folded peptide (zXPA-41) showed the acquisition of elements of secondary structure. Such a conclusion was confirmed with'H NMR data collected at 25°C, pH 6.3. Hα-secondary shifts and NOE patterns indicate that regions V102-C105 and G109-F112 form an anti-parallel β-sheet and residues N128-K137 form a nascent α-helix. Rapid exchange of most amide resonances between S115-C126 prohibited unambiguous assignment of all the proton resonances in this region. However, a 1.19 ppm downfield shift of the H^α resonance of T125 relative to the apo-zinc peptide, together with downfield shifted H^α resonances for the adjacent residues (P124 and L123), suggest a second β-sheet is present in the S115-C126 region. On the basis of structural similarities to GATA-1 (Science 267:438–446), a homology generated structure for zXPA-41 was made, using GATA-1 as the template, which satisfied all the observed NOEs. Using the hybrid homology-NMR based zXPA-41 structure and analogy to GATA-1, models for the role played by the zinc-binding core (101–141) of XPA in DNA damage recognition are proposed.     

Identification of Serpin Determinants of Specificity and Selectivity for Furin Inhibition through Studies of α1PDX (α1-Protease Inhibitor Portland)-Serpin B8 and Furin Active-site Loop Chimeras

α₁-Protease inhibitor Portland (α₁PDX) is an engineered serpin family inhibitor of the proprotein convertase (PC), furin, that exhibits high specificity but limited selectivity for inhibiting furin over other PC family proteases. Here, we characterize serpin B8, a natural inhibitor of furin, together with α₁PDX-serpin B8 and furin-PC chimeras to identify determinants of serpin specificity and selectivity for furin inhibition. Replacing reactive center loop (RCL) sequences of α₁PDX with those of serpin B8 demonstrated that both the P4–P1 RXXR recognition sequence as well as the P1′–P5′ sequence are critical determinants of serpin specificity for furin. Alignments of PC catalytic domains revealed four variable active-site loops whose role in furin reactivity with serpin B8 was tested by engineering furin-PC loop chimeras. The furin(298–300) loop but not the other loops differentially affected furin reactivity with serpin B8 and α₁PDX in a manner that depended on the serpin RCL-primed sequence. Modeling of the serpin B8-furin Michaelis complex identified serpin exosites in strand 3C close to the 298–300 loop whose substitution in α₁PDX differentially affected furin reactivity depending on the furin loop and serpin RCL-primed sequences. These studies demonstrate that RCL-primed residues, strand 3C exosites, and the furin(298–300) loop are critical determinants of serpin reactivity with furin, which may be exploited in the design of specific and selective α₁PDX inhibitors of PCs.     

Identification of an “α-helix-extended segment-α-helix” conformation of the sixth transmembrane domain in DMT1

DMT1 (divalent metal ion transporter 1) is one member of a family of proton-coupled transporters that facilitate the cellular absorption of divalent metal ions. A pair of mutation-sensitive and highly conserved histidines in the sixth transmembrane domain (TM6) of DMT1 was found to be important for proton-metal ion cotransport. In the present work, we investigate the structures and locations of the peptides from TM6 of DMT1 and its H267A and H272A mutants in SDS micelles by CD and NMR methods. The circular dichroism studies show that the α-helix is a predominant conformation for the wildtype peptide and H267A mutant in SDS micelles, whereas the helicity is evidently decreased for H272A mutant. The pH value has little effect on the α-helical contents of the three peptides. The NMR studies indicate that the wildtype peptide in SDS micelles forms an “α-helix-extended segment-α-helix” structure in which the His267 locates near the central part of the extended segment, while the His272 is involved in the α-helical folding. Both histidines are buried in SDS micelles as evidenced by their pK_a values. The structure of the wildtype peptide is evidently changed by the mutations of H267A and H272A. The H267A mutant forms an ordered structure consisting of an α-helix from the C-terminus to the central part and continuous turns in the residual part. The extended structure in the central part of the wildtype peptide is abolished by H267A mutation. The H272A mutation mainly induces unfolding of the short helix in the N-terminal side, while the short helix in the C-terminal side and unordered conformation in the central part remain. All the three peptides are embedded in SDS micelles, and the H267A mutant is inserted more deeply due to increasing hydrophobicity in the central part of the peptide. The specific “α-helix-extended segment-α-helix” structure of TM6 may have an important implication for the binding of the transporter to H⁺ and metal ions and the conformation change induced by the mutations of two highly conserved histidines may be correlated to the deficiency of the transport activity of DMT1.     

The 4b-4c loop of excitatory amino acid transporter 1 containing four critical residues essential for substrate transport

Abstract

In the mammalians, the 4b-4c loop of excitatory amino acid transporters (EAATs) spans more than 50 amino-acid residues that are absent in glutamate transporter homologue of Pyrococcus horikoshii (Glt_Ph). This part of insertion is unique for metazoans and indispensable to the localization of EAATs. The excitatory amino acid transporter (EAAT) 1 is one of the two glial glutamate transporters, which are responsible for efficiently clearing glutamate from the synaptic cleft to prevent neurotoxicity and cell death. Although the crystal structure of EAAT1_cryst (a human thermostable EAAT1) was resolved in 2017, the structure-function relationship of the 4b-4c loop has not been elucidated in EAAT1_cryst. To investigate the role of the 4b-4c loop, we performed alanine-scanning mutagenesis in the mutants and observed dramatically decreased transport activities in T192A, Y194A, N242A, and G245A mutants. The surface expression of T192A and Y194A mutants even decreased by more than 80%, and most of them were detained in the cytoplasm. However, when T192 and Y194 were substituted with conservative residues, the transport activities and the surface expressions of T192S and Y194F were largely recovered, and their kinetic parameters (K_m values) were comparable to the wild-type EAAT1 as well. In contrast, N242 and G245 substituted with conservative residues could not rescue the uptake function, suggesting that N242 and G245 may play irreplaceable roles in the glutamate uptake process. These results indicate that the 4b-4c loop of EAAT1 may not only affect the glutamate uptake activity, but also influence the surface localization of EAAT1 by T192 and Y194.

Communicated by Ramaswamy H. Sarma     

Design of peptide inhibitors for furin based on the C-terminal fragment of histone H1.2

The mammalian proprotein convertase furin has been found to play an important role in diverse physiological and pathological events, such as the activation of viral glycoproteins and bacterial exotoxins. Small, non-toxic and highly active, furin inhibitors are considered to be attractive drug candidates for diseases caused by virus and bacteria. In this study, a series of peptide inhibitors were designed and synthesized based on the C-terminal fragment of histone H1.2, which has an inhibitory effect on furin. Replacing the reactive site of inhibitors with the consensus substrate recognition sequence of furin has been found to increase inhibitory activity greatly. The most potent inhibitor, I4, with 14 amino acid residues has a Ki value of 17 nM for furin. Although most of the synthesized peptides were temporary inhibitors, the inhibitor I5, with nine amino acids, retained its full potency, even after a 3 h incubation period with furin at 37 degrees C. These inhibitors may potentially lead to the development of anti-viral and anti-bacterial drug compounds.     

Hydrogen bonding modulates binding of exogenous ligands in a myoglobin proximal cavity mutant.

In the sperm whale myoglobin mutant H93G, the proximal histidine is replaced by glycine, leaving a cavity in which exogenous imidazole can bind and ligate the heme iron (Barrick, D. (1994) Biochemistry 33, 6545-6554). Structural studies of this mutant suggest that serine 92 may play an important role in imidazole binding by serving as a hydrogen bond acceptor. Serine 92 is highly conserved in myoglobins, forming a well-characterized weak hydrogen bond with the proximal histidine in the native protein. We have probed the importance of this hydrogen bond through studies of the double mutants S92A/H93G and S92T/H93G incorporating exogenous imidazole and methylimidazoles. (1)H NMR spectra reveal that loss of the hydrogen bond in S92A/H93G does not affect the conformation of the bound imidazole. However, the binding constants for imidazoles to the ferrous nitrosyl complex of S92A/H93G are much weaker than in H93G. These results are discussed in terms of hydrogen bonding and steric packing within the proximal cavity. The results also highlight the importance of the trans diatomic ligand in altering the binding and sensitivity to perturbation of the ligand in the proximal cavity.     

Improving the catalytic efficiency of Bacillus pumilus CotA-laccase by site-directed mutagenesis

Bacterial laccases are potential enzymes for biotechnological applications because of their remarkable advantages, such as broad substrate spectrum, various reactions, high thermostability, wide pH range, and resistance to strongly alkaline environments. However, the use of bacterial laccases for industrialized applications is limited because of their low expression level and catalytic efficiency. In this study, CotA, a bacterial laccase from Bacillus pumilus, was engineered through presumptive reasoning and rational design approaches to overcome low catalytic efficiency and thermostability. L386W/G417L, a CotA double-mutant, was constructed through site-directed mutagenesis. The catalytic efficiency of L386W/G417L was 4.3 fold higher than that of wild-type CotA-laccase, but the thermostability of the former was decreased than that of the latter and other mutants. The half-life (t _1/2) of wild-type and G417L were 1.14 and 1.47 h, but the half-life of L386W/G417L was only 0.37 h when incubating the enzyme at 80 °C. Considering the high catalytic efficiency of L386W/G417L, we constructed L386W/G417L/G57F, another mutant, to improve thermostability. Results showed that the half-life of L386W/G417L/G57F was 0.54 h when incubating the enzyme at 90 °C for 2 h with about 34% residual activity, but the residual activity of L386W/G417L was less than 40% when incubating the enzyme at 90 °C for 5 min. L386W/G417L was more efficient in decolorizing various industrial dyes at pH 10 than other mutants. L386W/G417L/G57F also exhibited an efficient decolorization ability. L386W/G417L/G57F is appropriate for biotechnological applications because of its high activity and thermostability in decolorizing industrial dyes. CotA-laccase may be further subjected to molecular modification and be used as an enhancer to improve decolorization efficiency for the physical and chemical treatment of dye wastewater.

     

Determinants of Cation Permeation and Drug Sensitivity in Predicted Transmembrane Helix 9 and Adjoining Exofacial Re-entrant Loop 5 of Na+/H+ Exchanger NHE1

Mammalian Na⁺/H⁺ exchangers (NHEs) regulate numerous physiological processes and are involved in the pathogenesis of several diseases, including tissue ischemia and reperfusion injuries, cardiac hypertrophy and failure, and cancer progression. Hence, NHEs are being targeted for pharmaceutical-based clinical therapies, but pertinent information regarding the structural elements involved in cation translocation and drug binding remains incomplete. Molecular manipulations of the prototypical NHE1 isoform have implicated several predicted membrane-spanning (M) helices, most notably M4, M9, and M11, as important determinants of cation permeation and drug sensitivity. Here, we have used substituted-cysteine accessibility mutagenesis and thiol-modifying methanethiosulfonate (MTS) reagents to further probe the involvement of evolutionarily conserved sites within M9 (residues 342–363) and the adjacent exofacial re-entrant loop 5 between M9 and M10 (EL5; residues 364–415) of a cysteine-less variant of rat NHE1 on its kinetic and pharmacological properties. MTS treatment significantly reduced the activity of mutants containing substitutions within M9 (H353C, S355C, and G356C) and EL5 (G403C and S405C). In the absence of MTS, mutants S355C, G403C, and S405C showed modest to significant decreases in their apparent affinities for Na⁺_o and/or H⁺_i. In addition, mutations Y370C and E395C within EL5, whereas failing to confer sensitivity to MTS, nevertheless, reduced the affinity for Na⁺_o, but not for H⁺_i. The Y370C mutant also exhibited higher affinity for ethylisopropylamiloride, a competitive antagonist of Na⁺_o transport. Collectively, these results further implicate helix M9 and EL5 of NHE1 as important elements involved in cation transport and inhibitor sensitivity, which may inform rational drug design.     

The mitochondrial oxoglutarate carrier: cysteine-scanning mutagenesis of transmembrane domain IV and sensitivity of Cys mutants to sulfhydryl reagents.

Using a functional mitochondrial oxoglutarate carrier mutant devoid of Cys residues (C-less carrier), each amino acid residue in transmembrane domain IV and flanking hydrophilic loops (from T179 to S205) was replaced individually with Cys. The great majority of the 27 mutants exhibited significant oxoglutarate transport in reconstituted liposomes as compared to the activity of the C-less carrier. In contrast, Cys substitution for G183, R190, Q198, and Y202, in either C-less or wild-type carriers, yielded molecules with complete loss of oxoglutarate transport activity. G183 and R190 could be partially replaced only by Ala and Lys, respectively, whereas Q198 and Y202 were irreplaceable with respect to oxoglutarate transport. Of the single-Cys mutants tested, only T187C, A191C, V194C, and N195C were strongly inactivated by N-ethylmaleimide and by low concentrations of methanethiosulfonate derivatives. Oxoglutarate protects Cys residues at positions 187, 191, and 194 against reaction with N-ethylmaleimide. These positions as well as the residues found to be essential for the carrier activity, except Y202 which is located in the extramembrane loop IV-V, reside on the same face of transmembrane helix IV, probably lining part of a water-accessible crevice or channel between helices of the oxoglutarate carrier.     

Lysine 194 Is Functional in Isocitrate Lyase from Escherichia coli

Lysine 194 in conserved stretch 1 of tetrameric isocitrate lyase from Escherichia coli has been replaced by using the restriction-enzyme-site elimination method of directed mutagenesis. Expression of subunits of each variant and of wild-type (wt) enzyme was equivalent and all variants assembled into tetrameric proteins. The variants K194R and K194H had k_cat values relative to that of wt enzyme taken as 100 of 11 and 7, respectively. K _m values for Mg²⁺-D_s-isocitrate (in mM units) were: 0.13 for wt-enzyme; 0.12 for the K194R variant; and 0.55 for the K194H variant. Substitution at position 194 of Leu or Glu resulted in zero catalytic activity. These results establish that Lys 194 is another functional residue in conserved stretch one of isocitrate lyase from E. coli besides H184, K193, C195, and H197. Because K194 can be specifically replaced by the basic residues His and Arg with resultant lowered activity and by His with an increased K _m value, it may contribute to a cation center and facilitate substrate binding as well as orientation of the developing transition state. Received: 3 December 1996 / Accepted: 18 December 1996     

Genetic and population study of a Y-linked tetranucleotide repeat DNA polymorphism with a simple non-isotopic technique

A polymorphic microsatellite (Y-27H39) based on a (GATA) _n repeat was recently discovered on the short arm of the human Y chromosome. We have used a simple technique based on polymerase chain reaction amplification and native polyacrylamide gel electrophoresis followed by highly sensitive silver staining to study the inheritance, the genetic stability and the allele frequency distribution of this polymorphism in the Brazilian population. We have analyzed 100 randomly chosen Caucasian Brazilian father-son pairs with established paternity. Five alleles, four base-pairs apart, were easily distinguishable. Their frequencies were: A (186 bp), 0.19; B (190 bp), 0.49; C (194 bp), 0.24; D (198 bp), 0.07; E (202 bp), 0.01. In all father-son pairs, there was complete allelic concordance. From these data, the probability of discrimination for forensic cases and the average probability of exclusion for paternity cases were both calculated to be 0.66.     

Proteolytic Activation of the Spike Protein at a Novel RRRR/S Motif Is Implicated in Furin-Dependent Entry,Syncytium Formation,and Infectivity of Coronavirus Infectious Bronchitis Virus in Cultured Cells

The spike (S) protein of the coronavirus (CoV) infectious bronchitis virus (IBV) is cleaved into S1 and S2 subunits at the furin consensus motif RRFRR₅₃₇/S in virus-infected cells. In this study, we observe that the S2 subunit of the IBV Beaudette strain is additionally cleaved at the second furin site (RRRR₆₉₀/S) in cells expressing S constructs and in virus-infected cells. Detailed time course experiments showed that a peptide furin inhibitor, decanoyl-Arg-Val-Lys-Arg-chloromethylketone, blocked both viral entry and syncytium formation. Site-directed mutagenesis studies revealed that the S1/S2 cleavage by furin was not necessary for, but could promote, syncytium formation by and infectivity of IBV in Vero cells. In contrast, the second site is involved in the furin dependence of viral entry and syncytium formation. Mutations of the second site from furin-cleavable RRRR/S to non-furin-cleavable PRRRS and AAARS, respectively, abrogated the furin dependence of IBV entry. Instead, a yet-to-be-identified serine protease(s) was involved, as revealed by protease inhibitor studies. Furthermore, sequence analysis of CoV S proteins by multiple alignments showed conservation of an XXXR/S motif, cleavable by either furin or other trypsin-like proteases, at a position equivalent to the second IBV furin site. Taken together, these results suggest that proteolysis at a novel XXXR/S motif in the S2 subunit might be a common mechanism for the entry of CoV into cells.The surface glycoproteins of numerous pathogenic enveloped viruses are proteolytically matured during infection in the host or cultured cell lines and are essential for the initiation of infection (33). In many cases, this processing is carried out by cellular proprotein convertases (PCs), most commonly furin (reviewed in reference 46). Furin is a calcium-dependent serine protease that circulates between the trans-Golgi network, plasma membrane, and early endosome by association with exocytic and endocytic pathways (9, 39). This membrane-bound enzyme undergoes further processing and is secreted from cells in an active soluble form (49). Furin processes a wide variety of precursor proteins after the C-terminal arginine (R) residue in the preferred consensus motif RXR(K)R/X (K is lysine, X is any amino acid, and the slash [/] indicates the cleavage position) for viral fusion proteins (2, 32, 33). So far, seven PCs have been identified in mammalian cells, and they display similar, but not identical, specificities for basic motifs at the cleavage site of a substrate. Accumulated studies indicate that secretory PCs, such as furin, PC5, and PC7, are major candidates for processing surface glycoproteins of pathogenic viruses, such as human immunodeficiency virus types 1 and 2, avian influenza virus H5N1, Ebola virus, and respiratory syncytial virus (RSV) (2, 27).Coronavirus (CoV) spike (S) protein, a class I viral fusion protein (7), is responsible for viral attachment to and entry into target cells and for cell-to-cell spread during infection. Typical class I fusion proteins usually require processing at a position immediately upstream of the fusion peptide in order to expose the membrane-anchored subunit. However, in infectious bronchitis virus (IBV) and murine hepatitis virus (MHV), processing of the S protein by furin occurs at a position more than 200 amino acids away from the predicted fusion peptides (6). Furthermore, there is a tradeoff between the furin cleavability of S protein and heparin sulfate (HS) binding in certain CoV strains adapted to cultured cell lines (15, 17). Consequently, CoV S proteins may be proteolytically activated by other proteases to initiate virus-cell fusion. Recently, proteolytic activation by an endosomal protease, cathepsin L, and a membrane-bound protease, factor Xa, was reported to play a role in the entry of severe acute respiratory syndrome (SARS)-CoV (18, 45). Cathepsin is also implicated in the proteolytic activation of many CoV S proteins, including human CoV 229E, feline infectious peritonitis virus (FIPV) 1146, feline enteric CoV (FECV) 1683, and MHV strain 2 (MHV-2), but not for MHV A59 and human CoV NL63 (31, 41, 43, 45).The association of cell surface sialic acid and a low-pH environment were reported to be required for IBV entry (14, 51, 52). However, the factors that determine the infectivity of IBV for cultured cells have yet to be identified. Clinical and field isolates of IBV can be propagated only in embryonated chicken eggs or, transiently, in primary chicken embryo kidney cells. In contrast, IBV of Beaudette strain origin can be readily adapted to cultured cells, such as Vero and BHK-21, by serial passages (1, 22, 40), and hence, it is often used as an in vitro infection model of IBV. Studies with a recombinant infectious clone system demonstrated that IBV S protein is indeed the determinant of extended cell tropism (12). IBV S protein is usually cleaved into S1 and S2 subunits at the furin consensus motif, RRFRR₅₃₇/S (the position includes the signal peptide) in virus-infected cells (13). Interestingly, Beaudette and related strains carry a mutation at position 687 of the S protein from proline (P) to R, creating a novel furin site (RRRR₆₉₀/S or RRKR₆₉₀/S). The acquisition of an additional furin site in the fusion protein may increase cell-to-cell spread by further activation of the protein (23) or extend the host range by utilization of cell surface HS as an entry receptor (17). In this study, furin-mediated cleavage of the IBV S protein at two furin sites was observed in IBV-infected cells. Mutational analysis of the two furin sites revealed that the second site is implicated in the furin dependence of IBV entry and syncytium formation. In contrast, cleavage at the S1/S2 site by furin was not necessary for, but could promote, syncytium formation and the infectivity of IBV in Vero cells.     

The second life of antibodies

Antibodies (immunoglobulins, Ig) are used by the immune system to identify and neutralize foreign objects and are responsible for antigen-binding and effector functions. Immunoglobulin G (IgG) is the major serum immunoglobulin of a healthy human (～75% of the total Ig fraction). The discovery in 1970 of the endogenous tetrapeptide tuftsin (Thr-Lys-Pro-Arg, fragment 289–292 of the C_H2-domain of the heavy (H) chain of IgG), possessing both immunostimulatory and neurotrophic activities, was an impetus for the search for new biologically active peptides of immunoglobulin origin. As a result, fragments of the H-chain of IgG produced as a result of enzymatic cleavage of IgG within the antigen-antibody complex were discovered, synthesized, and studied. These fragments include rigin (341–344), immunorphin (364–373), immunocortin (11–20), and peptide p24 (335–358) and its fragments. In this review the properties of these peptides and their role in regulating the immune response are analyzed.     

Zinc coordination polymers with 1-(3-aminopropyl)-imidazole as bridging bidentate unit

The reaction of Zn(SCN)₂ with one or two equivalents of 1-(3-aminopropyl)-imidazole (api) yields the coordination polymers [Zn(SCN)₂(api)]_n (1) and [Zn(SCN)₂(api)₂]_n (2), respectively. Single-crystal X-ray diffraction analysis reveals one-dimensional polymeric chain structures for both compounds. The structure of 1 consists of tetrahedral Zn(SCN)₂(api)₂ units linked by one molecule 1-aminopropyl imidazole in an unsymmetric mode, i.e., each metal center is coordinated by an imidazole nitrogen as well as a nitrogen of the aminopropyl group. The metal ions in 2 display an octahedral coordination geometry with each Zn(SCN)₂(api)₂ unit linked by two molecules of the imidazole, thus, exhibiting two imidazole and two amino groups in the coordination sphere. The polymers were further characterized by IR-, ¹H NMR- and ¹³C NMR-spectroscopy.     

Intimate genetic relationships and fungicide resistance in multiple strains of Aspergillus fumigatus isolated from a plant bulb

Fungal infections are increasingly dangerous because of environmentally dispersed resistance to antifungal drugs. Azoles are commonly used antifungal drugs, but they are also used as fungicides in agriculture, which may enable enrichment of azole-resistant strains of the human pathogen Aspergillus fumigatus in the environment. Understanding of environmental dissemination and enrichment of genetic variation associated with azole resistance in A. fumigatus is required to suppress resistant strains. Here, we focused on eight strains of azole-resistant A. fumigatus isolated from a single tulip bulb for sale in Japan. This set includes strains with TR₃₄/L98H/T289A/I364V/G448S and TR₄₆/Y121F/T289A/S363P/I364V/G448S mutations in the cyp51A gene, which showed higher tolerance to several azoles than strains harbouring TR₄₆/Y121F/T289A mutation. The strains were typed by microsatellite typing, single nucleotide polymorphism profiles, and mitochondrial and nuclear genome analyses. The strains grouped differently using each typing method, suggesting historical genetic recombination among the strains. Our data also revealed that some strains isolated from the tulip bulb showed tolerance to other classes of fungicides, such as QoI and carbendazim, followed by related amino acid alterations in the target proteins. Considering spatial–temporal factors, plant bulbs are an excellent environmental niche for fungal strains to encounter partners, and to obtain and spread resistance-associated mutations.