Interaction of d-lactate dehydrogenase protein 2 (Dld2p) with F-actin: implication for an alternative function of Dld2p

Closing the gap: yeast electron-transferring flavoprotein links the oxidation of d-lactate and d-α-hydroxyglutarate to energy production via the respiratory chain

Electron-transferring flavoproteins (ETFs) have been found in all kingdoms of life, mostly assisting in shuttling electrons to the respiratory chain for ATP production. While the human (h) ETF has been studied in great detail, very little is known about the biochemical properties of the homologous protein in the model organism Saccharomyces cerevisiae (yETF). In view of the absence of client dehydrogenases, for example, the acyl-CoA dehydrogenases involved in the β-oxidation of fatty acids, d-lactate dehydrogenase 2 (Dld2) appeared to be the only relevant enzyme that is serviced by yETF for electron transfer to the mitochondrial electron transport chain. However, this hypothesis was never tested experimentally. Here, we report the biochemical properties of yETF and Dld2 as well as the electron transfer reaction between the two proteins. Our study revealed that Dld2 oxidizes d-α-hydroxyglutarate more efficiently than d-lactate exhibiting kcatapp /KMapp values of 1200 ± 300 m-1 ·s-1 and 11 ± 2 m-1 ·s-1 , respectively. As expected, substrate-reduced Dld2 very slowly reacted with oxygen or the artificial electron acceptor 2,6-dichlorophenol indophenol. However, photoreduced Dld2 was rapidly reoxidized by oxygen, suggesting that the reaction products, that is, α-ketoglutarate and pyruvate, 'lock' the reduced enzyme in an unreactive state. Interestingly, however, we could demonstrate that substrate-reduced Dld2 rapidly transfers electrons to yETF. Therefore, we conclude that the formation of a product-reduced Dld2 complex suppresses electron transfer to dioxygen but favors the rapid reduction in yETF, thus preventing the loss of electrons and the generation of reactive oxygen species.

Mitochondrial involvement to methylglyoxal detoxification: D-Lactate/Malate antiporter in Saccharomyces cerevisiae

Research during the last years has accumulated a large body of data that suggest that a permanent high flux through the glycolytic pathway may be a source of intracellular toxicity via continuous generation of endogenous reactive dicarbonyl compound methylglyoxal (MG). MG detoxification by the action of the glyoxalase system produces D-lactate. Thus, this article extends our previous work and presents new insights concerning D-lactate fate in aerobically grown yeast cells. Biochemical studies using intact functional mitochondrial preparations derived from Saccharomyces cerevisiae show that D-lactate produced in the extramitochondrial phase can be taken up by mitochondria, metabolised inside the organelles with efflux of newly synthesized malate. Experiments were carried out photometrically and the rate of malate efflux was measured by use of NADP(+) and malic enzyme and it depended on the rate of transport across the mitochondrial membrane. It showed saturation characteristics (K(m) = 20 μM; V(max) = 6 nmol min(-1) mg(-1) of mitochondrial protein) and was inhibited by α-cyanocinnamate, a non-penetrant compound. Our data reveal that reducing equivalents export from mitochondria is due to the occurrence of a putative D-lactate/malate antiporter which differs from both D-lactate/pyruvate antiporter and D-lactate/H(+) symporter as shown by the different V(max) values, pH profile and inhibitor sensitivity. Based on these results we propose that D-lactate translocators and D-lactate dehydrogenases work together for decreasing the production of MG from the cytosol, thus mitochondria could play a pro-survival role in the metabolic stress response as well as for D-lactate-dependent gluconeogenesis.

Investigation of Laser‐Microdissected Inclusion Bodies

We established a novel combinatorial method of laser microdissection system and immunoblot analysis in combination with a novel unfolding chaperone (oligomeric Aip2p/Dld2p) that enables us to examine the molecular profile of proteins in the microscopic regions of interest. As a model system for analyzing inclusion bodies associated with various diseases such as Alzheimer's disease, Parkinson's disease, and prion diseases including bovine spongiform encephalopathy (BSE), we applied this novel method to examine brain samples of patients with Pick's disease, a type of progressive presenile dementia with intraneuronal lesions denoted as Pick bodies (PBs) whose major structural components are tau proteins. After boiling in Laemmli's sample buffer according to the established immunoblotting procedures, 500-2000 PBs were initially applied onto SDS-PAGE gels; however, only faint signals were obtained. Remarkably, only one Pick body was sufficient to illustrate an immunoblot signal; this indicates that pretreatment with oligomeric Aip2p/Dld2p enhances the immunoblot sensitivity by more than a 100-fold. This unprecedented property of laser microdissection combined with oligomeric Aip2p/Dld2p may have further potential applications. For example, a number of proteomic strategies for such inclusion bodies depend on liquid chromatography-tandem mass spectrometry (LC-MS/MS); however, sample preparation methods typically involve the use of detergents and chaotropic agents that often interfere with chromatographic separation and/or electrospray ionization. However, the use of oligomeric Aip2p/Dld2p would not interfere with the LC-MS/MS procedures. Therefore, it might significantly facilitate nanoscale analysis, which is often hindered by the aggregation property of the target proteins present under various analytical conditions, particularly, when the sample protein is present in minor quantities.

More than a 100-fold increase in immunoblot signals of laser-microdissected inclusion bodies with an excessive aggregation property by oligomeric actin interacting protein 2/d-lactate dehydrogenase protein 2

We established a histobiochemical approach targeting micron-order inclusion bodies possessing extensive aggregation properties in situ by using a nonchemical denaturant (oligomeric actin interacting protein 2/d-lactate dehydrogenase protein 2 [Aip2p/Dld2p]) with the combinatorial method of laser-microdissection and immunoblot analysis. As a model, pick bodies were chosen and laser-microdissected from three different brain regions of two patients with Pick's disease. Initially, 500 to 2000 pick bodies were applied onto SDS-PAGE gels after boiling in Laemmli's sample buffer according to established immunoblotting procedures; however, only faint signals were obtained. Following negative results with chemical denaturants or detergent, including 6 M guanidine hydrochloride, 8 M urea, and 2% SDS, the laser-microdissected pick bodies were pretreated with oligomeric Aip2p/Dld2p, which possesses robust protein unfolding activity under biological conditions. Strikingly, only one pick body was sufficient to illustrate an immunoblot signal, indicating that pretreatment with oligomeric Aip2p/Dld2p enhanced the immunoblot sensitivity by more than 100-fold. Pretreatment with oligomeric Aip2p/Dld2p also allowed us to quantify the total protein content of pick bodies. Thus, use of oligomeric Aip2p/Dld2p significantly contributed toward the acquisition of information pertaining to the molecular profile of proteins possessing an extensive aggregation property, particularly in small amounts.

Oligomeric Aip2p/Dld2p modifies the protein conformation of both properly folded and misfolded substrates in vitro

Oligomeric actin-interacting protein 2 (Aip2p) [Nat. Struct. Biol. 2 (1995) 28]/D-lactate dehydrogenase protein 2 (Dld2p) [Yeast 15 (1999) 1377, Biochem. Biophys. Res. Commun. 295 (2002) 910] exhibits the unique grapple-like structure with an ATP-dependent opening [Biochem. Biophys. Res. Commun. 320 (2004) 1271], which is required for the F-actin conformation modifying activity in vitro and in vivo [Biochem. Biophys. Res. Commun. 319 (2004) 78]. To further investigate the molecular nature of oligomeric Aip2p/Dld2p, the substrate specificity of its binding and protein conformation modifying activity was examined. In the presence of 1mM ATP or AMP-PNP, oligomeric Aip2p/Dld2p bound to all substrates so far examined, and modified the conformation of actin, DNase I, the mature form of invertase, prepro-alpha-factor, pro-alpha-factor, and mitochondrial superoxide dismutase, as determined by the trypsin susceptibility assay. Of note, the activity could modify even the conformation of pathogenic highly aggregated polypeptides, such as recombinant prion protein in beta-sheet form, alpha-synuclein, and amyloid beta (1-42) in the presence of ATP. The in vivo protein conformation modifying activity, however, depends on the growth stage; the most significant substrate modification activity was observed in yeast cells at the log phase, suggesting the presence of a cofactor/s in yeast cells, where F-actin is supposed to be a major target in vivo. These data further support our previous notion that the oligomeric Aip2p/Dld2p may belong to an unusual class of molecular chaperones [Biochem. Biophys. Res. Commun. 320 (2004) 1271], which can target both properly folded and misfolded proteins in an ATP-dependent manner in vitro.

Oligomeric Aip2p/Dld2p forms a novel grapple-like structure and has an ATP-dependent F-actin conformation modifying activity in vitro

In order to investigate the molecular mechanism of the F-actin conformation modifying activity [Biochem. Biophys. Res. Commun. 319 (2004) 78] of actin-interacting protein 2 (Aip2p) [Nat. Struct. Biol. 2 (1995) 28]/D-lactate dehydrogenase protein 2 (Dld2p) [Yeast 15 (1999) 1377; Biochem. Biophys. Res. Commun. 295 (2002) 910], the ultrastructure and the regulatory mechanism of the activity were further examined. Interestingly, a novel oligomeric grapple-like structure of 10-12 subunits with an ATP-dependent opening was observed. ATP regulates the opening and closing of the "gate" that forms the opening within oligomeric Aip2p/Dld2p, where binding to the substrate occurs while in the open form. In the presence of ATP (open state of oligomeric Aip2p/Dld2p), oligomeric Aip2p/Dld2p bound the F-actin fiber within the opening, whereas in the absence of ATP (closed state of oligomeric Aip2p/Dld2p), no binding was observed. Simultaneously, the oligomeric Aip2p/Dld2p increased the trypsin susceptibility of F-actin in an ATP-dependent manner. Use of the non-hydrolyzable ATP analogue AMP-PNP yielded similar results to those observed with ATP, suggesting that ATP binding rather than ATP hydrolysis is required for the protein conformation modifying reaction of oligomeric Aip2p/Dld2p. Endogenous Aip2p/Dld2p purified from Saccharomyces cerevisiae also exhibited such protein conformation modifying activity, but monomeric Aip2p/Dld2p with a C-terminal coiled-coil region-truncation failed to exhibit the activity. These data suggest that the oligomerization of Aip2p/Dld2p, which exhibits the unique grapple-like structure with an ATP-dependent opening, is required for the F-actin conformation modifying activity.