Molecular chaperones: pathways and networks.
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Some proteins synthesized by growing eukaryotic cells are transferred along unidirectional pathways of molecular chaperones until the risk of aggregation has decreased and they can be released safely. Mature proteins denatured by stress may instead be handled by chaperones acting in branched, reversible networks. (+info)
Opposite behavior of two isozymes when refolding in the presence of non-ionic detergents.
(18/6127)
GroEL has a greater affinity for the mitochondrial isozyme (mAAT) of aspartate aminotransferase than for its cytosolic counterpart (cAAT) (Mattingly JR Jr, Iriarte A, Martinez-Carrion M, 1995, J Biol Chem 270:1138-1148), two proteins that share a high degree of sequence similarity and an almost identical spatial structure. The effect of detergents on the refolding of these large, dimeric isozymes parallels this difference in behavior. The presence of non-ionic detergents such as Triton X-100 or lubrol at concentrations above their critical micelle concentration (CMC) interferes with reactivation of mAAT unfolded in guanidinium chloride but increases the yield of cAAT refolding at low temperatures. The inhibitory effect of detergents on the reactivation of mAAT decreases progressively as the addition of detergents is delayed after starting the refolding reaction. The rate of disappearance of the species with affinity for binding detergents coincides with the slowest of the two rate-limiting steps detected in the refolding pathway of mAAT. Limited proteolysis studies indicate that the overall structure of the detergent-bound mAAT resembles that of the protein in a complex with GroEL. The mAAT folding intermediates trapped in the presence of detergents can resume reactivation either upon dilution of the detergent below its CMC or by adding beta-cyclodextrin. Thus, isolation of otherwise transient productive folding intermediates for further characterization is possible through the use of detergents. (+info)
Clusterin gene expression mediates resistance to apoptotic cell death induced by heat shock and oxidative stress.
(19/6127)
Clusterin is a widely expressed, well conserved, secreted glycoprotein, which is highly induced in tissues regressing as a consequence of apoptotic cell death in vivo. It has recently been shown that clusterin expression is only confined to surviving cells following the induction of apoptosis in vitro, suggesting that it is involved in cell survival rather than death. In the hypothesis that clusterin may be implicated in cellular responses to stress, clusterin gene expression was analyzed in the A431 human epidermoid cancer cell line following heat shock and oxidative stress. Our results show that both a transient heat shock (20 min at 42 degrees C) and various oxidative stresses, including hydrogen peroxide, superoxide anion, hyperoxia and UVA exposure, induce a strong increase in clusterin mRNA levels as assessed by northern blot. Nuclear run-on analysis suggests that transcriptional activation is involved in inducing clusterin mRNA in response to heat shock. Using pulse-chase analysis of control and heat shocked cells, it is shown that clusterin mRNA is translated and secreted, thus resulting in increased extracellular levels of the protein following heat shock. To investigate the function of clusterin in response to these stresses, clusterin anti-sense transfectants that stably express virtually no clusterin at the mRNA and protein level were generated in A431 cells. These anti-sense transfectants are shown to be highly sensitive to apoptotic cell death induced by heat shock or oxidative stress compared with wild-type A431 cells or control transfectants. Taken together, our results show that clusterin gene expression is induced in response to heat shock and oxidative stress in human A431 cells, and confers cellular protection against heat shock and oxidative stress. (+info)
The protein disulphide-isomerase family: unravelling a string of folds.
(20/6127)
The mammalian protein disulphide-isomerase (PDI) family encompasses several highly divergent proteins that are involved in the processing and maturation of secretory proteins in the endoplasmic reticulum. These proteins are characterized by the presence of one or more domains of roughly 95-110 amino acids related to the cytoplasmic protein thioredoxin. All but the PDI-D subfamily are composed entirely of repeats of such domains, with at least one domain containing and one domain lacking a redox-active -Cys-Xaa-Xaa-Cys- tetrapeptide. In addition to their known roles as redox catalysts and isomerases, the last few years have revealed additional functions of the PDI proteins, including peptide binding, cell adhesion and perhaps chaperone activities. Attention is now turning to the non-redox-active domains of the PDIs, which may play an important role in all of the known activities of these proteins. Thus the presence of both redox-active and -inactive domains within these proteins portends a complexity of functions differentially accommodated by the various family members. (+info)
Isolation and characterization of a second subunit of molecular chaperonin from Pyrococcus kodakaraensis KOD1: analysis of an ATPase-deficient mutant enzyme.
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The cpkA gene encoding a second (alpha) subunit of archaeal chaperonin from Pyrococcus kodakaraensis KOD1 was cloned, sequenced, and expressed in Escherichia coli. Recombinant CpkA was studied for chaperonin functions in comparison with CpkB (beta subunit). The effect on decreasing the insoluble form of proteins was examined by coexpressing CpkA or CpkB with CobQ (cobyric acid synthase from P. kodakaraensis) in E. coli. The results indicate that both CpkA and CpkB effectively decrease the amount of the insoluble form of CobQ. Both CpkA and CpkB possessed the same ATPase activity as other bacterial and eukaryal chaperonins. The ATPase-deficient mutant proteins CpkA-D95K and CpkB-D95K were constructed by changing conserved Asp95 to Lys. Effect of the mutation on the ATPase activity and CobQ solubilization was examined. Neither mutant exhibited ATPase activity in vitro. Nevertheless, they decreased the amount of the insoluble form of CobQ by coexpression as did wild-type CpkA and CpkB. These results implied that both CpkA and CpkB could assist protein folding for nascent protein in E. coli without requiring energy from ATP hydrolysis. (+info)
Glypican-1 is a VEGF165 binding proteoglycan that acts as an extracellular chaperone for VEGF165.
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Glypican-1 is a member of a family of glycosylphosphatidylinositol anchored cell surface heparan sulfate proteoglycans implicated in the control of cellular growth and differentiation. The 165-amino acid form of vascular endothelial growth factor (VEGF165) is a mitogen for endothelial cells and a potent angiogenic factor in vivo. Heparin binds to VEGF165 and enhances its binding to VEGF receptors. However, native HSPGs that bind VEGF165 and modulate its receptor binding have not been identified. Among the glypicans, glypican-1 is the only member that is expressed in the vascular system. We have therefore examined whether glypican-1 can interact with VEGF165. Glypican-1 from rat myoblasts binds specifically to VEGF165 but not to VEGF121. The binding has an apparent dissociation constant of 3 x 10(-10) M. The binding of glypican-1 to VEGF165 is mediated by the heparan sulfate chains of glypican-1, because heparinase treatment abolishes this interaction. Only an excess of heparin or heparan sulfates but not other types of glycosaminoglycans inhibited this interaction. VEGF165 interacts specifically not only with rat myoblast glypican-1 but also with human endothelial cell-derived glypican-1. The binding of 125I-VEGF165 to heparinase-treated human vascular endothelial cells is reduced following heparinase treatment, and addition of glypican-1 restores the binding. Glypican-1 also potentiates the binding of 125I-VEGF165 to a soluble extracellular domain of the VEGF receptor KDR/flk-1. Furthermore, we show that glypican-1 acts as an extracellular chaperone that can restore the receptor binding ability of VEGF165, which has been damaged by oxidation. Taken together, these results suggest that glypican-1 may play an important role in the control of angiogenesis by regulating the activity of VEGF165, a regulation that may be critical under conditions such as wound repair, in which oxidizing agents that can impair the activity of VEGF are produced, and in situations were the concentrations of active VEGF are limiting. (+info)
Interaction between the nucleotide exchange factor Mge1 and the mitochondrial Hsp70 Ssc1.
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Function of Hsp70s such as DnaK of the Escherichia coli cytoplasm and Ssc1 of the mitochondrial matrix of Saccharomyces cerevisiae requires the nucleotide release factors, GrpE and Mge1, respectively. A loop, which protrudes from domain IA of the DnaK ATPase domain, is one of six sites of interaction revealed in the GrpE:DnaK co-crystal structure and has been implicated as a functionally important site in both DnaK and Ssc1. Alanine substitutions for the amino acids (Lys-108 and Arg-213 of Mge1) predicted to interact with the Hsp70 loop were analyzed. Mge1 having both substitutions was able to support growth in the absence of the essential wild-type protein. K108A/R213A Mge1 was able to stimulate nucleotide release from Ssc1 and function in refolding of denatured luciferase, albeit higher concentrations of mutant protein than wild-type protein were required. In vitro and in vivo assays using K108A/R213A Mge1 and Ssc1 indicated that the disruption of contact at this site destabilized the interaction between the two proteins. We propose that the direct interaction between the loop of Ssc1 and Mge1 is not required to effect nucleotide release but plays a role in stabilization of the Mge1-Ssc1 interaction. The robust growth of the K108A/R213A MGE1 mutant suggests that the interaction between Mge1 and Ssc1 is tighter than required for function in vivo. (+info)
Activation of the grp78 and grp94 promoters by hepatitis C virus E2 envelope protein.
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The hepatitis C virus E1 and E2 envelope proteins are targeted to the endoplasmic reticulum, but instead of being secreted, they are retained in a pre-Golgi compartment, at least partly in a misfolded state. Since secretory proteins which are retained in the endoplasmic reticulum frequently can activate the transcription of intraluminal chaperone proteins, we measured the effect of the E1 and E2 proteins on the promoters of two such chaperones, GRP78 (BiP) and GRP94. We found that E2 but not E1 protein activates these two promoters, as assayed by a reporter gene system. Furthermore, E2 but not E1 protein induces the synthesis of GRP78 from the endogenous cellular gene. We also found that E2 but not E1 protein expressed in mammalian cells is bound tightly to GRP78. This association may explain the ability of E2 protein to activate transcription, since GRP78 has been postulated to be a sensor of stress in the endoplasmic reticulum. Since overexpression of GRP78 has been shown to decrease the sensitivity of cells to killing by cytotoxic T lymphocytes and to increase tumorigenicity and resistance to antitumor drugs, this activity of E2 protein may be involved in the pathogenesis of hepatitis C virus-induced diseases. (+info)