β-internexin is a microtubule-associated protein identical to the 70-kDa heat-shock cognate protein and the clathrin uncoating ATPase

The C. elegans UNC-23 protein, a member of the BCL-2-associated athanogene (BAG) family of chaperone regulators, interacts with HSP-1 to regulate cell attachment and maintain hypodermal integrity

Mutations in the unc-23 gene in the free-living nematode, Caenorhabditis elegans result in detachment and dystrophy of the anterior body wall musculature and a bent-head phenotype when grown on solid substrate. We have determined that the unc-23 gene product is the nematode ortholog of the human BAG-2 protein, a member of the Bcl-2 associated athanogene (BAG) family of molecular chaperone regulators. We show that a functional GFP-tagged UNC-23 protein is expressed throughout development in several tissues of the animal, including body wall muscle and hypodermis, and associates with adhesion complexes and attachment structures within these 2 tissues. In humans, the BAG protein family consists of 6 members that all contain a conserved 45 amino acid BAG domain near their C-termini. These proteins bind to and modulate the activity of the ATPase domain of the heat shock cognate protein 70, Hsc70. We have isolated missense mutations in the ATPase domain of the C. elegans heat shock 70 protein, HSP-1 that suppress the phenotype exhibited by unc-23(e25) mutant hermaphrodites and we show that UNC-23 and HSP-1 interact in a yeast-2-hybrid system. The interaction of UNC-23 with HSP-1 defines a role for HSP-1 function in the maintenance of muscle attachment during development.

Correction of cystathionine β-synthase deficiency in mice by treatment with proteasome inhibitors

Cystathionine beta-synthase (CBS) deficiency is an inborn error of metabolism characterized by extremely elevated levels of plasma total homocysteine. The vast majority of CBS-deficient patients have missense mutations located in the CBS gene that result in the production of either misfolded or unstable protein. Here, we examine the effect of proteasome inhibitors on mutant CBS function using two different mouse models of CBS deficiency. These mice lack mouse CBS and express a missense mutant human CBS enzyme (either p.I278T or p.S466L) that has less than 5% of normal liver CBS activity, resulting in a 10-30-fold elevation in plasma homocysteine levels. We show that treatment of these mice with two different proteasome inhibitors can induce liver Hsp70, Hsp40, and Hsp27, increase levels of active CBS, and lower plasma homocysteine levels to within the normal range. However, response rates varied, with 100% (8/8) of the p.S466L animals showing correction, but only 38% (10/26) of the p.I278T animals. In total, our data show that treatment with proteostasis modulators can restore significant enzymatic activity to mutant misfolded CBS enzymes and suggests that they may be useful in treating certain types of genetic diseases caused by missense mutations.

Proteome analysis of microtubule-associated proteins and their interacting partners from mammalian brain

The microtubule (MT) cytoskeleton is essential for a variety of cellular processes. MTs are finely regulated by distinct classes of MT-associated proteins (MAPs), which themselves bind to and are regulated by a large number of additional proteins. We have carried out proteome analyses of tubulin-rich and tubulin-depleted MAPs and their interacting partners isolated from bovine brain. In total, 573 proteins were identified giving us unprecedented access to brain-specific MT-associated proteins from mammalian brain. Most of the standard MAPs were identified and at least 500 proteins have been reported as being associated with MTs. We identified protein complexes with a large number of subunits such as brain-specific motor/adaptor/cargo complexes for kinesins, dynein, and dynactin, and proteins of an RNA-transporting granule. About 25% of the identified proteins were also found in the synaptic vesicle proteome. Analysis of the MS/MS data revealed many posttranslational modifications, amino acid changes, and alternative splice variants, particularly in tau, a key protein implicated in Alzheimer's disease. Bioinformatic analysis of known protein-protein interactions of the identified proteins indicated that the number of MAPs and their associated proteins is larger than previously anticipated and that our database will be a useful resource to identify novel binding partners.

Permanent brain ischemia induces marked increments in hsp72 expression and local protein synthesis in synapses of the ischemic hemisphere

Transient focal ischemia induced in rat brain by occlusion of the middle cerebral artery (MCAo) elicits a generalized induction of the 72 kDa heat-shock protein (hsp72) heralding functional recovery. As this effect implies activation of protein synthesis, and local systems of protein synthesis are present in brain synapses, and may be analyzed in preparations of brain synaptosomes, we evaluated hsp72 expression and protein synthesis in synaptosomal fractions of spontaneously hypertensive rats (SHRs) subjected to permanent MCAo. SHRs were randomly divided in ischemics and sham controls, anaesthesia controls and passive controls. Focal ischemia was induced under chloral hydrate anaesthesia by unilateral permanent MCAo. Protein synthesis was determined by [35S]methionine incorporation into synaptosomal proteins from ischemic and contralateral cortex/striatum, and from cerebellum. Hsp72 expression was measured in the same fractions by immunoblotting. Our data demonstrate that under these conditions synaptic hsp72 markedly increases in the ischemic hemisphere 1 and 2 days after MCAo, progressively declining in the following 2 days, while no significant change occurs in control rats. In addition, in the ischemic hemisphere the rate of synaptic protein synthesis increases more than two-fold between 1 and 4 days after MCAo, without showing signs of an impending decline. The present data provide the first demonstration that synaptic protein synthesis is massively involved in brain plastic events elicited by permanent focal ischemia.

Glucose regulated proteins 78 and 75 bind to the receptor for hyaluronan mediated motility in interphase microtubules

The receptor for hyaluronan mediated motility (RHAMM), which is a hyaluronan-binding protein, is a centrosomal and microtubal protein. Here, we have identified two RHAMM-binding proteins, glucose regulated protein (GRP) 78 and GRP75, using co-immunoprecipitation analysis. These two proteins directly bound to glutathione-S-transferase-RHAMM fusion proteins. By double immunostaining, GRP78 and GRP75 colocalized with RHAMM in interphase microtubules, but were separated in mitotic spindles. Prevention of microtubule polymerization by TN-16 and vincristine sulfate induced RHAMM overexpression without a significant change in GRP78/75. Taken together, GRP78/75 and RHAMM complexes may stabilize microtubules in the interphase, associated with a downregulation of RHAMM. These results reveal a new biochemical activity of RHAMM.

Polyanions and the proteome

The behavior of the proteome reflects spatial and temporal organization both within and without cells. We propose that various macromolecular entities possessing polyanionic character such as proteoglycans, lipid bilayer surfaces, microtubules, microfilaments, and polynucleotides may provide a functional network that mediates a variety of cellular phenomena. The interaction of proteins with this array of polyanions is characterized by a lower degree of specificity than seen with most commonly recognized macromolecular interactions. In this commentary, potential roles for this polyanion network in diverse functions such as protein/protein interactions, protein folding and stabilization, macromolecular transport, and various disease processes are all considered, as well as the use of polyanions as therapeutic agents. The role of small polyanions in the regulation of protein/polyanion interactions is also postulated. We provide preliminary experimental analysis of the extent to which proteins interact with polyanions inside cells using a combination of two-dimensional chromatographic and electrophoretic methods and antibody arrays. We conclude that many hundreds to thousands of such interactions are present in cells and argue that future understanding of the proteome will require that the "polyanion world" be taken into account.

Heat-Shock Proteins 70 kDa and 19 kDa are not Required for Induction of Embryogenesis of Brassica napus L. cv. Topas Microspores

It is currently accepted that 'stress' triggers induction of microspore embryogenesis, and for Brassica napus L. cv. Topas it is heat-shock. It has been postulated that the heat-shock proteins (HSPs) generated during heat stress have a central role in the induction mechanism. To test this hypothesis we developed a microspore induction procedure, using colchicine instead of heat treatment. The level of HSP70 increased significantly during and following the microspore heat treatment while sHSP19 expression was induced at the onset of heat-shock and declined after 8 h. In contrast, induction of embryogenesis with colchicine was not accompanied by elevation of HSP70 nor by induction of sHSP19, indicating that these HSPs are not required for induction of microspore embryogensis in this model system. These data refute the current hypothesis that HSPs have an essential role in triggering microspore embryogenesis.

Chaperones increase association of tau protein with microtubules

Molecular chaperones and their functions in protein folding have been implicated in several neurodegenerative diseases, including Parkinson's disease and Huntington's disease, which are characterized by accumulation of protein aggregates (e.g., alpha-synuclein and huntingtin, respectively). These aggregates have been shown in various experimental systems to respond to changes in levels of molecular chaperones suggesting the possibility of therapeutic intervention and a role for chaperones in disease pathogenesis. It remains unclear whether chaperones also play a role in Alzheimer's disease, a neurodegenerative disorder characterized by beta-amyloid and tau protein aggregates. Here, we report an inverse relationship between aggregated tau and the levels of heat shock protein (Hsp)7090 in tau transgenic mouse and Alzheimer's disease brains. In various cellular models, increased levels of Hsp70 and Hsp90 promote tau solubility and tau binding to microtubules, reduce insoluble tau and cause reduced tau phosphorylation. Conversely, lowered levels of Hsp70 and Hsp90 result in the opposite effects. We have also demonstrated a direct association of the chaperones with tau proteins. Our results suggest that up-regulation of molecular chaperones may suppress formation of neurofibrillary tangles by partitioning tau into a productive folding pathway and thereby preventing tau aggregation.

Starvation-induced lysosomal degradation of aldolase B requires glutamine 111 in a signal sequence for chaperone-mediated transport

Aldolase B is an abundant cytosolic protein found in all eukaryotic cells. Like many glycolytic enzymes, this protein was sequestered into lysosomes for degradation during nutrient starvation. We report here that the degradation of recombinant aldolase B was enhanced two-fold when rat and human hepatoma cells were starved for amino acid and serum. In addition, starvation-induced degradation of aldolase B was inhibited by chloroquine, an inhibitor of lysosomal proteinases and by 3-methyladenine, an inhibitor of autophagy. Aldolase B has three lysosomal targeting motifs (Q(12)KKEL, Q(58)FREL, and IKLDQ(111)) that have been proposed to interact with hsc73 thereby initiating its transport into lysosomes. In this study, we have mutated the essential glutamine residues in each of these hsc73-binding motifs in order to evaluate their roles in the lysosomal degradation of aldolase B during starvation. We have found that when glutamines 12 or 58 are mutated to asparagines enhanced degradation of aldolase B proceeded normally. However, when glutamine 111 was mutated to an asparagine or a threonine, starvation-induced degradation was completely suppressed. These mutations did not appear to alter the tertiary structure of aldolase B since enzymatic activity was not affected. Our results suggest that starvation-induced lysosomal degradation of aldolase B requires both autophagy and glutamine 111. We discuss the possible roles for autophagy and hsc73-mediated transport in the lysosomal sequestration of aldolase B.

Localization of the heat-shock protein Hsp70 to the synapse following hyperthermic stress in the brain

Heat-shock proteins are induced in response to cellular stress. Although heat-shock proteins are known to function in repair and protective mechanisms, their relationship to critical neural processes, such as synaptic function, has received little attention. Here we investigate whether the major heat-shock protein Hsp70 localizes to the synapse following a physiologically relevant increase in temperature in the mammalian nervous system. Our results indicate that hyperthermia-induced Hsp70 is associated with pre- and postsynaptic elements, including the postsynaptic density. The positioning of Hsp70 at the synapse could facilitate the repair of stress-induced damage to synaptic proteins and also contribute to neuroprotective events at the synapse.

Hsc40, a new member of the hsp40 family, exhibits similar expression profile to that of hsc70 in mammalian cells

Here, we report the identification and characterization of a new hsp40 family member, heat shock cognate 40 (hsc40), which may be a specific functional partner for hsc70. The hsc40 gene consists of five exons and four introns. The previously characterized hsp40 gene contains only three exons and two introns. Despite this difference in the numbers of exons and introns, the structures of these two genes are actually quite similar. If the first two exons and introns of the hsc40 gene are excluded, the rest is essentially the same as the whole hsp40 gene. The intron/exon boundaries of the hsc40 and hsp40 genes are conserved with respect to amino acid coding, indicating that the hsp40 gene might have arisen from a duplicated hsc40 gene, which developed a new transcription-regulatory mechanism in the second intron. Hsc40 is evolutionally conserved. There is over 95% sequence identity between the putative mouse and human hsc40 proteins. However, there is only 60% sequence identity between the hsc40 and hsp40 proteins from either human or mouse cells. Northern blot analysis of various tissues and cells in culture revealed that this gene was expressed under normal conditions, and its expression was further increased after various stress treatments. The expression pattern of the hsc40 gene is very similar to that of the hsc70 gene under both normal and stress conditions, whereas the hsp40 gene exhibited an expression pattern more similar to that of hsp70 genes on the same Northern blots. These results, considered along with the high rate of sequence conservation between the same proteins from different species and high rate of variation between the two different proteins from the same species, strongly suggest that hsc40 and hsp40 may perform different functions and/or have different specificities in the cell.

Intermediate filament interactions can be altered by HSP27 and alphaB-crystallin

HSP27 and alphaB-crystallin are both members of the small heat shock protein family. alphaB-crystalllin has been proposed to modulate intermediate filaments and recently a mutation in alphaB-crystallin has been identified as the genetic basis of desmin related myopathy. This disease is characterised in its pathology by aggregates of intermediate filaments associated with alphaB-crystallin. Here we report that HSP27 like alphaB-crystallin is associated with glial fibrillary acidic protein and vimentin intermediate filament networks in unstressed U373MG astrocytoma cells. HSP27 is also associated with keratin filaments in MCF7 cells, indicating that this association is not restricted to a particular intermediate filament type. The association of sHSPs with both the soluble and filamentous intermediate filament fractions of U373 cells was demonstrated biochemically. Heat shock or drug treatments induced a co-collapse of intermediate filaments and associated small heat shock proteins. These data show that the presence of HSP27 or alphaB-crystallin could not prevent filament collapse and suggest that the purpose of this association is more than just filament binding. Indeed, in U373MG cells the intermediate filament association with small heat shock proteins is similar to that observed for another protein chaperone, HSC70. In order to discern the effect of different chaperone classes on intermediate filament network formation and maintenance, several in vitro assays were assessed. Of these, falling ball viscometry revealed a specific activity of small heat shock proteins compared to HSC70 that was apparently inactive in this assay. Intermediate filaments form a gel in the absence of small heat shock proteins. In contrast, inclusion of alphaB-crystallin or HSP27 prevented gel formation but not filament assembly. The transient transfection of GFAP into MCF7 cells was used to show that the induction of a completely separate network of intermediate filaments resulted in the specific association of the endogenous HSP27 with these new GFAP filaments. These data lead us to propose that one of the major functions of the association of small heat shock proteins with intermediate filaments is to help manage the interactions that occur between filaments in their cellular networks. This is achieved by protecting filaments against those non-covalent interactions that result when they come into very close proximity as seen from the viscosity experiments and which have the potential to induce intermediate filament aggregation as seen in some disease pathologies.

Selected subunits of the cytosolic chaperonin associate with microtubules assembled in vitro

The molecular chaperone activities of the only known chaperonin in the eukaryotic cytosol (cytosolic chaperonin containing T-complex polypeptide 1 (CCT)) appear to be relatively specialized; the main folding substrates in vivo and in vitro are identified as tubulins and actins. CCT is unique among chaperonins in the complexity of its hetero-oligomeric structure, containing eight different, although related, gene products. In addition to their known ability to bind to and promote correct folding of newly synthesized and denatured tubulins, we show here that CCT subunits alpha, gamma, zeta, and theta also associated with in vitro assembled microtubules, i.e. behaved as microtubule-associated proteins. This nucleotide-dependent association between microtubules and CCT polypeptides (Kd approximately 0.1 microM CCT subunit) did not appear to involve whole oligomeric chaperonin particles, but rather free CCT subunits. Removal of the tubulin COOH termini by subtilisin digestion caused all eight CCT subunits to associate with the microtubule polymer, thus highlighting the non-chaperonin nature of the selective CCT subunit association with normal microtubules.

Presence of molecular chaperones, heat shock cognate (Hsc) 70 and heat shock proteins (Hsp) 40, in the postsynaptic structures of rat brain

The synaptic localization of molecular chaperones, heat shock cognate protein 70 (Hsc70) and Hsp40, was investigated immunohistochemically in the normal rat brain. Postsynaptic density (PSD) fractions contained a constitutive form of HSP70, heat shock cognate protein 70 (Hsc70 or p73) but not inducible form of HSP70 (p72). The immunoreactivities of Hsc70 (p73) were distributed throughout the rat brain, in neuronal somata, dendrites and axons. Their immunoreactivity in neurons was localized in the cytoplasmic matrix, dendrites, and spines at the electron microscopic level. Presynaptic terminals, but less frequently than postsynaptic staining, were also reactive. Postsynaptic areas immediately beneath the synaptic contact or PSDs were immunoreactive for Hsc70. The Hsp40 was highly concentrated in PSD fractions. The staining of Hsp40 immunoreactivity was punctate and distributed widely in the brain. Hsp40 immunoreactivity was localized in dendritic spines, especially in the subsynaptic web, with weak staining of PSDs at the electron microscopic level. Double immunofluorescent staining and confocal microscopy revealed that Hsc70 and Hsp40 were co-localized on somata and neuronal processes of cultured cerebral neurons, on which synaptophysin immunoreactive spots were scattered. These results suggest that Hsp40 and Hsc70 are co-localized at postsynaptic sites and postsynaptic chaperone activity may be mediated by these two heat shock proteins.

Loss of Hsp70-Hsp40 chaperone activity causes abnormal nuclear distribution and aberrant microtubule formation in M-phase of Saccharomyces cerevisiae

The 70-kDa heat shock proteins, hsp70, are highly conserved among both prokaryotes and eukaryotes, and function as chaperones in diverse cellular processes. To elucidate the function of the yeast cytosolic hsp70 Ssa1p in vivo, we characterized a Saccharomyces cerevisiae ssa1 temperature-sensitive mutant (ssa1-134). After shifting to the restrictive temperature (37 degreesC), ssa1-134 mutant cells showed abnormal distribution of nuclei and accumulated as large-budded cells with a 2 N DNA content. We observed more prominent mutant phenotypes using nocodazole-synchronized cells: when cells were incubated at the restrictive temperature following nocodazole treatment, viability was rapidly lost and abnormal arrays of bent microtubules were formed. Chemical cross-linking and immunoprecipitation analyses revealed that the interaction of mutant Ssa1p with Ydj1p (cytosolic DnaJ homologue in yeast) was much weaker compared with wild-type Ssa1p. These results suggest that Ssa1p and Ydj1p chaperone activities play important roles in the regulation of microtubule formation in M phase. In support of this idea, a ydj1 null mutant at the restrictive temperature was found to exhibit more prominent phenotypes than ssa1-134. Furthermore, both ssa1-134 and ydj1 null mutant cells exhibited greater sensitivity to anti-microtubule drugs. Finally, the observation that SSA1 and YDJ1 interact genetically with a gamma-tubulin, TUB4, supports the idea that they play a role in the regulation of microtubule formation.

Concomitant alterations in distribution of 70 kDa heat shock proteins, cytoskeleton and organelles in heat shocked 9L cells

Maintenance of cell architecture and positioning of organelles are major functions of the cytoskeleton. On the other hand, induction of heat shock proteins (HSPs) and reorganization of the cytoskeleton are the most significant changes in heat-shocked mammalian cells. We examine the alterations in HSP70 and its constitutively expressed cognate, HSC70, as well as the cytoskeleton and organelles in 9L rat brain tumor cells upon heat shock. We employed fluorescence microscopy and scanning electron microscopy to follow these changes. Levels of HSP70s were quantified by Western blotting. Accumulation of HSC70 was more transient and the protein translocated to and subsequently exited from the nucleus more rapidly than HSP70. Changes in actin microfilaments include the nuclear localization of actin fraction and disappearance of cytoplasmic microfilament bundles, while the cortical actin microfilaments were almost unaffected. Furthermore, microtubules retracted slightly from the cell periphery but remained largely unchanged. In contrast, the intermediate filaments collapsed into the perinuclear region. The mitochondria converted from filamentous into granular forms and clustered in a region overlapping with the collapsed intermediate filaments. All of the above alterations are reversible and largely reverted after 8 h of recovery. The effect on Golgi organization was very transient and the apparatus assumed a normal appearance within 4 h after the heat treatment. The ER, on the other hand, was totally unaffected by the heat treatment. These observations help correlate the sequential events following a stress like heat shock and suggest possible physiological functions of these essential constituents of a cell under stress.

An intralysosomal hsp70 is required for a selective pathway of lysosomal protein degradation

Previous studies have implicated the heat shock cognate (hsc) protein of 73 kD (hsc73) in stimulating a lysosomal pathway of proteolysis that is selective for particular cytosolic proteins. This pathway is activated by serum deprivation in confluent cultured human fibroblasts. We now show, using indirect immunofluorescence and laser scanning confocal microscopy, that a heat shock protein (hsp) of the 70-kD family (hsp70) is associated with lysosomes (ly-hsc73). An mAb designated 13D3 specifically recognizes hsc73, and this antibody colocalizes with an antibody to lgp120, a lysosomal marker protein. Most, but not all, lysosomes contain ly-hsc73, and the morphological appearance of these organelles dramatically changes in response to serum withdrawal; the punctate lysosomes fuse to form tubules. Based on susceptibility to digestion by trypsin and by immunoblot analysis after two-dimensional electrophoresis of isolated lysosomes and isolated lysosomal membranes, most ly-hsc73 is within the lysosomal lumen. We determined the functional importance of the ly-hsc73 by radiolabeling cellular proteins with [3H]leucine and then allowing cells to endocytose excess mAb 13D3 before measuring protein degradation in the presence and absence of serum. The increased protein degradation in response to serum deprivation was completely inhibited by endocytosed mAb 13D3, while protein degradation in cells maintained in the presence of serum was unaffected. The intralysosomal digestion of endocytosed [3H]RNase A was not affected by the endocytosed mAb 13D3. These results suggest that ly-hsc73 is required for a step in the degradative pathway before protein digestion within lysosomes, most likely for the import of substrate proteins.

Molecular chaperones and the centrosome. A role for TCP-1 in microtubule nucleation

Molecular chaperones play an important role in facilitating the proper maturation of many newly synthesized proteins. Here we provide evidence that molecular chaperones also participate in regulating the assembly of the microtubule cytoskeleton. Via indirect immunofluorescence analysis, both hsp 73 and TCP-1 localized within the centrosome in interphase and mitotic cells. These proteins, along with the centrosome-specific protein, pericentrin, were also present within an enriched preparation of centrosomes. Because the centrosome serves as an initiation site for microtubule growth, we examined the ability of cells to regrow their microtubule network in the presence of hsp 73 or TCP-1 specific antibodies. Purified tubulin and GTP were added to cells following the depolymerization and extraction of cellular microtubules. Microtubules were observed to nucleate off the centrosome using this system, even in the presence of anti-hsp 73 antibodies. Incubation with anti-TCP-1 antibodies, however, blocked microtubule regrowth off the centrosome. Similarly, anti-TCP-1 antibodies microinjected into living cells first treated with nocodazole also inhibited the regrowth of the microtubule network following removal of the microtubule poison. Our results complement earlier genetic studies in yeast implicating a role for TCP-1 in microtubule mediated processes, and may help to explain the previously reported mitotic and meiotic abnormalities associated with TCP-1 mutations.

Identification of a molecular chaperone in the eukaryotic flagellum and its localization to the site of microtubule assembly

Monoclonal antibodies that recognize HSP70 family members from evolutionarily divergent organisms were used to identify both constitutively-expressed and stress-inducible HSP70 proteins in the green alga Chlamydomonas. These monoclonal antibodies also cross-reacted with a 70 kDa flagellar protein that comigrated with the constitutively-expressed HSP70 isoform(s) present in the cell body; this is the first identification of a molecular chaperone within the eukaryotic flagellum. Fractionation experiments demonstrated that much of the flagellar HSP70 was bound to the ‘9+2’ microtubule axoneme. Incubation of isolated axonemes in ATP, but not AMP or AMP-PNP, caused significant release of the previously bound HSP70 as is characteristic of complexed HSP70s. Immunofluorescent localization in whole flagella showed that flagellar HSP70 was concentrated at the distal ends of flagella, sites of axonemal assembly in vivo. Extraction of axonemes under ionic conditions known to cause the release of capping structures that link the distal ends of the axonemal microtubules to the flagellar membrane also caused the release of axonemal-bound HSP70. Taken together, these results suggest a model in which an HSP70 chaperone may assist in targeting tubulin and other unassembled axonemal components to the flagellar tip where the chaperone may also participate in the assembly of the ‘9+2’ flagellar axoneme.

Cytoplasmic HSP70 homologues of pea: differential expression in vegetative and embryonic organs

Eukaryotes express several cytoplasmic HSP70 genes, and their encoded proteins participate in diverse cellular processes. Three cDNAs encoding highly expressed cytoplasmic HSP70 homologues from Pisum sativum were cloned and characterized. They were designated PsHSP71.2, PsHSC71.0, and PsHSP70b. These HSP70 genes have different expression profiles in leaves: PsHSP71.2 is observed only in response to heat stress, PsHSC71.0 is present constitutively, and PsHSP70b is weakly constitutively expressed, but induced strongly in response to heat stress. In addition to being heat induced, the PsHSP71.2 mRNA is also expressed in zygotic, but not maternal organs of developing pea seeds, while PsHSC71.0 and PsHSP70b mRNAs are present in maternal and zygotic organs throughout seed development. Immunoblot analysis of parallel protein samples detects a 70 kDa polypeptide in all samples, and a 72 kDa polypeptide that corresponds to the PsHSP71.2 gene product is observed in cotyledons beginning at mid-maturation and in axes beginning between late maturation and desiccation. This polypeptide is not detected in the seed coat. The 72 kDa polypeptide remains abundant in both cotyledons and axes through germination, but declines substantially between 48 and 72 h after the onset of inbibition. Differential control of HSP70 expression during heat stress, seed maturation, and germination is consistent with the hypothesis that there are functional distinctions between cytoplasmic HSP70s.

The 70-kDa heat shock proteins associate with glandular intermediate filaments in an ATP-dependent manner

Keratin polypeptides 8 and 18 (K8/18) are intermediate filament proteins expressed preferentially in glandular epithelia. We describe the identification, by co-immunoprecipitation from normal human colonic tissues and cultured cell lines, of the 70-kDa heat shock protein (hsp) and its related heat shock cognate protein as K8/18-associated proteins (hsp/c). The association is significant but sub-stoichiometric and occurs preferentially with the soluble rather than the cytoskeletal K8/18 fractions. Heat stress increases the level of soluble K8/18 in association with an increase in hsp70 levels and an increase in the stoichiometry of K8/18-hsp70 association. Identity of the associated proteins was confirmed by microsequencing of a tryptic digest of the purified associated protein and by using anti-hsp/c70-specific antibodies. The K8/18-hsp/c70 complex can be dissociated in a Mg-ATP-dependent manner that requires ATP hydrolysis. Binding of hsp to K8/18 can be reconstituted using purified bovine hsp70 and human K8/18 immunoprecipitates that have been depleted of bound hsp/c70 and increases slightly in the presence of ATP. The reconstituted K8/18-hsp70 complex can be again released in the presence of Mg-ATP. In addition, hsp70 binds to K8/18 without having a significant effect on in vitro filament assembly when added during or after assembly. Using an overlay assay, hsp70 binds exclusively to K8 in the presence of ATP. Our results show direct association of the hsp/c70 proteins with K8/18. This interaction may serve, at least in part, to regulate the function of these two abundant protein groups.

Interactions between brain mitochondria and cytoskeleton: evidence for specialized outer membrane domains involved in the association of cytoskeleton-associated proteins to mitochondria in situ and in vitro

The surface distribution of several proteins (porin, hexokinase, and two proteins associated with microtubules or actin filaments) on the outer membrane of brain mitochondria was analyzed by immunogold labelling of purified mitochondria in vitro. The results suggest the existence of specialized domains for the distribution of porin in the outer mitochondrial membrane. Similarities between the distribution of porin and the distribution of microtubule-associated proteins bound in vitro to mitochondria suggested that mitochondria and microtubules interact by binding microtubule-associated proteins to porin-containing domains of the outer membrane. This hypothesis was supported by biochemical studies on outer mitochondrial proteins involved in in vitro binding of cytoskeleton elements. In vitro interactions between mitochondria and microtubules or neurofilaments were analyzed by electron microscopy. These studies revealed cross-bridging between the outer membrane of mitochondria and the two cytoskeleton elements. Cross-bridging was influenced by ATP hydrolysis and by several proteins associated with the surface of mitochondria or with microtubules. In addition, unidentified proteins which were recognized by antibodies to all intermediate filaments subunits were associated either with the mitochondrial surface or with microtubules. This data suggest the participation of additional cytoplasmic proteins in the interactions between cytoskeleton elements and mitochondria.

cDNA cloning and expression of stress-inducible rat hsp70 in normal and injured rat brain

A reverse transcriptase-polymerase chain reaction (RT-PCR) product obtained from ischemic rat brain RNA was used to screen a rat ischemic forebrain cDNA library for a cDNA clone containing the entire open reading frame for the inducible hsp70. The coding sequence for the rat hsp70 cDNA demonstrated significant similarities with the human hsp70 of Hunt and Morimoto (Proc Natl Acad Sci 82:6455-6459, 1985) and the mouse hsp70 of Hunt and Calderwood (Gene 87:199-204, 1990). The rat inducible hsp70 and constitutive hsc73 sequences are distinct. There was a low level of hsp70 mRNA expression in normal rat brain as in found in other tissues. hsp70 mRNA was markedly induced in rat brain 8 hours following global ischemia and kainic acid-induced seizures. Northern blots showed a approximately 2.9kb hsp70 mRNA band from control, kainic acid, and ischemic brains. RT-PCR confirmed the presence of hsp70 mRNA in normal rat brain. Since there are at least five human and six mouse inducible hsp70 genes known, many other rat hsp70 genes probably exist that could function in different cells or organelles or be induced under different circumstances.

The 70-kDa heat shock cognate protein (HSC70) is a major constituent of the central nervous system and is up-regulated only at the mRNA level in acute experimental autoimmune encephalomyelitis

The expression of the 70-kDa heat shock cognate (HSC70) and stress-inducible (HSP70) proteins, and their mRNAs, was examined in experimental autoimmune encephalomyelitis, a model of inflammatory demyelination in the CNS. This study was undertaken as an extension of previous work demonstrating an abrupt decline in mRNA levels of both glial fibrillary acidic protein and the low-molecular-weight neurofilament subunit in experimental autoimmune encephalomyelitis spinal cord at 12 days after inoculation, the height of inflammation and clinical signs. Using the same total RNA preparations as our previous study, we report here that mRNA levels for HSC70 increased approximately sixfold over control values at the same time that glial fibrillary acidic protein and low-molecular-weight neurofilament subunit messages decreased and were similar to controls by 21 days after inoculation. In situ hybridization experiments showed that HSC70 mRNA was predominantly expressed in neurons and that the influx of inflammatory cells into the CNS was not responsible for the large increase in HSC70 message. Despite this elevation in mRNA, only small (if any) increases in protein levels for HSC70 were detected by both western blotting and in vitro cell-free translation systems. However, by quantitative immunoblotting, we determined that constitutive levels of HSC70 comprised a substantial portion of CNS proteins, representing 2-3% of the total protein content of spinal cord. Immunohistochemical staining illustrated that the distribution of HSC70 was consistent with that of its message. In contrast, no HSP70 mRNA or protein was detected in either control or experimental animals.

Synthesis of heat shock/stress proteins during cellular injury

Several conclusions can be drawn from available data on the expression of stress proteins in brain with respect to their utility as markers of cellular injury. First, it is evident that all cell types in brain are capable of expressing stress proteins, although there is striking specificity in the population responding to a given insult. The apparent hierarchy of responsiveness indicated by hsp72 expression correlates well with the relative vulnerability of specific cell populations in a given model. With increasing severity of injury there can be an attenuation of the translational component of the stress response, in that hsp72 immunoreactivity fails to accumulate even though its mRNA is abundantly expressed. For this reason, hsp72 immunoreactivity provides an index of cell populations that have responded to an insult with a functional stress response. Such a response is not sufficient to guarantee survival, since many CA1 neurons that show significant hsp72 staining are eventually lost after global ischemia in the rat. However, brief insults that result in expression of hsp72 and other proteins encoded by induced mRNAs do result in tolerance to subsequent insults. Future studies may be expected to reveal the contributions of specific gene products to the tolerant state. Meanwhile, complementary evaluations of hsp72 mRNA and protein expression provide practical means of identifying cell populations responding to diverse injuries.

Cytoskeletal thermotolerance in NRK cells

We investigated the effect of heat shock on the cytoskeletons (actin stress fibres, intermediate filaments and microtubules) and the relation between thermotolerance of cytoskeletons and thermotolerance as defined by clonogenic survival in normal rat kidney (NRK) cells. Upon heating at 45 degrees C for 15 min, almost all actin stress fibres were disrupted. During the recovery period after heat shock the stress fibres were reorganized rapidly and returned to their preheated state by 16 h. When the cells were heated again at 45 degrees C for 15 min, the disruption of stress fibres was less severe, indicating thermotolerant state of stress fibres. However, the thermotolerant state of stress fibres induced by prior treatment with sodium arsenite or by heating at 42 degrees C for 2 h was less evident than that induced by prior heat shock at 45 degrees C. The intermediate filaments in NRK cells were very heat-resistant; no apparent changes were observed even after heat shock at 45 degrees C for 60 min, and prior conditioning treatments could not induce thermotolerance of microtubules. All prior conditioning treatments could induce heat shock proteins and thermotolerance as defined by clonogenic survival. From these results it is suggested that there is no correlation between cytoskeletal thermotolerance and either heat shock protein level or thermotolerance as defined by clonogenic survival in NRK cells.

Stress protein and proto-oncogene expression as indicators of neuronal pathophysiology after ischemia

Induction of hsp70 mRNA and protein appear to provide useful markers for delineating stages in the progression of neuronal pathophysiology after ischemia. Detection of hsp70 encoded by the induced mRNA is dependent on complex interactions between the time course of mRNA expression and recovery of protein synthesis in a given neuron population, and perhaps other factors relating to specific aspects of hsp70 physiology, during recirculation intervals of hours to days. Transient mRNA expression and subsequent detection of immunoreactive hsp70 protein appear to identify neurons more likely to survive ischemia and other insults, while prolonged expression of hsp70 mRNA is associated with more severe neuronal injury. Fos and Jun immunoreactivities are also increased after ischemia, and provide indexes of functional gene expression during earlier recirculation periods. The accumulation of Fos immunoreactivity in particular designates neurons in which rapid recovery of protein synthesis during 1-3 h recirculation has allowed translation of the very transiently expressed c-fos mRNA. Jun-like immunoreactivity allows an evaluation of events at later recirculation intervals, and provides a clear demonstration of synthesis and accumulation of induced protein in CA1 neurons at 6 h following 2 min ischemia. Detailed understanding of the significance of such interactions between transcriptional and translational events will continue to evolve as information accumulates regarding the expression of additional mRNAs and proteins after ischemia. The present demonstration that Jun-like immunoreactivity accumulates in CA1 neurons after brief ischemia indicates that widespread changes in gene expression, expected as a consequence of such primary effects on transcription factor activity, are likely to contribute to the phenomenon of induced ischemic tolerance and to other persistent changes in the brain following diverse insults.

The postsynaptic density: constituent and associated proteins characterized by electrophoresis, immunoblotting, and peptide sequencing

The proteins of the postsynaptic density (PSD) fraction of cerebral cortex were resolved by two-dimensional electrophoresis (2DE) and more than 30 proteins identified by characteristic 2DE mobility, immunoblotting with specific antibodies, and N-terminal and peptide sequencing. The PSD fraction is enriched for spectrin, actin, tublin and microtubule associated protein II, myosin, enzymes of glycolysis, creatine kinase, elongation factor 1 alpha, and receptor protein. The three neurofilament proteins are detected but a 58-kDa protein is prominent and is, by peptide sequencing, the bovine homolog of the recently cloned 66-kDa neurofilament protein; in contrast to the latter, however, it is enriched in cerebrum compared with spinal cord. A 68-kDa protein is identified as a member of the hsp70/BiP family of proteins. A protein, designated dynamin, indicating its putative role as a microtubule motor, is identified as a major protein, is found, however, greatly enriched in the particulate fraction, and is significantly denaturant and detergent insoluble. A protein designated N-ethylmaleimide-sensitive factor is also detected. Thus, two proteins implicated in vesicular transport are present in the PSD fraction. Seven polyclonal antibodies were produced to 2DE separated and electroeluted proteins of the PSD and were identified by peptide sequence analysis and 2DE profile as the hsp70/BiP homologous protein, the novel neurofilament protein synapsin IIa, pyruvate kinase, dynamin, aconitase and an unknown contaminating protein, and a 115-kDa protein that by subcellular fractionation and immunoblotting is a diagnostic PSD molecule. In addition, peptide sequences are obtained for four additional higher molecular weight proteins of the PSD that are not related at the level of primary structure to any known proteins.

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.

Heat Shock Proteins in Hypoxic-Ischemic Brain Injury: A Perspective

There is much to suggest that the induction of heat shock protein synthesis is an important response to injury and stress in the brain. The role of heat shock proteins in neurological disease has been approached from two points-of-view. First, the induction and synthesis of specific proteins after brain cell injury provide a window through which insight on the regulation of gene expression in pathological tissue can be obtained. These studies have broad implications for understanding pathophysiological mechanisms of disease. Second, putative cell protective effects of heat shock proteins in brain tissue provide insight into biochemical mechanisms of selective neuronal vulnerability. These studies have extremely important clinical implications since cell sensitivity to injury can seemingly be modified. The role of heat shock proteins in hypoxic-ischemic brain injury is discussed forthwith.

HSP70-related 65 kDa protein of beet yellows closterovirus is a microtubule-binding protein

Beet yellows virus (BYV) genome encodes a 65 kDa protein homologous to the HSP70 family of cellular heat-shock proteins (Agranovsky, A.A., Boyko, V.P., Karasev, A.V., Koonin, E.V. and Dolja, V.V. (1991) J. Mol. Biol. 217, 603-610). The respective gene was cloned and expressed in vitro yielding a product of the expected size (p65). This product was found to bind to the purified microtubules with a binding constant of 4 x 10(-7) M. The binding of p65 was stimulated if ATP presented in the translation mixture was hydrolyzed by apyrase. Removal of the short C-terminal domains of alpha- and beta-tubulin by subtilisin digestion abolished the binding, demonstrating its specificity. The possible role of p65 association with microtubules in the movement of virus within and/or between plant cells is proposed.

The effect of carboxyl-terminal deletions on the nuclear transport rate of rat hsc70

Rat brain hsc70 is a constitutively expressed member of the 70-kDa family of heat shock proteins that is capable of bidirectional transport across the nuclear envelope when microinjected into Xenopus oocytes [1]. The objective of this study was to identify domains involved in its bidirectional transport. Limited proteolytic digestion with chymotrypsin generated three major truncated proteins of approximately 67.5, 59.5, and 56.5 kDa. Reactivity with NH2-terminal-specific antibodies showed that carboxyl-terminal fragments were removed. Nuclear uptake studies were performed by microinjecting 125I-labeled proteins into the cytoplasm and determining their subsequent nucleocytoplasmic distribution. The accumulation rates, while faster than bovine serum albumin controls, were inversely related to the size of the truncated proteins and greatly reduced compared to undigested hsc70. Nuclear efflux was assayed by microinjecting labeled proteins directly into oocyte nuclei. The relative efflux rates of the truncated polypeptides were less than the undigested protein, and, as observed for uptake, were inversely related to size. These results indicate that the carboxyl-terminal domain of hsc70 is involved in its bidirectional exchange.

Axonal transport of two major components of the ubiquitin system: free ubiquitin and ubiquitin carboxyl-terminal hydrolase PGP 9.5

Ubiquitin (Ub), a stress protein thought to target abnormal proteins for degradation, is present in abnormal structures that occur in neuronal perikarya and axons of degenerative diseases including Alzheimer disease. To begin to assess the role of the Ub system in the axon, we studied expression and axonal transport of Ub and other stress proteins, as well as of Ub carboxyl-terminal hydrolase PGP 9.5, in the rat visual system in normal conditions and following heat-shock (HS). In the retina, both the constitutive and inducible forms of HSPs 70 were expressed under normal conditions, while in the superior colliculus the inducible form was detected only following HS. Ub, PGP 9.5 and HSPs 70 were transported in the axon exclusively with the slow component b (SCb), known to carry cytoskeletal and cytoplasmic proteins. The exceedingly long time needed for stress proteins to reach distant axonal locales at the rate of SCb (approximately 3 mm/day) makes it unlikely that they could contribute significantly to the stress response at those sites.

Two 68-kDa proteins in slow axonal transport belong to the 70-kDa heat shock protein family and the annexin family

The major 68-kDa protein found selectively in the faster of the two subcomponents of slow axonal transport [group IV or slow component b (SCb)] in the rat sciatic nerve has been characterized. It was found to contain two distinct classes of proteins, S1 and S2, both of which have isoelectric points of 5.7, but differ in their solubility in the presence of calcium. The S1 protein, which contributes up to 70% of the 68-kDa component, was soluble in the presence or absence of calcium, whereas the S2 protein was bound to the cytoskeleton in a calcium-dependent manner. Further characterization of the two proteins by peptide mapping and immunological methods revealed that the S1 protein belonged to a family of proteins related to the 70-kDa heat shock protein, whereas the S2 protein was identical to 68-kDa calelectrin (annexin VI). Selective occurrence in SCb of these proteins with potential abilities to regulate protein-protein or protein-membrane interactions suggests that they may play important roles in the control of cytoskeletal organization in the axon, because SCb contains mainly cytoskeletal proteins in a more dynamic form compared with the slowest rate component, slow component a, which is enriched in the stably polymerized form of these proteins.

Regulation of heat shock protein synthesis in rat astrocytes

Rat forebrain astrocytes synthesize heat shock proteins with molecular weights 97, 89, 70, 68, and 30-34 kilodaltons. The stress inducible 68-kDa heat shock protein (HSP-68) was vigorously expressed by astrocytes in culture after a 45 degrees C, 20 min heat shock. HSP-68 synthesis was poorly inducible by a second heat shock given 16 hr after the initial heat shock. Decreased [35S]methionine incorporation into HSP-68 correlated with low levels of HSP-68 mRNA present after the second heat shock. The data suggest that control of HSP-68 mRNA levels by transcriptional/posttranscriptional mechanisms is a major site for regulation of HSP-68 synthesis.

Secondary structure of the mammalian 70-kilodalton heat shock cognate protein analyzed by circular dichroism spectroscopy and secondary structure prediction

Heat shock proteins are rapidly synthesized when cells are exposed to stressful agents that cause protein damage. The 70-kDa heat shock induced proteins and their closely related constitutively expressed cognate proteins bind to unfolded and aberrant polypeptides and to hydrophilic peptides. The structural features of the 70-kDa heat shock proteins that confer the ability to associate with diverse polypeptides are unknown. In this study, we have used circular dichroism (CD) spectroscopy and secondary structure prediction to analyze the secondary structure of the mammalian 70-kDa heat shock cognate protein (hsc 70). The far-ultraviolet CD spectrum of hsc 70 indicates a large fraction of alpha-helix in the protein and resembles the spectra one obtains from proteins of the alpha/beta structural class. Analysis of the CD spectra with deconvolution methods yielded estimates of secondary structure content. The results indicate about 40% alpha-helix and 20% aperiodic structure within hsc 70 and between 16-41% beta-sheet and 21-0% beta-turn. The Garnier-Osguthorpe-Robson method of secondary structure prediction was applied to the rat hsc 70 amino acid sequence. The predicted estimates of alpha-helix and aperiodic structure closely matched the values derived from the CD analysis, whereas the predicted estimates of beta-sheet and beta-turn were midway between the CD-derived values. Present evidence suggests that the polypeptide ligand binding domain of the 70-kDa heat shock protein resides within the C-terminal 160 amino acids [Milarski, K. L., & Morimoto, R. I. (1989) J. Cell Biol. 109, 1947-1962].(ABSTRACT TRUNCATED AT 250 WORDS)

Three-dimensional structure of the ATPase fragment of a 70K heat-shock cognate protein

The three-dimensional structure of the amino-terminal 44K ATPase fragment of the 70K bovine heat-shock cognate protein has been solved to a resolution of 2.2 A. The ATPase fragment has two structural lobes with a deep cleft between them; ATP binds at the base of the cleft. Surprisingly, the nucleotide-binding 'core' of the ATPase fragment has a tertiary structure similar to that of hexokinase, although the remainder of the structures of the two proteins are completely dissimilar, suggesting that both the phosphotransferase mechanism and the substrate-induced conformational change intrinsic to the hexokinases may be used by the 70K heat shock-related proteins.

Cyclic AMP-dependent protein phosphorylation in isolated neuronal growth cones from developing rat forebrain

We have shown recently that neuronal growth cones isolated from developing rat forebrain possess an appreciable activity of adenylate cyclase, which produces cyclic AMP and can be stimulated by various neurotransmitter receptor agonists and by forskolin. To investigate cyclic AMP-mediated biochemical mechanisms in isolated growth cones, we have centered the present study on cyclic AMP-dependent protein phosphorylation. One-dimensional gel electrophoretic analysis showed that cyclic AMP analogs increased incorporation of 32P into several phosphoproteins in molecular mass ranges of 50-58 and 76-82 kilodaltons, including those of 82, 76, and 51 kilodaltons. Two-dimensional electrophoresis, using isoelectric focusing in the first dimension, resolved phosphorylated alpha- and beta-tubulin species, actin, a very acidic protein (isoelectric point 4.0) with a molecular mass of 93 kilodaltons, and two proteins (x and x') closely neighboring beta-tubulin. Two other phosphoproteins seen in the gels had molecular masses of 56 and 51 kilodaltons (respective isoelectric points, 4.5 and 4.4) and, along with the 93-kilodalton phosphoprotein, were highly enriched in the isolated growth cones. Only the tubulin and actin species were major proteins in the isolated growth cones. Cyclic AMP analogs enhanced incorporation of 32P into phosphoproteins x and x', and, as assessed by immunoprecipitation, into beta-tubulin. Peptide digest experiments suggested that phosphoproteins x and x' are unrelated to beta-tubulin. Nonequilibrium two-dimensional electrophoresis resolved many phosphoproteins, of which a 79- and 75-kilodalton doublet, a 74-kilodalton species, and a 58-kilodalton doublet showed enhanced incorporation of 32P in the presence of cyclic AMP.