Glutathione conjugates recognize the Rossmann fold of glyceraldehyde-3-phosphate dehydrogenase

D-Glyceraldehyde-3-Phosphate Dehydrogenase Structure and Function

Aside from its well-established role in glycolysis, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has been shown to possess many key functions in cells. These functions are regulated by protein oligomerization , biomolecular interactions, post-translational modifications , and variations in subcellular localization . Several GAPDH functions and regulatory mechanisms overlap with one another and converge around its role in intermediary metabolism. Several structural determinants of the protein dictate its function and regulation. GAPDH is ubiquitously expressed and is found in all domains of life. GAPDH has been implicated in many diseases, including those of pathogenic, cardiovascular, degenerative, diabetic, and tumorigenic origins. Understanding the mechanisms by which GAPDH can switch between its functions and how these functions are regulated can provide insights into ways the protein can be modulated for therapeutic outcomes.

Impaired GAPDH-induced mitophagy contributes to the pathology of Huntington's disease

Mitochondrial dysfunction is implicated in multiple neurodegenerative diseases. In order to maintain a healthy population of functional mitochondria in cells, defective mitochondria must be properly eliminated by lysosomal machinery in a process referred to as mitophagy. Here, we uncover a new molecular mechanism underlying mitophagy driven by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) under the pathological condition of Huntington’s disease (HD) caused by expansion of polyglutamine repeats. Expression of expanded polyglutamine tracts catalytically inactivates GAPDH (iGAPDH), which triggers its selective association with damaged mitochondria in several cell culture models of HD. Through this mechanism, iGAPDH serves as a signaling molecule to induce direct engulfment of damaged mitochondria into lysosomes (micro-mitophagy). However, abnormal interaction of mitochondrial GAPDH with long polyglutamine tracts stalled GAPDH-mediated mitophagy, leading to accumulation of damaged mitochondria, and increased cell death. We further demonstrated that overexpression of inactive GAPDH rescues this blunted process and enhances mitochondrial function and cell survival, indicating a role for GAPDH-driven mitophagy in the pathology of HD.

Structural analysis of glyceraldehyde-3-phosphate dehydrogenase functional diversity

Multifunctional proteins provide a new mechanism to expand exponentially cell information and capability beyond that indicated by conventional gene analyses. As such, examination of their structure-function relationships provides a means to define the mechanisms through which cells accomplish critical yet disparate activities required for cell viability and survival. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) may be considered the quintessential multidimensional protein which exhibits a variety of functions unrelated to its classical role in energy production. This review discusses new insights into the structure-function mechanisms through which defined GAPDH amino acid domains are utilized for its diverse activities, the importance of its post-translational modification, and, intriguingly, the logic inherent in the presence or the absence of specific signaling domains.

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and Alzheimer's disease

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a ubiquitous enzyme that catalyzes the sixth step of glycolysis and thus, serves to break down glucose for energy production. Beyond the traditional aerobic metabolism of glucose, recent studies have highlighted additional roles played by GAPDH in non-metabolic processes, such as control of gene expression and redox post-translational modifications. Neuroproteomics have revealed high affinity interactions between GAPDH and Alzheimer's disease-associated proteins, including the β-amyloid, β-amyloid precursor protein and tau. This neuronal protein interaction may lead to impairment of the GAPDH glycolytic function in Alzheimer's disease and may be a forerunner of its participation in apoptosis. The present review examines the crucial implication of GAPDH in neurodegenerative processes and clarifies its role in apoptotic cell death.

Glyceraldehyde-3-phosphate dehydrogenase regulates cyclooxygenase-2 expression by targeting mRNA stability

Cyclooxygenase (COX)-2 is an inducible inflammatory protein whose expression is partially regulated at the post-transcriptional level. We investigated whether glyceraldehyde-3-phosphate dehydrogenase (GAPDH) binds to the AU-rich element (ARE) of COX-2 mRNA for its degradation. Knockdown of GAPDH in hepa1c1c7 cells significantly enhanced COX-2 expressions. Recombinant GAPDH bound to the COX-2 ARE within the first 60 nucleotides of the 3'-UTR via the NAD(+) binding domain. Interestingly, a C151S GAPDH mutant retained binding activity. Confocal microscopy observation revealed that LPS exposure reduced the localization of GAPDH in nuclei. Our results indicate that GAPDH negatively regulates COX-2 by binding to its ARE.

On the functional diversity of glyceraldehyde-3-phosphate dehydrogenase: biochemical mechanisms and regulatory control

BACKGROUND

New studies provide evidence that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is not simply a classical glycolytic protein of little interest. Instead, it is a multifunctional protein with significant activity in a number of fundamental cell pathways. GAPDH is a highly conserved gene and protein, with a single mRNA transcribed from a unique gene. Control mechanisms must exist which regulate its functional diversity.

SCOPE OF REVIEW

This review focuses on new, timely studies defining not only its diverse activities but also those which define the regulatory mechanisms through which those functions may be controlled. The reader is referred to the author's prior review for the consideration of past reports which first indicated GAPDH multiple activities (Sirover, Biochim. Biophys. Acta 1432 (1999) 159-184.)

CONCLUSIONS

These investigations demonstrate fundamental roles of GAPDH in vivo, dynamic changes in its subcellular localization, and the importance of posttranslational modifications as well as protein:protein interactions as regulatory control mechanisms.

GENERAL SIGNIFICANCE

GAPDH is the prototype "moonlighting" protein which exhibits activities distinct from their classically identified functions. Their participation in diverse cell pathways is essential. Regulatory mechanisms exist which control those diverse activities as well as changes in their subcellular localization as a consequence of those new functions.

Oxidatively modified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and Alzheimer's disease: many pathways to neurodegeneration

Recently, the oxidoreductase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), has become a subject of interest as more and more studies reveal a surfeit of diverse GAPDH functions, extending beyond traditional aerobic metabolism of glucose. As a result of multiple isoforms and cellular locales, GAPDH is able to come in contact with a variety of small molecules, proteins, membranes, etc., that play important roles in normal and pathologic cell function. Specifically, GAPDH has been shown to interact with neurodegenerative disease-associated proteins, including the amyloid-beta protein precursor (AbetaPP). Studies from our laboratory have shown significant inhibition of GAPDH dehydrogenase activity in Alzheimer's disease (AD) brain due to oxidative modification. Although oxidative stress and damage is a common phenomenon in the AD brain, it would seem that inhibition of glycolytic enzyme activity is merely one avenue in which AD pathology affects neuronal cell development and survival, as oxidative modification can also impart a toxic gain-of-function to many proteins, including GAPDH. In this review, we examine the many functions of GAPDH with respect to AD brain; in particular, the apparent role(s) of GAPDH in AD-related apoptotic cell death is emphasized.

Knock down of cytosolic phospholipase A2: an antisense oligonucleotide having a nuclear localization binds a C-terminal motif of glyceraldehyde-3-phosphate dehydrogenase

We have previously shown that an antisense, effective in the knock down of cytosolic phospholipase A2 (cPLA2), localizes mainly in the nucleus of human endothelial cells and monocytes and that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is involved in its nuclear localization. In this study, we clarify how GAPDH participates in the nuclear localization of this antisense oligodeoxynucleotide (ODN) directed against cPLA2 mRNA. A central TAAAT motif providing specificity and high affinity binding was assumed to interact with the enzyme Rossmann fold region on the basis of competition to this site by NAD+. To asses whether the TAAAT motif interacts directly with the enzyme Rossmann fold region, we evaluated the binding to GAPDH of different oligonucleotides and the effect of competitors such as NAD+, NADH, mononucleotides, DNA, polyribonucleic acids and polyanions. We found that the dissociation constant for TAAAT containing oligonucleotides was three--to fivefold higher with respect to oligo not containing this motif. By covalently linking 32P-labeled cPLA2p(N)16 to GAPDH and after executing hydrolysis with hydroxylamine, the labeling was exclusively found in the C-terminal domain (aa 286-334). These results indicate that the antisense oligonucleotide interacts with a site not having a defined function but which can be negatively allosterically regulated when NAD+ or polynucleotides are bound to Rossmann fold.

NADPH dependent activation of microsomal glutathione transferase 1

Microsomal glutathione transferase 1 (MGST1) can become activated up to 30-fold by several mechanisms in vitro (e.g. covalent modification by reactive electrophiles such as N-ethylmaleimide (NEM)). Activation has also been observed in vivo during oxidative stress. It has been noted that an NADPH generating system (g.s.) can activate MGST1 (up to 2-fold) in microsomal incubations, but the mechanism was unclear. We show here that NADPH g.s treatment impaired N-ethylmaleimide activation, indicating a shared target (identified as cysteine-49 in the latter case). Furthermore, NADPH activation was prevented by sulfhydryl compounds (glutathione and dithiothreitol). A well established candidate for activation would be oxidative stress, however we could exclude that oxidation mediated by cytochrome P450 2E1 (or flavine monooxygenase) was responsible for activation under a defined set of experimental conditions since superoxide or hydrogen peroxide alone did not activate the enzyme (in microsomes prepared by our routine procedure). Actually, the ability of MGST1 to become activated by hydrogen peroxide is critically dependent on the microsome preparation method (which influences hydrogen peroxide decomposition rate as shown here), explaining variable results in the literature. NADPH g.s. dependent activation of MGST1 could instead be explained, at least partly, by a direct effect observed also with purified enzyme (up to 1.4-fold activation). This activation was inhibited by sulfhydryl compounds and thus displays the same characteristics as that of the microsomal system. Whereas NADPH, and also ATP, activated purified MGST1, several nucleotide analogues did not, demonstrating specificity. It is thus an intriguing possibility that MGST1 function could be modulated by ligands (as well as reactive oxygen species) during oxidative stress when sulfhydryls are depleted.

New insights into an old protein: the functional diversity of mammalian glyceraldehyde-3-phosphate dehydrogenase

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was considered a classical glycolytic protein examined for its pivotal role in energy production. It was also used as a model protein for analysis of protein structure and enzyme mechanisms. The GAPDH gene was utilized as a prototype for studies of genetic organization, expression and regulation. However, recent evidence demonstrates that mammalian GAPDH displays a number of diverse activities unrelated to its glycolytic function. These include its role in membrane fusion, microtubule bundling, phosphotransferase activity, nuclear RNA export, DNA replication and DNA repair. These new activities may be related to the subcellular localization and oligomeric structure of GAPDH in vivo. Furthermore, other investigations suggest that GAPDH is involved in apoptosis, age-related neurodegenerative disease, prostate cancer and viral pathogenesis. Intriguingly, GAPDH is also a unique target of nitric oxide. This review discusses the functional diversity of GAPDH in relation to its protein structure. The mechanisms through which mammalian cells may utilize GAPDH amino acid sequences to provide these new functions and to determine its intracellular localization are considered. The interrelationship between new GAPDH activities and its role in cell pathologies is addressed.

ATP-Dependent human erythrocyte glutathione-conjugate transporter. I. Purification, photoaffinity labeling, and kinetic characteristics of ATPase activity

Dinitrophenyl S-glutathione (DNP-SG) ATPase is a 38 kDa membrane protein expressed in erythrocytes and other tissues. Although stimulation of ATP hydrolysis catalyzed by DNP-SG ATPase has been demonstrated in the presence of several structurally unrelated amphiphilic ions, structural and functional properties of this protein have not been well-defined. In the present study, we have developed an improved protocol for the purification of DNP-SG ATPase and investigated its kinetic and substrate-binding properties. The purification procedure was based on highly specific elution of the 38 kDa protein from DNP-SG affinity resin in the presence of ATP. The protein could not be eluted using either ADP or adenosine-5'-[beta,gamma-methylene]triphosphate (methylene-ATP), a nonhydrolyzable analogue of ATP. Doxorubicin (DOX), a weakly basic anthracycline chemotherapy agent, was found to be the preferred activator for stimulation of ATP hydrolysis by the enzyme. ATP binding to the enzyme was demonstrated using 8-azido-ATP photoaffinity labeling and binding of trinitrophenyl (TNP)-ATP, a fluorescent analogue of ATP. The photoaffinity labeling of DNP-SG ATPase (38 kDa) was saturable with respect to 8-azido ATP (Kd = 2 microM), indicating that the enzyme was capable of specific and saturable binding to ATP. DNP-SG binding was evident from the purification procedure itself and was also demonstrable by quenching of tryptophan fluorescence. Results of quenching of tryptophan fluorescence as well as radioactive isotope-binding studies indicated that DOX was bound to the purified protein as well.

Mutations in the glutathione-gated KefC K+ efflux system of Escherichia coli that cause constitutive activation

The kefC gene of Escherichia coli encodes a potassium efflux system that is gated by glutathione (GSH) and by GSH adducts. Independently isolated kefC mutations that result in spontaneous activation of the efflux system have been analyzed. Three mutations affect residues located adjacent to the conserved Rossman fold in the carboxyl-terminal domain. Two mutations lie in a sequence predicted to form a cytoplasmically located loop in the membrane domain of KefC. All of the mutants retain normal regulation by the YabF protein and by GSH adducts. A mutation in the Rossman fold, R416S, alters the normal regulation of KefC by GSH. In contrast to the wild-type protein, which is inactive in the presence of GSH, the R416S mutant is only active in the presence of GSH or its analogue, ophthalmic acid. Other mutations in this region or elsewhere in the protein have their spontaneous activity augmented by depletion of the GSH pool. These data identify a specific role for the carboxyl-terminal domain of KefC in regulating KefC activity and are discussed in the light of recent data that suggest that GSH adducts can bind within a Rossman fold.