Heat shock protein 70 binds to human apurinic/apyrimidinic endonuclease and stimulates endonuclease activity at abasic sites

The interaction of human heat shock protein 70 (HSP70) with human apurinic/apyrimidinic endonuclease (HAP1) was demonstrated by coimmunoprecipitation. A combination of HSP70 and HAP1 also caused a shift in the electrophoretic mobility of a DNA fragment containing an apurinic/apyrimidinic site. The functional consequence of the HSP70/HAP1 interaction was a 10-100-fold enhancement of endonuclease activity at abasic sites. The physical and functional interaction between HSP70 and HAP1 did not require the addition of ATP. The association of HSP70 and a key base excision repair enzyme suggests a role for heat shock proteins in promoting base excision repair. These findings provide a possible mechanism by which HSP70 protects cells against oxidative stress.     

Isolation and characterization of human HSP70 expressed in Escherichia coli

A plasmid containing the human HSP70 gene was used to transfect and express the protein in Escherichia coli. The bacterial product was a fusion protein containing 640 amino acids of HSP70, plus 33 additional NH2 terminal amino acids; 12 from the bacterial expression vector and 21 from a 5' human sequence that is not normally translated. It was partially purified by ion-exchange and ATP-Sepharose affinity column chromatography. The bacterially produced human HSP70 protein was then compared with HSP70 obtained from cultured 293 cells. Both shared the same staphylococcal V8 protease peptide fragment pattern, ATP binding, and a weak ATPase activity (about 10-15 nmol ATP hydrolyzed per milligram protein per minute at 30 degrees C). The bacterially produced human HSP70 protein differed in its V8 protease pattern with an E. coli ATP-binding protein that corresponded in molecular mass to the E. coli dnaK gene product. Mutants in the human HSP70 gene were constructed which significantly reduced a predicted major alpha-helical domain in the HSP70 molecule that has partial homology to an ATP-binding site of several protein kinases. One HSP70 mutant clone contained a deletion of 20% at the NH2 terminus, and expressed a 57-kDa product, while the other was missing the middle 50% of the gene (40-kDa product). Neither protein fragment bound to an ATP affinity column, suggesting that ATP binding to HSP70 may be conformationally affected by a region about 20% internal to the NH2 terminal end of the molecule. Recently, a similar location of the ATP-binding site has been reported by Milarski and Morimoto (27).     

Adenovirus capsid proteins interact with HSP70 proteins after penetration in human or rodent cells

Soon after penetration of adenovirus serotype 2 in BHK-21 and HeLa cells, HSP70 and HSC70 proteins become associated with the viral capsid. By analysis with a polyclonal antibody derived from a fusion protein containing the C-terminal domain, 290 amino acids of HSP70, and using both immunological methods and infected cells fractionation we observed that a significant amount of HSP70 proteins moved to the nucleus and colocalized with the adenovirus particles. HSP70 proteins of infected cells were isolated as a complex cross-linked with intracytoplasmic adenovirus type 2. By coprecipitation, using a polyclonal-specific antiserum derived from the fusion protein, or two different monoclonal-specific antisera, we showed that HSP70 and HSC70 proteins were associated with hexon, the major adenovirus capsid protein.     

Characterization of a DNA binding domain in the C-terminus of HIV-1 integrase by deletion mutagenesis.

The integrase (IN) protein of human immunodeficiency virus type 1 (HIV-1) catalyzes site-specific cleavage of 2 bases from the viral long terminal repeat (LTR) sequence yet it binds DNA with little DNA sequence specificity. We have previously demonstrated that the C-terminal half of IN (amino acids 154-288) possesses a DNA binding domain. In order to further characterize this region, a series of clones expressing truncated forms of IN as N-terminal fusion proteins in E.coli were constructed and analyzed by Southwestern blotting. Proteins containing amino acids 1-263, 1-248 and 170-288 retained the ability to bind DNA, whereas a protein containing amino acids 1-180 showed no detectable DNA binding. This defines a DNA binding domain contained within amino acids 180-248. This region contains an arrangement of 9 lysine and arginine residues each separated by 2-4 amino acids (KxxxKxxxKxxxxRxxxRxxRxxxxKxxxKxxxK), spanning amino acids 211-244, which is conserved in all HIV-1 isolates. A clone expressing full-length IN with a C-terminal fusion of 16 amino acids was able to bind DNA comparably to a cloned protein with a free C-terminus, and an IN-specific monoclonal antibody which recognizes an epitope contained within amino acids 264-279 was unable to block DNA binding, supporting the evidence that a region necessary for binding lies upstream of amino acid 264.     

Site-directed mutagenesis of the human DNA repair enzyme HAP1: identification of residues important for AP endonuclease and RNase H activity.

HAP1 protein, the major apurinic/apyrimidinic (AP) endonuclease in human cells, is a member of a homologous family of multifunctional DNA repair enzymes including the Escherichia coli exonuclease III and Drosophila Rrp1 proteins. The most extensively characterised member of this family, exonuclease III, exhibits both DNA- and RNA-specific nuclease activities. Here, we show that the RNase H activity characteristic of exonuclease III has been conserved in the human homologue, although the products resulting from RNA cleavage are dissimilar. To identify residues important for enzymatic activity, five mutant HAP1 proteins containing single amino acid substitutions were purified and analysed in vitro. The substitutions were made at sites of conserved amino acids and targeted either acidic or histidine residues because of their known participation in the active sites of hydrolytic nucleases. One of the mutant proteins (replacement of Asp-219 by alanine) showed a markedly reduced enzymatic activity, consistent with a greatly diminished capacity to bind DNA and RNA. In contrast, replacement of Asp-90, Asp-308 or Glu-96 by alanine led to a reduction in enzymatic activity without significantly compromising nucleic acid binding. Replacement of His-255 by alanine led to only a very small reduction in enzymatic activity. Our data are consistent with the presence of a single catalytic active site for the DNA- and RNA-specific nuclease activities of the HAP1 protein.     

Cloning of the HSP70 gene from Halobacterium marismortui: relatedness of archaebacterial HSP70 to its eubacterial homologs and a model for the evolution of the HSP70 gene.

Heat shock induces the synthesis of a set of proteins in Halobacterium marismortui whose molecular sizes correspond to the known major heat shock proteins. By using the polymerase chain reaction and degenerate oligonucleotide primers for conserved regions of the 70-kDa heat shock protein (HSP70) family, we have successfully cloned and sequenced a gene fragment containing the entire coding sequence for HSP70 from H. marismortui. HSP70 from H. marismortui shows between 44 and 47% amino acid identity with various eukaryotic HSP70s and between 51 and 58% identity with its eubacterial and archaebacterial homologs. On the basis of a comparison of all available HSP70 sequences, we have identified a number of unique sequence signatures in this protein family that provide a clear distinction between eukaryotic organisms and prokaryotic organisms (archaebacteria and eubacteria). The archaebacterial (viz., H. marismortui and Methanosarcina mazei) HSP70s have been found to contain all of the signature sequences characteristic of eubacteria (particularly the gram-positive bacteria), which suggests a close evolutionary relationship between these groups. In addition, detailed analyses of HSP70 sequences that we have carried out have revealed a number of additional novel features of the HSP70 protein family. These include (i) the presence of an insertion of about 25 to 27 amino acids in the N-terminal quadrants of all known eukaryotic and prokaryotic HSP70s except those from archaebacteria and the gram-positive group of bacteria, (ii) significant sequence similarity in HSP70 regions comprising its first and second quadrants from organisms lacking the above insertion, (iii) highly significant similarity between a protein, MreB, of Escherichia coli and the N-terminal half of HSP70s, (iv) significant sequence similarity between the N-terminal quadrant of HSP70 (from gram-positive bacteria and archaebacteria) and the m-type thioredoxin of plant chloroplasts. To account for these and other observations, a model for the evolution of HSP70 proteins involving gene duplication is proposed. The model proposes that HSP70 from archaebacteria (H. marismortui and M. mazei) and the gram-positive group of bacteria constitutes the ancestral form of the protein and that all other HSP70s (viz., other eubacteria as well as eukaryotes) containing the insert have evolved from this ancient protein.     

Molecular characterization of heat shock protein 70 genes in the liver of three warm freshwater fishes with differential tolerance to microcystin-LR

Heat shock protein 70 (HSP70) protect cell from oxidative stress by preventing the irreversible loss of vital proteins and facilitating their subsequent regeneration. Silver carp (Hypophthalmichthys molitrix), grass carp (Ctenopharyngodon idellus), and Nile tilapia (Oreochromis nilotica) are three warm freshwater fishes with differential tolerance to microcystin-LR (MC-LR). Full-length cDNAs encoding the HSP70 were cloned from the livers of the three fishes. The HSP70 cDNAs of silver carp, grass carp, and Nile tilapia were 2356, 2348, and 2242 bp in length and contained an open-reading frame of 1950 bp (encoding a polypeptide of 649 amino acids), 1950 bp (649 amino acids), and 1917 bp (638 amino acids), respectively. Like mammalian HSP70, the HSP70 of the three fish was also composed of an ATPase domain from residues 1 to 383 (44 kDa), substrate peptide binding domain from residues 384 to 544 (18 kDa), and a C-terminus domain from residues 545 to 649 (10 kDa). The relatively high conservation of HSP70 sequences among different vertebrates is consistent with their important role in fundamental cellular processes. Using beta-actin as an external control, RT-PCR within the exponential phase was conducted to determine the constitutive and inducible expression level of HSP70 gene among the three fishes (6-12 g) intraperitoneally injected with MC-LR (50 μg kg(-1) body weight). Both constitutive and inducible liver mRNA levels of the fish HSP70 genes showed positive relationships with their tolerance to MC-LR: highest in Nile tilapia, followed by silver carp, and lowest in grass carp. The differential expression pattern of liver HSP70 genes in the three fish indicated a potential role of HSP70 in the detoxification process of MC-LR.     

Characterisation of PHSP1, a cDNA encoding a mitochondrial HSP70 fromPisum sativum

A pea cDNA clone,PHSP1, encoding a member of the HSP70 gene family has been isolated. DNA sequence analysis indicates that the protein encoded byPHSP1 is a homologue of the mitochondrial HSP70 proteins, SSP1 fromSchizosaccharomyces pombe and SSC1 fromS. cerevisiae. It contains an amino-terminal extension of 50 amino acids, rich in basic and hydroxyl amino acids, similar to other plant mitochondrial leader sequences. Western blot analysis indicates that the PHSP1 protein is associated only with mitochondria and not with any other sub-cellular organelle or cytoplasm. Further confirmation of its location within mitochondria was obtained fromin vitro protein translocation experiments into purifiedPisum sativum mitochondria. It was observed that the precursor protein was efficiently imported and that it is processed to produce a protein with anM _r of the anticipated size of the mature protein. Results are discussed with respect to the structure and function of the mitochondrial HSP70 protein.Abbreviations mtHSP70 mitochondrial HSP70 - ER endoplasmic reticulum - nt nucleotide - IgG immunoglobulin G - BiP immunoglobulin-binding protein - hsc heat shock cognate     

Inhibition of human immunodeficiency virus type 1 infection of human CD4+ T cells by microbial HSP70 and the peptide epitope 407-426

Human immunodeficiency virus type 1 (HIV-1) virions contain heat shock proteins (HSP), but these proteins have received limited attention. The objectives of this study were to establish if the microbial 70-kDa HSP exerts an inhibitory effect on the HIV-1 infection of human CD4+ T cells, to identify an inhibitory peptide epitope within the sequence of HSP70, and to evaluate the kinetic features of any inhibitory activity. The results of these studies suggest that microbial HSP70 exerts dose-dependent inhibition on CCR5 (R5) strains of clades B, C, and D of HIV-1 infecting human CD4+ T cells. The site of the HIV-1-inhibitory function was identified within the C-terminal peptide binding domain of HSP70, and the function is expressed by the peptide epitope comprising amino acids 407 to 426. The mechanism of inhibition of HIV-1 infectivity by HSP70 is blocking of the CCR5 coreceptors directly and indirectly by inducing CC chemokines and APOBEC3G. The inhibitory effect of HSP70, its C-terminal fragment, or peptide 407-426 may make HSP70 useful as a microbicidal agent. A potentiating noncognate inhibition of HIV-1 infectivity by combined treatment with HSP70 and monoclonal or polyclonal antibody to CCR5 was demonstrated. This novel strategy may be utilized in therapeutic immunization against HIV-1 infection.     

Mutational analysis of the human HSP70 protein: distinct domains for nucleolar localization and adenosine triphosphate binding

The human HSP70 gene was modified in vitro using oligonucleotide-directed mutagenesis to add sequences encoding a peptide from the testis-specific form of human lactate dehydrogenase (LDH) to the carboxy terminus of HSP70. The peptide-tagged HSP70 can be distinguished from the endogenous HSP70 protein using an LDH peptide-specific antiserum in indirect immunofluorescence assays of cells transiently transfected with an expression vector containing the tagged HSP70 gene regulated by the human HSP70 promoter. A series of deletion mutants within the HSP70 protein coding region were generated. Using double-label indirect immunofluorescence with the LDH peptide-specific antiserum and HSP70-specific mAbs, we compared the intracellular distribution of the deletion mutants to that of endogenous HSP70. We have determined that sequences in the carboxy terminus of HSP70 are necessary for proper nucleolar localization after heat shock. In contrast, sequences in the amino terminus of HSP70 are responsible for the ATP-binding ability of the protein. Mutants that were unable to bind ATP, however, still displayed nucleolar association, indicating that ATP binding is apparently not required for interaction with substrate. Additional support that HSP70 appears to be composed of at least two domains follows from the results of trypsin digestions of wild type and mutant HSP70. Protease digestion of the mutant HSP70 proteins identified a region of HSP70 that, when deleted, affected HSP70 conformation.     

Characterization of multiple members of the HSP70 family in platyfish culture cells: molecular evolution of stress protein HSP70 in vertebrates

A shift from 28 to 37 degrees C in the incubation temperature of a culture of the platyfish fibroblast cell line, EHS cells (platyfish fibroblast cell line), induced a set of stress proteins. A two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) analysis showed that the cells expressed three genetically distinct forms of heat-shock protein 70 (HSP70) family proteins: heat-inducible forms of HSP70, the constitutively expressed heat-shock cognate protein 70 (HSC70) and its phosphorylated isoform, and the glucose-regulated protein 78 (GRP78). Three different clones encoding two major isoforms of heat-inducible HSP70, platyfish HSP70-1 and HSP70-2, and of the HSC70 were isolated from a platyfish cDNA library. We compared the deduced amino acid sequences of the platyfish HSP70 and HSC70 proteins with those of other vertebrates. Phylogenetic analysis showed that vertebrate HSP70 could be classified into four cluster groups: (a) fish HSP70, with two isoforms of heat-inducible HSP70 in fish, fish HSP70-1 and HSP70-2; (b) the mammalian testis-specific HSP70-related protein HST70; (c) the mammalian heat-inducible HSP70B'; and (d) the mammalian major histocompatibility complex (MHC)-linked HSP70, including the MHC-linked heat-inducible HSP70 and the testis-specific HSP70-related protein. These findings suggest that vertebrate HSP70 was derived from a single ancestral HSP70 gene during vertebrate evolution and that multiple copies of heat-inducible HSP70 were probably evolved during genetic divergence in fish and higher vertebrates.     

The induction mechanism of the molecular chaperone HSP70 in the gastric mucosa by Geranylgeranylacetone (HSP-inducer)

To elucidate the induction mechanism of HSP70 by geranylgeranylacetone (GGA), we investigated GGA specific binding proteins using a GGA-affinity column. Alteration of chaperone activity of HSP70 and binding affinity of HSP70 to heat shock factor-1 (HSF-1) was evaluated in the presence or absence of GGA. The binding domain of HSP70 to GGA was also analyzed. A 70-kDa protein eluted by 10 mM GGA from the GGA-affinity column was identical to constitutively expressed HSP70 on immunoblotting. GGA-binding domain of HSP70 was C-terminal of the protein as peptide-binding domain (HSP70C). The chaperone activity of HSP70 and recombinant HSP70C was suppressed by GGA. Furthermore, dissociation of the HSP70 from HSF-1 was observed in the presence of GGA. GGA preferentially binds to the C-terminal of HSP70 which binds to HSF-1. After dissociation of HSP70, free HSF-1 could acquire the ability to bind to HSE (the promoter region of HSP70) gene.     

Sec2 protein contains a coiled-coil domain essential for vesicular transport and a dispensable carboxy terminal domain

SEC2 function is required at the post-Golgi apparatus stage of the yeast secretory pathway. The SEC2 sequence encodes a protein product of 759 amino acids containing an amino terminal region that is predicted to be in an alpha-helical, coiled-coil conformation. Two temperature-sensitive alleles, sec2-41 and sec2-59, encode proteins truncated by opal stop codons and are suppressible by an opal tRNA suppressor. Deletion analysis indicates that removal of the carboxyl terminal 251 amino acids has no apparent phenotype, while truncation of 368 amino acids causes temperature sensitivity. The amino terminal half of the protein, containing the putative coiled-coil domain, is essential at all temperatures. Sec2 protein is found predominantly in the soluble fraction and displays a native molecular mass of greater than 500 kD. All phenotypes of the temperature-sensitive sec2 alleles are partially suppressed by duplication of the SEC4 gene, but the lethality of a sec2 disruption is not suppressed. The sec2-41 mutation exhibits synthetic lethality with the same subset of the late acting sec mutants as does sec4-8 and sec15-1. The Sec2 protein may function in conjunction with the Sec4 and Sec15 proteins to control vesicular traffic.     

Intrinsically disordered proteins as molecular shields

The broad family of LEA proteins are intrinsically disordered proteins (IDPs) with several potential roles in desiccation tolerance, or anhydrobiosis, one of which is to limit desiccation-induced aggregation of cellular proteins. We show here that this activity, termed molecular shield function, is distinct from that of a classical molecular chaperone, such as HSP70 - while HSP70 reduces aggregation of citrate synthase (CS) on heating, two LEA proteins, a nematode group 3 protein, AavLEA1, and a plant group 1 protein, Em, do not; conversely, the LEA proteins reduce CS aggregation on desiccation, while HSP70 lacks this ability. There are also differences in interaction with client proteins - HSP70 can be co-immunoprecipitated with a polyglutamine-containing client, consistent with tight complex formation, whereas the LEA proteins can not, although a loose interaction is observed by F?rster resonance energy transfer. In a further exploration of molecular shield function, we demonstrate that synthetic polysaccharides, like LEA proteins, are able to reduce desiccation-induced aggregation of a water-soluble proteome, consistent with a steric interference model of anti-aggregation activity. If molecular shields operate by reducing intermolecular cohesion rates, they should not protect against intramolecular protein damage. This was tested using the monomeric red fluorescent protein, mCherry, which does not undergo aggregation on drying, but the absorbance and emission spectra of its intrinsic fluorophore are dramatically reduced, indicative of intramolecular conformational changes. As expected, these changes are not prevented by AavLEA1, except for a slight protection at high molar ratios, and an AavLEA1-mCherry fusion protein is damaged to the same extent as mCherry alone. A recent hypothesis proposed that proteomes from desiccation-tolerant species contain a higher degree of disorder than intolerant examples, and that this might provide greater intrinsic stability, but a bioinformatics survey does not support this, since there are no significant differences in the degree of disorder between desiccation tolerant and intolerant species. It seems clear therefore that molecular shield function is largely an intermolecular activity implemented by specialist IDPs, distinct from molecular chaperones, but with a role in proteostasis.     

Cloning and nucleotide sequence of a hsp70 gene from Streptomyces griseus

The cultivation of Streptomyces griseus 2247 at the growth-limited temperature (37°C) or in liquid medium containing 5% ethanol (toxic for growth) revealed the presence of heat-induced proteins in the total cellular proteins. Among them, a 70 kDal protein was isolated and its N-terminal amino acid sequence was determined. The 70 kDal protein possessed a possible ATP-binding site in the N-terminus, which was conserved among the HSP70 family. A DNA fragment encoding the HSP70 homologue was isolated from a genomic library of S. griseus 2247 strain using an oligonucleotide probe based on the N-terminal amino acid sequence of the 70 kDal protein. DNA sequence analysis of the cloned gene revealed an open reading frame consisting of 618 amino acid residues. The deduced amino acid sequence is highly homologous to the HSP70 family proteins; it is 59.8 % identical to Clostridium perfringens HSP70, 59.7% to the Bacillus megaterium DnaK protein, 58.4% to the Methanosarcina mazei DnaK protein, 58.1% to Synechocystis HSP70, 52.8% to the DnaK protein of Escherichia coli, and about 50% to some of the mitochondrial heat shock proteins. The cloned gene could encode the HSP70 of S. griseus.