Protein cleavage in virus-infected cells

A variety of proteins, including viral precursor polypeptides, were bound to a solid support and used in a sensitive assay for proteolytic enzymes in HeLa cells. A trypsin-like endoprotease, present on ribosomes of HeLa cells, loses activity after picornavirus infection. The decline follows synthesis and processing of a viral protein. Inhibition of cellfree activity of HeLa protease occurs when protein trypsin inhibitors or double-stranded RNA are added. After the mid-point of infection, protease activity with enhanced specificity for viral substrates is detected. The new protease has a pH optimum and heat stability different from endogenous host enzymes, and is synthesized following infection. A viral mutant was isolated which produces a temperature-sensitive protease. The results indicate that a poliovirus gene product participates enzymatically in the final cleavages of some polioviral proteins. A model for the regulation of poliovirus replication based on specific proteolysis is presented.     

Functional analysis of a human A(1) adenosine receptor/green fluorescent protein/G(i1)alpha fusion protein following stable expression in CHO cells

The nuclear lamina protein, lamin A is produced by proteolytic cleavage of a 74 kDa precursor protein, prelamin A. The conversion of this precursor to mature lamin A is mediated by a specific endoprotease, prelamin A endoprotease. Subnuclear fractionation indicates that the prelamin A endoprotease is localized at the nuclear membrane. The enzyme appears to be an integral membrane protein, as it can only be removed from the nuclear envelope with detergent. It is effectively solubilized by the detergent n-octyl-beta-D-glucopyranoside and can be partially-purified (approximately 1200-fold) by size exclusion and cation exchange (Mono S) chromatography. Prelamin A endoprotease from HeLa cells was eluted from Mono S with 0.3 M sodium chloride as a single peak of activity. SDS-PAGE analysis of this prelamin A endoprotease preparation shows that it contains one major polypeptide at 65 kDa and smaller amounts of a second 68 kDa polypeptide. Inhibition of the enzyme activity in this preparation by specific serine protease inhibitors is consistent with the enzyme being a serine protease.     

Delay of Iris flower senescence by protease inhibitors

Visible senescence of the flag tepals in Iris x hollandica (cv. Blue Magic) was preceded by a large increase in endoprotease activity. Just before visible senescence about half of total endoprotease activity was apparently due to cysteine proteases, somewhat less than half to serine proteases, with a minor role of metalloproteases. Treatment of isolated tepals with the purported serine protease inhibitors AEBSF [4-(2-aminoethyl)-benzenesulfonyl fluoride] or DFP (diisopropyl-fluorophosphate) prevented the increase in endoprotease activity and considerably delayed or prevented the normal senescence symptoms. The specific cysteine protease-specific E-64d reduced maximum endoprotease activity by 30%, but had no effect on the time to visible senescence. Zinc chloride and aprotinin reduced maximum endoprotease activity by c. 50 and 40%, respectively, and slightly delayed visible senescence. A proteasome inhibitor (Z-leu-leu-Nva-H) slightly delayed tepal senescence, which indicates that protein degradation in the proteasome may play a role in induction of the visible senescence symptoms. It is concluded that visible senescence is preceded by large-scale protein degradation, which is apparently mainly due to cysteine- and serine protease activity, and that two (unspecific) inhibitors of serine proteases considerably delay the senescence symptoms.     

ompT encodes the Escherichia coli outer membrane protease that cleaves T7 RNA polymerase during purification.

Bacteriophage T7 RNA polymerase is stable in Escherichia coli but very susceptible to cleavage by at least one endoprotease after cell lysis. The major source of this endoprotease activity was found to be localized to the outer membrane of the cell. A rapid whole-cell assay was developed to screen different strains for the presence of this proteolytic activity. Using this assay, we identified some common laboratory strains that totally lack the protease. Genetic and Southern analyses of these null strains allowed us to conclude that the protease that cleaves T7 RNA polymerase is OmpT (formerly termed protein a), a known outer membrane endoprotease, and that the null phenotype results from deletion of the OmpT structural gene. A recombinant plasmid carrying the ompT gene enables these deletion strains to synthesize OmpT and converts them to a protease-positive phenotype. The plasmid led to overproduction of OmpT protein and protease activity in the E. coli K-12 and B strains we used, but only weak expression in the E. coli C strain, C1757. This strain-dependent difference in ompT expression was investigated with respect to the known influence of envZ on OmpT synthesis. A small deletion in the ompT region of the plasmid greatly diminishes the amount of OmpT protein and plasmid-encoded protease present in outer membranes. Use of ompT deletion strains for production of T7 RNA polymerase from the cloned gene has made purification of intact T7 RNA polymerase routine. Such strains may be useful for purification of other proteins expressed in E. coli.     

Cell-free protein synthesis in t,e rabbit liver ribosomal system. II. Large scale preparation of purified 80S ribosomes.

A procedure for the preparation of a large quantity of biologically active, highly purified ribosomes from rabbit liver is described. The method employs polyethylene glycol-dextran sulfate parition and DEAE-cellulose chromatography to overcome the limitations encountered in conventional procedures. The entire process takes only 48 h to obtain 10,000 A(260) units of ribosomes. The ribosomes thus obtained are predominantly 78S particles with a constant protein-RNA ratio of 0.95. The ribosomes are free from RNase, amino-acyl-tRNA synthetase, and amino-acyl-tRNA: protein transferase activity. The protein synthesizing activity is dependent on added mRNA and protein factors. These ribosomes are stable for prolonged periods of storage in a liquid nitrogen refrigerator.     

Bimodal protein targeting through activation of cryptic mitochondrial targeting signals by an inducible cytosolic endoprotease

Bimodal targeting of the endoplasmic reticular protein, cytochrome P4501A1 (CYP1A1), to mitochondria involves activation of a cryptic mitochondrial targeting signal through endoprotease processing of the protein. Here, we characterized the endoprotease that regulates mitochondrial targeting of CYP1A1. The endoprotease, which was induced by beta-naphthoflavone, was a dimer of 90 kDa and 40 kDa subunits, each containing Ser protease domains. The purified protease processed CYP1A1 in a sequence-specific manner, leading to its mitochondrial import. The glucocorticoid receptor, retinoid X receptor, and p53 underwent similar processing-coupled mitochondrial transport. The inducible 90 kDa subunit was a limiting factor in many cells and some tissues and, thus, regulates the mitochondrial levels of these proteins. A number of other mitochondria-associated proteins with noncanonical targeting signals may also be substrates of this endoprotease. Our results describe a new mechanism of mitochondrial protein import that requires an inducible cytoplasmic endoprotease for activation of cryptic mitochondrial targeting signals.     

Synthesis of rabbit globin in a cell-free protein synthesis system utilizing sea urchin egg and zygote ribosomes

Reports of the reduced ability of sea urchin egg ribosomes to participate in synthetic mRNA-directed protein synthesis have fostered the suggestion that the low protein synthesis rate of eggs is due to ribosome-associated inhibitors. To test this hypothesis with a natural message, we have isolated 80S ribosomes and microsomal ribosomes of sea urchin eggs and zygotes and compared their activity at synthesizing protein from rabbit α and β globin mRNA in a Krebs II ascites tumor cell-free system. Both egg and zygote 80S ribosomes responded to added mRNA and were shown to synthesize complete α and β globin chains by CM-cellulose chromatography. In most cases, the activity of the egg ribosomes was in comparable instances higher than the zygote ribosomes. Attempts to determine the cause of this difference indicated that it was not a function of K⁺ or Mg²⁺ concentration, type of tRNA used, or ribosomal wash proteins. From these studies it is apparent that sea urchin egg ribosomes are functional at a level equivalent to or better than zygote ribosomes, and it appears that the lack of protein synthetic activity in unfertilized eggs is not due to the presence of a population of inhibited ribosomes.     

Effect of food deprivation and hypophysectomy on in vitro protein synthesis by membrain-bound and free hepatic ribosomes.

The protein synthetic activities of membrane-bound and free hepatic ribosomes isolated from intact rats fed ad libitum, and normal rats subjected to food restriction to match that of hypophysectomised (Hx) rats were compared to the in vitro protein synthetic capacity of hepatic ribosomes isolated from Hx rats. Hypophysectomy resulted in decreased protein synthetic ability of bound ribosomes, whether protein synthesis was directed by endogenous messenger RNA (mRNA) (p less than 0.05) or by polyuridylic acid (polyU) (p less than 0.01). In contrast, the protein synthetic activity of free hepatic ribosomes from Hx rats was reduced when protein synthesis was directed by endogenous mRNA (p less than 0.05) but, when polyU was substituted as the messenger, the protein synthetic activity of these free ribosomes was equal to that of control rats. On the other hand the effects of food restriction on hepatic ribosomal function could be clearly differentiated from the effects observed following hypophysectomy. Thus, the reduced protein synthetic activity of hepatic bound ribosomes isolated from food restricted normal rats was not demonstrable, when polyU was used to direct protein synthesis. Further, food restriction had no effect on the protein synthetic activity of free hepatic ribosomes, and this was true when protein synthesis was directed by either endogenous or artificial messenger. It is concluded that hypophysectomy reduces the protein synthetic ability of both bound and free hepatic ribosomes, and this change of ribosomal function of Hx rats cannot be attributed to their decreased food intake.     

Ribosomes from a relC mutant strain of Escherichia coli show altered activity with bacterial release factor-1

Ribosomes from a relC mutant of Escherichia coli, JF505, are altered in the large subunit protein L11. This protein has abnormal mobility on gel electrophoresis. The ribosomes have a lowered specific activity for release factor-1 which is intermediate between that found for ribosomes containing normal L11 and that for L11 lacking ribosomes. JF505 ribosomes are as sensitive to inactivation of in vitro termination by thiostrepton as normal ribosomes when the antibiotic is added in dimethylsulphoxide but less sensitive when it is added in ethanol.     

Bovine mitochondrial ribosomes. Elongation factor specificity

The activity of bovine mitochondrial ribosomes with elongation factors from a variety of sources including the mitochondria of lower eukaryotes, chloroplasts, Gram-negative bacteria, Gram-positive bacteria, and the eukaryotic cell cytoplasm has been investigated. Bovine mitochondrial ribosomes are active with homologous mitochondrial elongation factor (EF)-G but display no activity with the mitochondrial or chloroplast translocases from the lower eukaryote Euglena gracilis, with Escherichia coli or Bacillus subtilis EF-G or with cytoplasmic EF-2. In contrast to the results obtained with the translocases, E. coli EF-Tu, B. subtilis EF-Tu, and Euglena chloroplast EF-Tu all function to a significant extent on the mitochondrial ribosomes. Cytoplasmic EF-1 has barely detectable activity on the animal mitochondrial ribosomes. The polymerization of phenylalanine by these ribosomes is dependent on poly(U), displays a rather broad Mg2+ optimum around 12 mM, and proceeds most rapidly at low monovalent ion concentrations.     

Evidence for the presence of an inhibitor on ribosomes in mouse L cells infected with mengovirus.

After infection of mouse L cells with mengovirus, there is a rapid inhibition of protein synthesis, a concurrent disaggregation of polysomes, and an accumulation of 80S ribosomes. These 80S ribosomes could not be chased back into polysomes under an elongation block. The infected-cell 80S-ribosome fraction contained twice as much initiator methionyl-tRNA and mRNA as the analogous fraction from uninfected cells. Since the proportion of 80S ribosomes that were resistant to pronase digestion also increased after infection, these data suggest that the accumulated 80S ribosomes may be in the form of initiation complexes. The specific protein synthetic activity of polysomal ribosomes also decreased with time of infection. However, the transit times in mock-infected and infected cells remained the same. Cell-free translation systems from infected cells reflected the decreased protein synthetic activity of intact cells. The addition of reticulocyte initiation factors to such systems failed to relieve the inhibition. Fractionation of the infected-cell lysate revealed that the ribosomes were the predominant target affected. Washing the infected-cell ribosomes with 0.5 M KCI restored their translational activity. In turn, the salt wash from infected-cell ribosomes inhibited translation in lysates from mock-infected cells. The inhibitor in the ribosomal salt wash was temperature sensitive and micrococcal nuclease resistant. A model is proposed wherein virus infection activates (or induces the synthesis of) an inhibitor that binds to ribosomes and stops translation after the formation of the 80S-ribosome initiation complex but before elongation. The presence of such an inhibitor on ribosomes could prevent them from being remobilized into polysomes in the presence of an inhibitor of polypeptide elongation.     

Protein synthesis kinetics with ribosomes from temperature-sensitive mutants of Escherichia coli.

The kinetics of MS2 ribonucleic acid (RNA) directed protein synthesis have been investigated at seven temperatures between 30 and 47 degrees C by using ribosomes isolated from a wild type strain and seven temperature-sensitive mutants of Escherichia coli. The amount of MS2 coat protein formed at each temperature was determined by gel electrophoresis of the products formed with control ribosomes. With ribosomes from each of the mutant strains, the activation energy required to drive protein synthesis below the maximum temperature (up to 40 degrees C) was increased relative to the control (wild type) activity. Preincubation of the ribosomes at 44 degrees C revealed the kinetics of thermal inactivation, with ribosomes from each of the mutants having a half-life for inactivation less than that of the control ribosomes. A good correlation was observed between the relative activity of the different ribosomes at 44 degrees C and their relative rate of thermal inactivation. Mixing assays allowed the identification of a temperature-sensitive ribosomal subunit for each of the mutants. Defects in one or more of three specific steps in protein synthesis (messenger RNA binding, transfer RNA binding, transfer RNA binding, and subunit reassociation) were identified for the ribosomes from each mutant. The relationship between temperature sensitivity and protein synthesis in these strains is discussed.     

Human furin is a calcium-dependent serine endoprotease that recognizes the sequence Arg-X-X-Arg and efficiently cleaves anthrax toxin protective antigen.

Previous work demonstrated that human furin is a predominantly Golgi membrane-localized endoprotease that can efficiently process precursor proteins at paired basic residues (-Lys-Arg- or -Arg-Arg-) in transfected cells. Anion-exchange chromatography of culture supernatant from cells expressing a soluble truncated form of human furin resulted in a greatly enriched preparation of the endoprotease (approximately 70% pure as determined by protein staining). Enzymatic studies show that furin is a calcium-dependent (K0.5 = 200 microM) serine endoprotease which has greater than 50% of maximal activity between pH 6.0 and 8.5. The inhibitor sensitivity of furin suggests that it is similar to, yet distinct from, other calcium-dependent proteases. Evidence that furin may require a P4 Arg in fluorogenic peptide substrates suggested that this enzyme might cleave the protective antigen (PA) component of anthrax toxin at the sequence -Arg-Lys-Lys-Arg-. Indeed, PA was cleaved by purified furin at the proposed consensus site (-Arg-X-Lys/Arg-Arg decreases-) at a rate (8 mumol/min/mg total protein) 400-fold higher than that observed with synthetic peptides. In addition, the processing of mutant PA molecules with altered cleavage sites suggests that furin-catalyzed endoproteolysis minimally requires an -Arg-X-X-Arg- recognition sequence for efficient cleavage. Together, these results support the hypothesis that furin processes protein precursors containing this cleavage site motif in the exocytic pathway and in addition, raises the possibility that the enzyme also cleaves extracellular substrates, including PA.     

Extraction from free ribosomes of a factor mediating ribosome detachment from rough microsomes.

A salt extract of rabbit reticulocyte free monosomes or polysomes contains a factor with an activity that detaches membrane bound monosomes but not polysomes from dog pancreas rough microsomes. It is proposed that this activity, referred to as detachment factor, functions in the dissociation of membrane bound ribosomes from the microsomal membrane after each round of translation. In addition to free ribosomes, the factor is also present in a ribosome-free, high speed supernatant, the cell fractionation equivalent of the cytosol. The factor can be extracted from free ribosomes of a variety of tissues and species, and is able to function on ribosome membrane junctions homologous as well as heterologous with respect to its source.     

The role of ethanol extractable proteins from the 80S rat liver ribosome

80S rat liver ribosomes have been extracted with fifty percent ethanol at varying salt concentrations. The resulting 80S core ribosomes have lost almost all of their protein synthesis activity. The protein synthesis activity could be partially regained when the ethanol extracted proteins were reconstituted with the core ribosomes; however, reconstitution of the ribosome dependent EF-II GTP hydrolysis activity could not be detected. The ethanol extracts were found to contain only a few proteins, one or more of which we believe is necessary for the binding of elongation factor-II.     

Hormonal regulation of the number and activity of ribosomes in mammary gland explants of mice

Possible effects of insulin, hydrocortisone and prolactin on the number and activity of ribosomes enganged in protein biosynthesis in mammary gland explants were explored. The rate and extent of [3H]-puromycin attachment to nascent peptides was used to assess, respectively, the activity and number of ribosomes engaged in protein biosynthesis. None of the hormones altered the number of ribosomes engaged in protein biosynthesis. In addition, of the hormones tested, only insulin appeared to accelerate the rate at which ribosomes carried out the translocation process. The early (1 hr) effect of insulin on protein biosynthesis in the mammary gland would therefore appear to occur via an activation of ribosomal activity. In contrast, the early (6 hr.) effect of prolactin on protein biosynthesis would appear to be exclusively via an RNA-DNA dependent mechanism.     

Pokeweed antiviral protein region Gly209-Lys225 is critical for RNA N-glycosidase activity of the prokaryotic ribosome

Pokeweed antiviral protein (PAP) isolated from Phytolacca americana is a ribosome-inactivating protein (RIP) that has RNA N-glycosidase (RNG) activity towards both eukaryotic and prokaryotic ribosomes. In contrast, karasurin-A (KRN), a RIP from Trichosanthes kirilowii var. japonica, is active only on eukaryotic ribosomes. Stepwise selection of chimera proteins between PAP and KRN indicated that the C-terminal region of PAP (residues 209–225) was critical for RNG activity toward prokaryotic ribosomes. When the region of PAP (residues 209–225) was replaced with the corresponding region of KRN the PAP chimera protein, like KRN, was active only on eukaryotic ribosomes. Furthermore, insertion of the region of PAP (residues 209–225) into the KRN chimera protein resulted not only in the detectable RNG activity toward prokaryotic ribosome, but also activity toward the eukaryotic ribosomes as well that was seven-fold higher than for the original KRN. In this study, the possibility of genetic manipulation of the activity and substrate specificity of RIPs is demonstrated.     

Dual initiation sites of protein synthesis on foot-and-mouth disease virus RNA are selected following internal entry and scanning of ribosomes in vivo.

The initiation of protein synthesis on foot-and-mouth disease virus RNA occurs at two sites separated by 84 nucleotides. Immediately upstream from the first of these sites is the internal ribosome entry site (IRES), which directs the translation of this RNA to be cap-independent. The utilization of these two initiation sites has been examined using artificial fusion genes in vivo under a variety of conditions. Additional in-frame AUG codons have been introduced between these two authentic start sites to determine the mechanism by which ribosomes recognize the second start site. The results indicate that following internal entry of ribosomes on the 5' side of the first initiation codon, many fail to initiate protein synthesis at this position and scan along the RNA to the second initiation site. In the presence or absence of the IRES both initiation sites are efficiently used but the utilization of the two sites is slightly biased towards the second initiation site by the IRES. Furthermore, in the presence of the IRES, protein synthesis initiates at both sites independently of the activity of the cap-binding complex.     

The effect of Escherichia coli ribosomal protein S1 on the translational specificity of bacterial ribosomes

Ribosomes from Gram-negative bacteria such as Escherichia coli exhibit non-specific translation of bacterial mRNAs. That is, they are able to translate mRNAs from a variety of sources in a manner independent of the "strength" of the Shine-Dalgarno region, in contrast to ribosomes from many Gram-positive bacteria, such as Bacillus subtilis, which show specific translation in only being able to translate other Gram-positive mRNA, or mRNAs that have "strong" Shine-Dalgarno regions. There is an evolutionary correlation between the translational specificity and the absence of a protein analogous to E. coli ribosomal protein S1. The specificity observed with B. subtilis ribosomes is a function of their 30 S subunit which lacks S1; translation of Gram-negative mRNA can occur with heterologous ribosomes containing the 30 S subunit of E. coli ribosomes and the 50 S subunit of B. subtilis ribosomes. However, the addition of E. coli S1 alone to B. subtilis ribosome does not overcome their characteristic inability to translate mRNA from Gram-negative organisms. By contrast, the removal of S1 from E. coli ribosomes results in translational behavior similar to that shown by B. subtilis ribosomes in that the S1-depleted E. coli ribosomes can translate mRNA from Gram-positive sources in the absence of added S1, although addition of S1 stimulates further translation of such mRNAs by the E. coli ribosomes.