The Function of Ile-X-Ile Motif in the Oligomerization and Chaperone-Like Activity of Small Heat Shock Protein AgsA at Room Temperature

Small heat shock proteins assemble as large oligomers in vitro and exhibit ATP-independent chaperone activities. Ile-X-Ile motif is essential in both the function and oligomer formation. AgsA of Salmonella enterica serovar Typhimurium has been demonstrated to adopt large oligomeric structure and possess strong chaperone activity. Size exclusion chromatography, non-denaturing pore gradient PAGE, and negatively stain electron microscopic analysis of the various C-terminal truncated mutants were performed to investigate the role of Ile-X-Ile motif in the oligomer assembly of AgsA. By measuring the ability to prevent insulin from aggregating induced by TCEP, the chaperone-like activity of AgsA and the C-terminal truncated mutants at room temperature were determined. We found that the truncated mutants with Ile-X-Ile motif partially or fully deleted lost the ability to form large oligomers. Contrast to wild type AgsA which displayed weak chaperone-like activity, those mutants shown significantly enhanced activities at room temperature. In summary, biochemical experiment, activity assay and electron microscopic analysis suggested that Ile-X-Ile motif is essential in oligomer assembly of AgsA and might take the role of an inhibitor for its chaperone-like activity at room temperature.     

Cloning and truncation modification of trehalose-6-phosphate synthase gene from Selaginella pulvinata

A homologous sequence was amplified from resurrection plant Selaginella pulvinta by RACE technique, proved to be the full-length cDNA of trehalose-6-phosphate synthase gene by homologous alignment and yeast complementation assay, and nominated as SpTPS1 gene. The open reading frame of this gene was truncated 225 bp at the 5′-end, resulting the N-terminal truncation modification of 75 amino acids for its encoding protein. The TPS1 deletion mutant strain YSH290 of the brewer's yeast transformed by the truncated gene SpTPS1Δ and its original full-length version restored growth on the medium with glucose as a sole carbon source and displayed growth curves with no significant difference, indicating their encoding proteins functioning as TPS enzyme. The TPS activity of the mutant strain transformed by the truncated gene SpTPS1Δ was about six fold higher than that transformed by its original version, reasoning that the extra N-terminal extension of the full-length amino acid sequence acts as an inhibitory domain to trehalose synthesis. However, the trehalose accumulation of the mutant strain transformed by the truncated gene SpTPS1Δ was only 8% higher than that transformed by its original version. This result is explained by the feedback balance of trehalose content coordinated by the comparative activities between trehalose synthase and trehalase. The truncated gene SpTPS1Δ is suggested to be used in transgenic operation, together with the inhibition of trehalase activity by the application of validamycin A or genetic deficiency of the endogenous trehalase gene, for the enhancement of trehalose accumulation and improvement of abiotic tolerance in transgenic plants.     

In vivo analysis of the overlapping functions of DnaK and trigger factor

Trigger factor (TF) is a ribosome-bound protein that combines catalysis of peptidyl-prolyl isomerization and chaperone-like activities in Escherichia coli. TF was shown to cooperate with the DnaK (Hsp70) chaperone machinery in the folding of newly synthesized proteins, and the double deletion of the corresponding genes (tig and dnaK) exhibited synthetic lethality. We used a detailed genetic approach to characterize various aspects of this functional cooperation in vivo. Surprisingly, we showed that under specific growth conditions, one can delete both dnaK and tig, indicating that bacterial survival can be maintained in the absence of these two major cytosolic chaperones. The strain lacking both DnaK and TF exhibits a very narrow temperature range of growth and a high level of aggregated proteins when compared to either of the single mutants. We found that, in the absence of DnaK, both the N-terminal ribosome-binding domain and the C-terminal domain of unknown function are essential for TF chaperone activity. In contrast, the central PPIase domain is dispensable. Taken together, our data indicate that under certain conditions, folding of newly synthesized proteins in E. coli is not totally dependent on an interaction with either TF and/or DnaK, and suggest that additional chaperones may be involved in this essential process.     

C-terminal lysine truncation increases thermostability and enhances chaperone-like function of porcine alphaB-crystallin

The carboxyl-terminal segment of alpha-crystallin, a major lens protein of all vertebrates, has a short and flexible peptide extension of about 20 amino acid residues that are very susceptible to proteolytic truncation and modifications under physiological conditions. To investigate its role in crystallin aggregation and chaperone-like activity, we constructed a mutant of porcine alphaB-crystallin with C-terminal lysine truncated end, which unexpectedly showed better chaperone-like function than wild-type alphaB-crystallin. From circular dichroism (CD) spectra, we show that the mutant possesses similar secondary and tertiary structures to those of native purified and recombinant alphaB-crystallins. Analytical ultracentrifugation revealed that the truncated mutant was smaller than wild-type alphaB-crystallin in aggregation size and mass. The observed higher thermostability and anti-thermal aggregation propensity of the truncated alphaB-crystallin mutant than wild-type alphaB-crystallin are in contrast to the prevailing notion that mutations at the C-terminal lysines of alphaB-crystallin result in substantial loss of chaperone-like activity, despite the overall preservation of secondary structure. The detailed characterization of the C-terminal deletion mutants may provide some deeper insight into the chaperoning mechanism of the structurally related small heat-shock protein family.     

Structural and functional implications of C-terminal regions of alpha-synuclein

Aggregation of alpha-synuclein is thought to play a major role in the pathogenesis of Parkinson's disease (PD), which is characterized by the presence of intracytoplasmic Lewy bodies (LB) in the brain. alpha-Synuclein and its deletion mutants are largely unfolded proteins with random coil structures as revealed by CD spectra, fluorescence spectra, gel filtration chromatography, and ultracentrifugation. On the basis of its highly unfolded and flexible conformation, we have investigated the chaperone-like activity of alpha-synuclein in vitro. In our experiments, alpha-synuclein inhibited the aggregation of model substrates and protected the catalytic activity of alcohol dehydrogenase and rhodanese during heat stress. In addition, alpha-synuclein inhibited the initial aggregation of reduced/denatured lysozyme on the refolding pathway. Interestingly, deletion of the C-terminal regions led to the abolishment of chaperone activity, although largely unstructured conformations are maintained. Moreover, alpha-synuclein could inhibit the aggregation of various Escherichia coli cellular proteins during heat stress, and C-terminal deletion mutants could not provide any protection to these cellular proteins. Results with synthetic C-terminal peptides and C-terminal deletion mutants suggest that the second acidic repeat, (125)YEMPSEEGYQDYEPEA(140), is important for the chaperone activity of alpha-synuclein, and C-terminal deletion leads to the facilitated aggregation with the elimination of chaperone activity.     

Effect of phosphorylation on alpha B-crystallin: differences in stability, subunit exchange and chaperone activity of homo and mixed oligomers of alpha B-crystallin and its phosphorylation-mimicking mutant

Phosphorylation appears to be one of the modulators of chaperone functions of small heat shock proteins. However, the role of phosphorylation is not completely understood. We have investigated the structural and functional consequences of a phosphorylation-mimicking mutation in αB-crystallin, a small heat shock protein with chaperone activity. We have used a phosphorylation-mimicking mutant, 3DαB-crystallin, in which all the three phosphorylatable serine residues are replaced with aspartic acid. 3DαB-Crystallin showed enhanced chaperone-like activity towards DTT-induced aggregation of insulin, heat-induced aggregation of citrate synthase and SDS-induced amyloid fibril formation of α-synuclein. Fluorescence and circular dichroism spectroscopic studies showed that 3DαB-crystallin exhibits lower stability towards urea-induced denaturation compared to αB-crystallin. Subunit exchange studies using fluorescence resonance energy transfer showed that 3DαB-crystallin exhibits an observable increase in subunit exchange compared to αB-crystallin. Since only part of αB-crystallin is phosphorylated in vivo, our subunit exchange studies indicate that formation of mixed oligomers between the unphosphorylated and phosphorylated subunits are likely to play a role in vivo. Our study shows that mixed-oligomer formation modulates the chaperone-like activity. We propose that the degree of phosphorylation of the αB-crystallin oligomers and temperature are key modulators to achieve a wide range of chaperone capabilities of the small heat shock protein, αB-crystallin.     

The C-terminal domain of Escherichia coli Hfq is required for regulation

The Escherichia coli RNA chaperone Hfq is involved in riboregulation of target mRNAs by small trans-encoded non-coding (ncRNAs). Previous structural and genetic studies revealed a RNA-binding surface on either site of the Hfq-hexamer, which suggested that one hexamer can bring together two RNAs in a pairwise fashion. The Hfq proteins of different bacteria consist of an evolutionarily conserved core, whereas there is considerable variation at the C-terminus, with the γ- and β-proteobacteria possessing the longest C-terminal extension. Using different model systems, we show that a C-terminally truncated variant of Hfq (Hfq₆₅), comprising the conserved hexameric core of Hfq, is defective in auto- and riboregulation. Although Hfq₆₅ retained the capacity to bind ncRNAs, and, as evidenced by fluorescence resonance energy transfer assays, to induce structural changes in the ncRNA DsrA, the truncated variant was unable to accommodate two non-complementary RNA oligonucleotides, and was defective in mRNA binding. These studies indicate that the C-terminal extension of E. coli Hfq constitutes a hitherto unrecognized RNA interaction surface with specificity for mRNAs.     

Purification and characterization of the wild type and truncated human cystathionine beta-synthase enzymes expressed in E. coli

In this paper, we describe the expression and characterization of recombinant human cystathionine β-synthase (CBS) in Escherichia coli. We have used a glutathione-S-transferase (GST) fusion protein vector and incorporated a cleavage site with a long hinge region which allows for the independent folding of CBS and its fusion partner. In addition, our construct has the added benefit of yielding a purified CBS which only contains one extra glycine amino acid residue at the N-terminus. In our two-step purification procedure we are able to obtain a highly pure enzyme in sufficient quantities for crystallography and other physical chemical methods. We have investigated the biochemical and catalytic properties of purified full-length human CBS and of two truncation mutants lacking the C-terminal domain or both the N-terminal heme-binding and the C-terminal regulatory regions. Specifically, we have determined the pH optima of the different CBS forms and their kinetic and spectral properties. The full-length and the C-terminally truncated enzyme had a broad pH 8.5 optimum while the pH optimum of the N- and C- terminally truncated enzyme was sharp and shifted to pH 9. Furthermore, we have shown unequivocally that CBS binds one mole of heme per subunit by determining both the heme and the iron content of the enzyme. The activity of the enzyme was unaffected by the redox status of the heme iron. Finally, we show that CBS is stimulated by S-adenosyl- l-methionine but not its analogs.     

Selection for transport competence of C-terminal polypeptides derived from Escherichia coli hemolysin: the shortest peptide capable of autonomous HIyB/HIyD-dependent secretion comprises the C-terminal 62 amino acids of HlyA

Escherichia coli hemolysin (HlyA) is secreted by a specific export machinery which recognizes a topogenic secretion signal located at the C-terminal end of HlyA. This signal sequence has been variously defined as comprising from 27 to about 300 amino acids at the C-terminus of HlyA. We have used here a combined genetic and immunological approach to select for C-terminal HlyA peptides that are still secretion-component. A deletion library of HlyA mutant proteins was generated in vitro by successive degradation of hy1A from the 5′ end with exonuclease III. Secretion competence was tested by immunoblotting of the supernatant of each clone with an antiserum raised against a C-terminal portion of hemolysin. It was found that the hemolysin secretion system has no apparent size limitation for HlyA proteins over a range from 1024 to 62 amino acids. The smallest autonomously secretable peptide isolated in this selection procedure consists of the C-terminal 62 amino acids of HlyA. This sequence is shared by all secretion-competent, truncated HlyA proteins, which suggests that secretion of the E.coli hemolysin is strictly post-translational. The capacity of the hemolysin secretion machinery was found to be unsaturated by the steady-state level of its natural HlyA substrate and large amounts of truncated HlyA derivatives could still be secreted in addition to full-length HlyA.     

Structure and activity of ClpB from Escherichia coli. Role of the amino-and -carboxyl-terminal domains

ClpB is a member of a protein-disaggregating multi-chaperone system in Escherichia coli. The mechanism of protein-folding reactions mediated by ClpB is currently unknown, and the functional role of different sequence regions in ClpB is under discussion. We have expressed and purified the full-length ClpB and three truncated variants with the N-terminal, C-terminal, and a double N- and C-terminal deletion. We studied the protein concentration-dependent and ATP-induced oligomerization of ClpB, casein-induced activation of ClpB ATPase, and ClpB-assisted reactivation of denatured firefly luciferase. We found that both the N- and C-terminal truncation of ClpB strongly inhibited its chaperone activity. The reasons for such inhibition were different, however, for the N- and C-terminal truncation. Deletion of the C-terminal domain inhibited the self-association of ClpB, which led to decreased affinity for ATP and to decreased ATPase and chaperone activity of the C-terminally truncated variants. In contrast, deletion of the N-terminal domain did not inhibit the self-association of ClpB and its basal ATPase activity but decreased the ability of casein to activate ClpB ATPase. These results indicate that the N-terminal region of ClpB may contain a functionally significant protein-binding site, whereas the main role of the C-terminal region is to support oligomerization of ClpB.     

Directed Evolution of the DnaK Chaperone: Mutations in the Lid Domain Result in Enhanced Chaperone Activity

We improved the DnaK molecular chaperone system for increased folding efficiency towards two target proteins, by using a multi-parameter screening procedure. First, we used a folding-deficient C-terminal truncated chloramphenicol acetyl transferase (CAT_Cd9) to obtain tunable selective pressure for enhanced DnaK chaperon function in vivo. Second, we screened selected clones in vitro for CAT_Cd9 activity after growth under selective pressure. We then analyzed how these variants performed as compared to wild type DnaK towards folding assistance of a second target protein; namely, chemically denatured firefly luciferase. A total of 11 single point DnaK mutants and 1 truncated variant were identified using CAT_Cd9 as the protein target, while 4 of the 12 selected variants showed improved luciferase refolding in vitro. This shows that improving the DnaK chaperone by using a certain target substrate protein, does not necessarily result in a loss or reduction in its ability to assist other proteins. Of the 12 identified mutations, half were clustered in the nucleotide binding domain, and half in the lid domain (LD) of DnaK. The truncated variant is characterized by a 35-residue C-terminal truncation (Cd35) and exhibited the highest improvement for luciferase refolding. Cd35 showed a 7-fold increase in initial refolding rate for denatured luciferase and resulted in a 5-fold increase in maximal luminescence as compared to wild type DnaK. Given that the best in vitro performing mutants contained LD substitutions, and that the LD is not involved in ATP binding, ATP hydrolysis or client protein association, but is involved in allosteric regulation of the chaperone cycle, we propose that improved DnaK variants result in changes to allosteric domain communication, ultimately retuning the ATP-dependent chaperone cycle.     

Functional characterization of artemin, a ferritin homolog synthesized in Artemia embryos during encystment and diapause

Oviparously developing embryos of the crustacean Artemia franciscana encyst and enter diapause, exhibiting a level of stress tolerance seldom seen in metazoans. The extraordinary stress resistance of encysted Artemia embryos is thought to depend in part on the regulated synthesis of artemin, a ferritin superfamily member. The objective of this study was to better understand artemin function, and to this end the protein was synthesized in Escherichia coli and purified to apparent homogeneity. Purified artemin consisted of oligomers approximately 700 kDa in molecular mass that dissociated into monomers and a small number of dimers upon SDS/PAGE. Artemin inhibited heat-induced aggregation of citrate synthase in vitro, an activity characteristic of molecular chaperones and shown here to be shared by apoferritin and ferritin. This is the first report that apoferritin/ferritin may protect cells from stress other than by iron sequestration. Stably transfected mammalian cells synthesizing artemin were more resistant to heat and H(2)O(2) than were cells transfected with vector only, actions also shared by molecular chaperones such as the small heat shock proteins. The data indicate that artemin is a structurally modified ferritin arising either from a common ancestor gene or by duplication of the ferritin gene. Divergence, including acquisition of a C-terminal peptide extension and ferroxidase center modification, eliminated iron sequestration, but chaperone activity was retained. Therefore, because artemin accumulates abundantly during development, it has the potential to protect embryos from stress during encystment and diapause without adversely affecting iron metabolism.     

The structural stability and chaperone activity of artemin,a ferritin homologue from diapause-destined <Emphasis Type="Italic">Artemia</Emphasis> embryos,depend on different cysteine residues

Diapause-destined embryos of the crustacean, Artemia franciscana, accumulate large amounts of an oligomeric, heat-stable, molecular chaperone termed artemin, a cysteine-enriched ferritin homologue. In this study, cysteines 22, 61, 166, and 172 of artemin were substituted with alanines, respectively yielding ArtC22A, ArtC61A, ArtC166A, and ArtC172A. Wild-type and modified artemins were synthesized in transformed bacteria and purified. As measured by heat-induced denaturation of citrate synthase in vitro, each substitution reduced chaperone activity, with ArtC172A the least active. Protein modeling indicated that C172 is close to a region of surface hydrophobicity, also present in ferritin, suggesting that this site contributes to chaperone activity. Only slight differences in oligomer molecular mass were apparent between artemin variants, but ArtC22A and ArtC61A displayed significantly reduced thermostability, perhaps due to the disruption of an inter-subunit disulphide bridge. In contrast, ArtC172A was thermostable, reflecting the location of C172 on the oligomer surface and that it contributes minimally to artemin stabilization. To our knowledge, this is the initial study of structure/function relationships within a ferritin homologue of importance in diapause and the first to indicate that a defined region of hydrophobicity contributes to artemin and ferritin chaperoning.     

Structural and functional studies of FkpA from Escherichia coli, a cis/trans peptidyl-prolyl isomerase with chaperone activity

The protein FkpA from the periplasm of Escherichia coli exhibits both cis/trans peptidyl-prolyl isomerase (PPIase) and chaperone activities. The crystal structure of the protein has been determined in three different forms: as the full-length native molecule, as a truncated form lacking the last 21 residues, and as the same truncated form in complex with the immunosuppressant ligand, FK506. FkpA is a dimeric molecule in which the 245-residue subunit is divided into two domains. The N-terminal domain includes three helices that are interlaced with those of the other subunit to provide all inter-subunit contacts maintaining the dimeric species. The C-terminal domain, which belongs to the FK506-binding protein (FKBP) family, binds the FK506 ligand. The overall form of the dimer is V-shaped, and the different crystal structures reveal a flexibility in the relative orientation of the two C-terminal domains located at the extremities of the V. The deletion mutant FkpNL, comprising the N-terminal domain only, exists in solution as a mixture of monomeric and dimeric species, and exhibits chaperone activity. By contrast, a deletion mutant comprising the C-terminal domain only is monomeric, and although it shows PPIase activity, it is devoid of chaperone function. These results suggest that the chaperone and catalytic activities reside in the N and C-terminal domains, respectively. Accordingly, the observed mobility of the C-terminal domains of the dimeric molecule could effectively adapt these two independent folding functions of FkpA to polypeptide substrates.     

The peptidyl-prolyl isomerase and chaperone Par27 of Bordetella pertussis as the prototype for a new group of parvulins

Proteins that pass through the periplasm in an unfolded state are highly sensitive to proteolysis and aggregation and, therefore, often require protection by chaperone-like proteins. The periplasm of Gram-negative bacteria is well equipped with ATP-independent chaperones and folding catalysts, including peptidyl-prolyl isomerases (PPIases). The filamentous hemagglutinin of Bordetella pertussis, which is secreted by the two-partner secretion pathway, crosses the periplasm in an unfolded conformation. By affinity chromatography, we identified a new periplasmic PPIase of the parvulin family, Par27, which binds to an unfolded filamentous hemagglutinin fragment. Par27 differs from previously characterized bacterial and eukaryotic parvulins. Its central parvulin-like domain is flanked by atypical N- and C-terminal extensions that are found in a number of putative PPIases present mostly in β proteobacteria. Par27 displays both PPIase and chaperone activities in vitro. In vivo, Par27 might function as a general periplasmic chaperone in B. pertussis.     

Mouse Hsp25, a small shock protein. The role of its C-terminal extension in oligomerization and chaperone action.

Under conditions of cellular stress, small heat shock proteins (sHsps), e.g. Hsp25, stabilize unfolding proteins and prevent their precipitation from solution. 1H NMR spectroscopy has shown that mammalian sHsps possess short, polar and highly flexible C-terminal extensions. A mutant of mouse Hsp25 without this extension has been constructed. CD spectroscopy reveals some differences in secondary and tertiary structure between this mutant and the wild-type protein but analytical ultracentrifugation and electron microscopy show that the proteins have very similar oligomeric masses and quaternary structures. The mutant shows chaperone ability comparable to that of wild-type Hsp25 in a thermal aggregation assay using citrate synthase, but does not stabilize alpha-lactalbumin against precipitation following reduction with dithiothreitol. The accessible hydrophobic surface of the mutant protein is less than that of the wild-type protein and the mutant is also less stable at elevated temperature. 1H NMR spectroscopy reveals that deletion of the C-terminal extension of Hsp25 leads to induction of extra C-terminal flexibility in the molecule. Monitoring complex formation between Hsp25 and dithiothreitol-reduced alpha-lactalbumin by 1H NMR spectroscopy indicates that the C-terminal extension of Hsp25 retains its flexibility during this interaction. Overall, these data suggest that a highly flexible C-terminal extension in mammalian sHsps is required for full chaperone activity.     

Probing the impact of the echinT C-terminal domain on structure and catalysis

Histidine triad nucleotide binding protein (Hint) is considered as the ancestor of the histidine triad protein superfamily and is highly conserved from bacteria to humans. Prokaryote genomes, including a wide array of both Gram-negative bacteria and Gram-positive bacteria, typically encode one Hint gene. The cellular function of Hint and the rationale for its evolutionary conservation in bacteria have remained a mystery. Despite its ubiquity and high sequence similarity to eukaryote Hint1 [Escherichia coli Hint (echinT) is 48% identical with human Hint1], prokaryote Hint has been reported in only a few studies. Here we report the first conformational information on the full-length N-terminal and C-terminal residues of Hint from the E. coli complex with GMP. Structural analysis of the echinT-GMP complex reveals that it crystallizes in the monoclinic space group P2₁ with four homodimers in the asymmetric unit. Analysis of electron density for both the N-terminal residues and the C-terminal residues of the echinT-GMP complex indicates that the loops in some monomers can adopt more than one conformation. The observation of conformational flexibility in terminal loop regions could explain the presence of multiple homodimers in the asymmetric unit of this structure. To explore the impact of the echinT C-terminus on protein structure and catalysis, we conducted a series of catalytic radiolabeling and kinetic experiments on the C-terminal deletion mutants of echinT. In this study, we show that sequential deletion of the C-terminus likely has no effect on homodimerization and a modest effect on the secondary structure of echinT. However, we observed a significant impact on the folding structure, as reflected by a significant lowering of the T_m value. Kinetic analysis reveals that the C-terminal deletion mutants are within an order of magnitude less efficient in catalysis compared to wild type, while the overall kinetic mechanism that proceeds through a fast step, followed by a rate-limiting hydrolysis step, was conserved. Nevertheless, the ability of the C-terminal deletion mutants to hydrolyze lysyl-AMP generated by LysU was greatly impaired. Taken together, our results highlight the emerging role of the C-terminus in governing the catalytic function of Hints.     

Heterologous Expression and Characterization of the Manganese-Oxidizing Protein from Erythrobacter sp. Strain SD21

The manganese (Mn)-oxidizing protein (MopA) from Erythrobacter sp. strain SD21 is part of a unique enzymatic family that is capable of oxidizing soluble Mn(II). This enzyme contains two domains, an animal heme peroxidase domain, which contains the catalytic site, followed by a C-terminal calcium binding domain. Different from the bacterial Mn-oxidizing multicopper oxidase enzymes, little is known about MopA. To gain a better understanding of MopA and its role in Mn(II) oxidation, the 238-kDa full-length protein and a 105-kDa truncated protein containing only the animal heme peroxidase domain were cloned and heterologously expressed in Escherichia coli. Despite having sequence similarity to a peroxidase, hydrogen peroxide did not stimulate activity, nor was activity significantly decreased in the presence of catalase. Both pyrroloquinoline quinone (PQQ) and hemin increased Mn-oxidizing activity, and calcium was required. The K_m for Mn(II) of the full-length protein in cell extract was similar to that of the natively expressed protein, but the K_m value for the truncated protein in cell extract was approximately 6-fold higher than that of the full-length protein, suggesting that the calcium binding domain may aid in binding Mn(II). Characterization of the heterologously expressed MopA has provided additional insight into the mechanism of bacterial Mn(II) oxidation, which will aid in understanding the role of MopA and Mn oxidation in bioremediation and biogeochemical cycling.     

C-terminal truncation of a bovine B12 trafficking chaperone enhances the sensitivity of the glutathione-regulated thermostability

The human B₁₂ trafficking chaperone hCblC is well conserved in mammals and non-mammalian eukaryotes. However, the C-terminal ∼40 amino acids of hCblC vary significantly and are predicted to be deleted by alternative splicing of the encoding gene. In this study, we examined the thermostability of the bovine CblC truncated at the C-terminal variable region (t-bCblC) and its regulation by glutathione. t-bCblC is highly thermolabile (T_m = ∼42℃) similar to the full-length protein (f-bCblC). However, t-bCblC is stabilized to a greater extent than f-bCblC by binding of reduced glutathione (GSH) with increased sensitivity to GSH. In addition, binding of oxidized glutathione (GSSG) destabilizes t-bCblC to a greater extent and with increased sensitivity as compared to f-bCblC. These results indicate that t-bCblC is a more sensitive form to be regulated by glutathione than the full-length form of the protein. [BMB Reports 2013; 46(3): 169-174]