Molecular cloning and in vivo expression of a precursor to rat mitochondrial aspartate aminotransferase

Strangers in strange lands: mitochondrial proteins found at extra-mitochondrial locations

The mitochondrial proteome is estimated to contain ∼1100 proteins, the vast majority of which are nuclear-encoded, with only 13 proteins encoded by the mitochondrial genome. The import of these nuclear-encoded proteins into mitochondria was widely believed to be unidirectional, but recent discoveries have revealed that many these ‘mitochondrial’ proteins are exported, and have extra-mitochondrial activities divergent from their mitochondrial function. Surprisingly, three of the exported proteins discovered thus far are mitochondrially encoded and have significantly different extra-mitochondrial roles than those performed within the mitochondrion. In this review, we will detail the wide variety of proteins once thought to only reside within mitochondria, but now known to ‘emigrate’ from mitochondria in order to attain ‘dual citizenship’, present both within mitochondria and elsewhere.

FAT/CD36 expression is not ablated in spontaneously hypertensive rats

There is doubt whether spontaneously hypertensive rats (SHR; North American strain) are null for fatty acid translocase (FAT/CD36). Therefore, we examined whether FAT/CD36 is expressed in heart, muscle, liver and adipose tissue in SHR. Insulin resistance was present in SHR skeletal muscle. We confirmed that SHR expressed aberrant FAT mRNAs in key metabolic tissues; namely, the major 2.9 kb transcript was not expressed, but 3.8 and 5.4 kb transcripts were present. Despite this, FAT/CD36 protein was expressed in all tissues, although there were tissue-specific reductions in FAT/CD36 protein expression and plasmalemmal content, ranging from 26-85%. Fatty acid transport was reduced in adipose tissue (-50%) and was increased in liver (+47%). Normal rates of fatty acid transport occurred in heart and muscle, possibly due to compensatory upregulation of plasmalemmal fatty acid binding protein (FABPpm) in red (+123%) and white muscle (+110%). In conclusion, SHRs (North American strain) are not a natural FAT/CD36 null model, the North American strain of SHR express FAT/CD36, albeit at reduced levels.

Quantitative proteomics identifies a change in glial glutamate metabolism at the time of female puberty

Mammalian puberty requires activation of luteinizing hormone-releasing hormone (LHRH) neurons. In turn, these neurons are controlled by transsynaptic and glia-to-neuron communication pathways, which employ diverse cellular proteins for proper function. We have now used a high throughput relative quantitative proteomics technique to identify such proteins. We selected the method of two-dimensional liquid chromatography tandem mass spectrometry (2DLC-MS/MS) and cleavable isotope-coded affinity tags (cICAT), to both identify and quantify individual proteins within a complex protein mixture. The proteins used derived from the hypothalamus of juvenile (25-day-old) and peripubertal (first proestrus, LP) female rats, and their identity was established by analyzing their mass spectra via database searching. Five proteins involved in glutamate metabolism were detected and two of them appeared to be differentially expressed. They were selected for further analysis, because of their importance in controlling glutamate synthesis and degradation, and their preferential expression in astroglial cells. One, glutamate dehydrogenase (GDH) catalyzes glutamate synthesis; its hypothalamic content detected by 2DLC-MS/MS increases at first proestrus. The other, glutamine synthetase (GS), catalyzes the metabolism of glutamate to glutamine; its content decreases in proestrus. Western blot analysis verified these results. Because these changes suggested an increased glutamate production at puberty, we measured glutamate release from hypothalamic fragments from juvenile 29-day old rats, and from rats treated with PMSG to induce a premature proestrus surge of luteinizing hormone (LH). To determine the net output of glutamate in the absence of re-uptake we used the excitatory amino acid transporter (EAAT) inhibitor l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC). PDC elicited significantly more glutamate- and LHRH-release from the proestrus hypothalamus. Thus, an increase excitatory drive to the LHRH neuronal network provided by glutamatergic inputs of glial origin, is an event contributing to the pubertal activation of LHRH secretion.

Tissue-Specific and Fatty Acid Transporter-Specific Changes in Heart and Soleus Muscle Over a 1-yr Period

Rates of fatty acid oxidation increase rapidly in both rat heart and skeletal muscle in the early postnatal period. Therefore, we examined in heart and soleus muscle, (a) whether there were rapid changes in fatty acid transporter (FAT/CD36, FABPpm) mRNA and protein expression early in life (days 10 -36) and thereafter (days 84, 160, 365), and (b) whether the rates of fatty acid transport and the plasmalemmal content of FAT/CD36 and FABPpm were altered. Protein expression was altered rapidly from day 10-36 in both heart (FAT/CD36 only, +21%, P < 0.05)) and soleus muscle (FAT/CD36 + 100%, P < 0.05; FABPpm -20%, P < 0.05), with no further changes thereafter (P < 0.05). Rates of fatty acid transport (day 10 vs day 160) were increased in heart (+33%, P < 0.05) and muscle (+85%, P < 0.05), and were associated with concomitant increases in plasmalemmal FABPpm (+44%, P < 0.05) and FAT/CD36 (+16%, P < 0.05) in the heart, and only plasmalemmal FAT/CD36 in muscle (+90%, P < 0.05). Therefore, known changes in the rates of fatty acid oxidation in heart and muscle early in life appear to be accompanied by a concurrent upregulation in the rates of fatty acid transport and the expression of FAT/CD36 in heart and muscle, as well as an increase in plasmalemmal FAT/CD36 and FABPpm in the heart, and only plasmalemmal FAT/CD36 in soleus muscle. We speculate that the rapid upregulation of fatty acid transport rates in heart and muscle are needed to support the increased rates of fatty oxidation that have been previously observed in these tissues.

Identification of Hsc70 binding sites in mitochondrial aspartate aminotransferase

Hsc70 binds acid-unfolded mitochondrial aspartate aminotransferase (mAAT), forming either soluble or insoluble complexes depending on the relative concentrations of the proteins. Using partial proteolysis of Hsc70-mAAT complexes in combination with MALDI-TOF mass spectrometry, we have identified several potential Hsc70-binding regions in the mAAT polypeptide. Only one mAAT peptide was found bound to Hsc70 in the insoluble complexes while nine peptides arising from eight sequence regions of mAAT were found associated with Hsc70 in the soluble complexes. Most of these binding sites map to secondary structure elements, particularly alpha-helix, that are partly exposed on the surface of the folded structure. These results suggest that these peptide regions must not only be exposed but still in a flexible extended conformation in the mAAT folding intermediates recognized by Hsc70. Thus, for mAAT the discrimination between native and non-native structures by Hsc70 may rely more on the level of structure of the binding sites than on their degree of exposure to the solvent in the native structure.

Differential effects of contraction and PPAR agonists on the expression of fatty acid transporters in rat skeletal muscle

We have examined over the course of a 1-week period the independent and combined effects of chronically increased muscle contraction and the peroxisome proliferator-activated receptor (PPAR)alpha and PPARgamma activators, Wy 14,643 and rosiglitazone, on the expression and plasmalemmal content of the fatty acid transporters, FAT/CD36 and FABPpm, as well as on the rate of fatty acid transport. In resting muscle, the activation of either PPARalpha or PPARgamma failed to induce the protein expression of FAT/CD36. PPARalpha activation also failed to induce the protein expression of FABPpm. In contrast, PPARgamma activation induced the expression of FABPpm protein (40%; P < 0.05). Chronic muscle contraction increased the protein expression of FAT/CD36 (approximately 50%; P < 0.05), whereas FABPpm was slightly increased (12%; P < 0.05). Neither PPARalpha nor PPARgamma activation altered the contraction-induced expression of FAT/CD36 or FABPpm. Changes in protein expression of FAT/CD36 or FABPpm, induced by either contractions or by administration of rosiglitazone, were largely attributable to increased transcription. The contraction-induced increments in FAT/CD36 were accompanied by parallel increments in plasmalemmal FAT/CD36 and in rates of fatty acid transport (P < 0.05). Up-regulation of FABPpm expression was, however, accompanied by a reduction in plasmalemmal FABPpm, which did not affect the rates of long chain fatty acid (LCFA) transport. These studies have shown that in skeletal muscle (i) neither PPARalpha nor PPARgamma activation alters FAT/CD36 expression, (ii) PPARgamma activation selectively up-regulates FABPpm expression and (iii) contraction-induced up-regulation of LCFA transport does not appear to occur via activation of either PPARalpha or PPARgamma.

Overexpression of membrane-associated fatty acid binding protein (FABPpm) in vivo increases fatty acid sarcolemmal transport and metabolism

Fatty acid translocase (FAT/CD36) is a key fatty acid transporter in skeletal muscle. However, the effects on fatty acid transport by another putative fatty acid transporter, plasma membrane-associated fatty acid binding protein (FABPpm), have not been determined in mammalian tissue. We examined the functional effects of overexpressing FABPpm on the rates of 1) palmitate transport across the sarcolemma and 2) palmitate metabolism in skeletal muscle. One muscle (soleus) was transfected with pTracer containing FABPpm cDNA. The contralateral muscle served as control. After injecting the FABPpm cDNA, muscles were electroporated. FABPpm overexpression was directly related to the quantity of DNA administered. Electrotransfection (200 μg/muscle) rapidly induced FABPpm protein overexpression ( day 1, +92%, P < 0.05), which was further increased during the next few days ( days 3–7; range +142% to +160%, P < 0.05). Sarcolemmal FABPpm was comparably increased ( day 7, +173%, P < 0.05). Neither FAT/CD36 expression nor sarcolemmal FAT/CD36 content was altered. FABPpm overexpression increased the rates of palmitate transport (+79%, P < 0.05). Rates of palmitate incorporation into phospholipids were also increased +36%, as were the rates of palmitate oxidation (+20%). Rates of palmitate incorporation into triacylglycerol depots were not altered. These studies demonstrate that in mammalian tissue FABPpm overexpression increased the rates of palmitate transport across the sarcolemma, an effect that is independent of any changes in FAT/CD36. However, since the overexpression of plasmalemmal FABPpm (+173%) exceeded the effects on the rates of palmitate transport and metabolism, it appears that the overexpression of FABPpm alone is not sufficient to induce completely parallel increments in palmitate transport and metabolism. This suggests that other mechanisms are required to realize the full potential offered by FABPpm overexpression.

The Nature of the Rate-limiting Steps in the Refolding of the Cofactor-dependent Protein Aspartate Aminotransferase

The refolding of mitochondrial aspartate aminotransferase (mAAT; EC 2.6.1.1) has been studied following unfolding in 6 m guanidine hydrochloride for different periods of time. Whereas reactivation of equilibrium-unfolded mAAT is sigmoidal, reactivation of the short term unfolded protein displays a double exponential behavior consistent with the presence of fast and slow refolding species. The amplitude of the fast phase decreases with increasing unfolding times (k approximately 0.75 min(-1) at 20 degrees C) and becomes undetectable at equilibrium unfolding. According to hydrogen exchange and stopped-flow intrinsic fluorescence data, unfolding of mAAT appears to be complete in less than 10 s, but hydrolysis of the Schiff base linking the coenzyme pyridoxal 5'-phosphate (PLP) to the polypeptide is much slower (k approximately 0.08 min(-1)). This implies the existence in short term unfolded samples of unfolded species with PLP still attached. However, since the disappearance of the fast refolding phase is about 10-fold faster than the release of PLP, the fast refolding phase does not correspond to folding of the coenzyme-containing molecules. The fast refolding phase disappears more rapidly in the pyridoxamine and apoenzyme forms of mAAT, both of which lack covalently attached cofactor. Thus, bound PLP increases the kinetic stability of the fast refolding unfolding intermediates. Conversion between fast and slow folding forms also takes place in an early folding intermediate. The presence of cyclophilin has no effect on the reactivation of either equilibrium or short term unfolded mAAT. These results suggest that proline isomerization may not be the only factor determining the slow refolding of this cofactor-dependent protein.

Changes in fatty acid transport and transporters are related to the severity of insulin deficiency

We have examined the effects of streptozotocin (STZ)-induced diabetes (moderate and severe) on fatty acid transport and fatty acid transporter (FAT/CD36) and plasma membrane-bound fatty acid binding protein (FABPpm) expression, at the mRNA and protein level, as well as their plasmalemmal localization. These studies have shown that, with STZ-induced diabetes, 1) fatty acid transport across the plasma membrane is increased in heart, skeletal muscle, and adipose tissue and is reduced in liver; 2) changes in fatty acid transport are generally not associated with changes in fatty acid transporter mRNAs, except in the heart; 3) increases in fatty acid transport in heart and skeletal muscle occurred with concomitant increases in plasma membrane FAT/CD36, whereas in contrast, the increase and decrease in fatty acid transport in adipose tissue and liver, respectively, were accompanied by concomitant increments and reductions in plasma membrane FABPpm; and finally, 4) the increases in plasma membrane transporters (FAT/CD36 in heart and skeletal muscle; FABPpm in adipose tissue) were attributable to their increased expression, whereas in liver, the reduced plasma membrane FABPpm appeared to be due to its relocation within the cell in the face of slightly increased expression. Taken together, STZ-induced changes in fatty acid uptake demonstrate a complex and tissue-specific pattern, involving different fatty acid transporters in different tissues, in combination with different underlying mechanisms to alter their surface abundance.

Increased rates of fatty acid uptake and plasmalemmal fatty acid transporters in obese Zucker rats

Giant vesicles were used to study the rates of uptake of long-chain fatty acids by heart, skeletal muscle, and adipose tissue of obese and lean Zucker rats. With obesity there was an increase in vesicular fatty acid uptake of 1.8-fold in heart, muscle and adipose tissue. In some tissues only fatty acid translocase (FAT) mRNA (heart, +37%; adipose, +80%) and fatty acid-binding protein (FABPpm) mRNA (heart, +148%; adipose, +196%) were increased. At the protein level FABPpm expression was not changed in any tissues except muscle (+14%), and FAT/CD36 protein content was altered slightly in adipose tissue (+26%). In marked contrast, the plasma membrane FAT/CD36 protein was increased in heart (+60%), muscle (+80%), and adipose tissue (+50%). The plasma membrane FABPpm was altered only in heart (+50%) and adipose tissues (+70%). Thus, in obesity, alterations in fatty acid transport in metabolically important tissues are not associated with changes in fatty acid transporter mRNAs or altered fatty acid transport protein expression but with their increased abundance at the plasma membrane. We speculate that in obesity fatty acid transporters are relocated from an intracellular pool to the plasma membrane in heart, muscle, and adipose tissues.

Metabolic adaptation of the hypertrophied heart: role of the malate/aspartate and alpha-glycerophosphate shuttles

Activation of the malate/aspartate and alpha -glycerophosphate shuttles (the NADH shuttles) has been identified in glycolytically active newborn myocardium. The goal of this study was to determine if the NADH shuttles and their regulatory genes are activated in hypertrophied myocardium as substrate utilization shifts away from fatty acids and toward glucose and lactate. Capacity of the shuttles was determined in cardiac mitochondria isolated one week, one month, and three months following aortic banding or sham operation. Myocardial steady-state mRNA and protein levels of regulatory enzymes were also measured. Despite a significant increase in left ventricular mass and activation of the atrial natriuretic peptide gene, no change in malate/aspartate nor alpha -glycerophosphate shuttle capacity was found at any of the three time points studied. Reactivation of the genes encoding the regulatory inner mitochondrial membrane proteins was not found in the hypertrophied myocardium, though these genes were down regulated one week following aortic-banding. These results suggest that sufficient malate/aspartate and alpha -glycerophosphate shuttle capacity exists in cardiac mitochondria to accommodate increased shuttle flux as hypertrophied myocardium becomes more glycolytically active.

Interaction of the precursor to mitochondrial aspartate aminotransferase and its presequence peptide with model membranes

The possible contribution of the mature portion of a mitochondrial precursor protein to its interaction with membrane lipids is unclear. To address this issue, we examined the interaction of the precursor to mitochondrial aspartate aminotransferase (pmAAT) and of a synthetic peptide corresponding to the 29-residue presequence peptide (mAAT-pp) with anionic phospholipid vesicles. The affinity of mAAT-pp and pmAAT for anionic vesicles is nearly identical. Results obtained by analyzing the effect of mAAT-pp or full-length pmAAT on either the permeability or microviscosity of the phospholipid vesicles are consistent with only a shallow insertion of the presequence peptide in the bilayer. Analysis of the quenching of Trp-17 fluorescence by brominated phospholipids reveals that this presequence residue inserts to a depth of approximately 9 A from the center of the bilayer. Furthermore, in membrane-bound pmAAT or mAAT-pp, both Arg-8 and Arg-28 are accessible to the solvent. These results suggest that the presequence segment lies close to the surface of the membrane and that the mature portion of the precursor protein has little effect on the affinity or mode of binding of the presequence to model membranes. In the presence of vesicles, mAAT-pp adopts considerable alpha-helical structure. Hydrolysis by trypsin after Arg-8 results in the dissociation of the remaining 21-residue C-terminal peptide fragment from the membrane bilayer, suggesting that the N-terminal portion of the presequence is essential for membrane binding. Based on these results, we propose that the presequence peptide may contain dual recognition elements for both the lipid and import receptor components of the mitochondrial membrane.

Gene expression of receptors and enzymes involved in GABAergic and glutamatergic neurotransmission in the CNS of rats behaviourally dependent on ethanol

The steady state levels of the messenger RNA (mRNA) of eight GABA(A) receptor subunits, five glutamate receptor subunits and seven enzymes involved in the synthesis of glutamate and GABA were measured in eight regions of rat brain in a recently developed animal model of 'behavioural dependence' on ethanol. 'Behavioural dependence' including loss of control was induced by offering the rats the choice between ethanol and water over a 9-month period (Group A). This group was compared with a group given the choice between ethanol and water for only 2 months (not yet 'behaviourally dependent', Group B), a group forced to consume ethanol as sole fluid over a 9-month period (also not 'behaviourally dependent', Group C) and ethanol-naive control rats (Group D). All groups were sacrificed 1 month after the ethanol was withdrawn. The mRNA concentrations of all eight GABA receptor subunits, four out of the five subunits of different glutamate receptors and those of seven enzymes involved in GABA and glutamate production were reduced almost exclusively in the parieto-occipital cortex in Groups A and B, but not Group C. These data suggest that the synthesis of glutamate and GABA and the activities of their respective neurons are selectively impaired in the parieto-occipital cortex in the groups having consumed ethanol in a free-choice design, in which its rewarding properties can better take effect than after forced administration. As the parieto-occipital cortex is believed to contain emotional memory structures, it may be hypothesized that the glutamatergic and GABAergic neuronal systems in this area are involved in the development of memory for reward from ethanol. However, they are not specifically associated with 'behavioural dependence'.

Stability and release requirements of the complexes of GroEL with two homologous mammalian aminotransferases

The mitochondrial (mAAT) and cytosolic (cAAT) homologous isozymes of aspartate aminotransferase are two relatively large proteins that in their nonnative states interact very differently with GroEL. MgATP alone can increase the rate of GroEL-assisted reactivation of cAAT, yet the presence of GroES is mandatory for mAAT. Addition of an excess of a denatured substrate accelerates reactivation of cAAT in the presence of GroEL, but has no effect on mAAT. These competition studies suggest that the more stringent substrate mAAT forms a thermodynamically stable complex with GroEL, while rebinding affects the slow reactivation kinetics of cAAT with GroEL alone. However, the competitor appears to accelerate the release of cAAT from GroEL, most likely by displacing bound cAAT from the GroEL cavity. Moreover, cAAT, but not mAAT, shows a time-dependent increase in protease resistance while bound to GroEL at low temperature. These results suggest that folding and release of cAAT from GroEL in the absence of cofactors may occur stepwise with certain interactions being broken and reformed until the protein escapes binding. The distinct behavior of these two isozymes most likely results from differences in the structure of the nonnative states that bind to GroEL.

Fatty acid transport regulatory proteins in the developing rat placenta and in trophoblast cell culture models

The placenta forms a selective barrier that is able to transport nutrients that are of critical use to the fetus. Delivery of essential fatty acids to the fetus is dependent upon transplacental transport and provides the backbone for the biosynthesis of biological membranes, myelin and various signalling molecules. The primary objective of this research was to elucidate the expression patterns of genes that regulate fatty acid transport across the placenta. Several fatty acid transport regulatory genes have been identified in the rat including; cytoplasmic heart fatty acid binding protein (hFABP), plasma membrane fatty acid binding protein (FABPpm), fatty acid translocase (FAT) and fatty acid transport protein (FATP). In this study, we have elucidated temporal and spatial expression patterns for these genes in the rat placenta and in cell culture models of the rat placenta by Northern blot, RT-PCR, Western blot and/or by in situ hybridization analyses. Expression of hFABP was specific to the labyrinth zone, the main barrier and site of transplacental transport in the rat placenta. In addition, the levels of hFABP expression increased with gestational age, suggesting a growing requirement for fatty acid transport with advancing stages of pregnancy. FABPpm, FAT and FATP are expressed in both the junctional and labyrinth zones of the rat placenta. FAT was predominantly localized to the labyrinth zone by in situ hybridization analysis. The placental cell expression patterns of the genes involved in fatty acid transport were supported by our observations of HRP-1 (labyrinth zone) and Rcho-1 (junctional zone) trophoblast cell culture models. Given their cell surface location, we predict that FABPpm, FAT and FATP potentially participate in placental fatty acid uptake. The predominant expression of hFABP and FAT in the labyrinth zone of the chorioallantoic placenta implicates hFABP and FAT in the transplacental movement of fatty acids from maternal to fetal compartments.

Thyroid hormone regulation of the NADH shuttles in liver and cardiac mitochondria

Thyroid hormone can potentially regulate the malate/aspartate and alpha-glycerophosphate shuttle pathways in cardiac mitochondria either directly, by altering gene expression, or indirectly, by increasing myocardial workload. The goal of the current study was to determine the influence of thyroid hormone on the NADH shuttles in cardiac and liver mitochondria. Malate/aspartate and alpha-glycerophosphate shuttle capacities were significantly increased in cardiac mitochondria from adult rats treated for 9 days with T3 compared to saline-treated controls. Liver mitochondria demonstrated a significant increase in alpha-glycerophosphate and no change in malate/aspartate shuttle capacity. T3 increased steady-state mRNA levels and activity of mitochondrial alpha-glycerophosphate dehydrogenase in both myocardium and liver. Quantitative immunoblot studies demonstrated a significant increase in aspartate-glutamate carrier levels in T3-treated myocardium suggesting a regulatory role of the aspartate/glutamate carrier in T3-treated hearts. Thyroid hormone effects on the NADH shuttles are tissue-specific. Changes in the NADH shuttles in the presence of thyroid hormone excess occur both directly at the gene level and indirectly as an adaptive response.

Selective up-regulation of fatty acid uptake by adipocytes characterizes both genetic and diet-induced obesity in rodents

Long chain fatty acid transport is selectively up-regulated in adipocytes of Zucker fatty rats, diverting fatty acids from sites of oxidation toward storage in adipose tissue. To determine whether this is a general feature of obesity, we studied [(3)H]oleate uptake by adipocytes and hepatocytes from 1) homozygous male obese (ob), diabetic (db), fat (fat), and tubby (tub) mice and from 2) male Harlan Sprague-Dawley rats fed for 7 weeks a diet containing 55% of calories from fat. V(max) and K(m) were compared with controls of the appropriate background strain (C57BL/6J or C57BLKS) or diet (13% of calories from fat). V(max) for adipocyte fatty acid uptake was increased 5-6-fold in ob, db, fat, and tub mice versus controls (p < 0.001), whereas no differences were seen in the corresponding hepatocytes. Similar changes occurred in fat-fed rats. Of three membrane fatty acid transporters expressed in adipocytes, plasma membrane fatty acid-binding protein mRNA was increased 9-11-fold in ob and db, which lack a competent leptin/leptin receptor system, but was not increased in fat and tub, i.e. in strains with normal leptin signaling capability; fatty acid translocase mRNA was increased 2.2-6.5-fold in tub, ob, and fat adipocytes, but not in db adipocytes; and only marginal changes in fatty acid transport protein 1 mRNA were found in any of the mutant strains. Adipocyte fatty acid uptake is generally increased in murine obesity models, but up-regulation of individual transporters depends on the specific pathophysiology. Leptin may normally down-regulate expression of plasma membrane fatty acid binding protein.

Divergent Hsc70 binding properties of mitochondrial and cytosolic aspartate aminotransferase. Implications for their segregation to different cellular compartments

Cytosolic Hsc70 discriminates between the homologous mitochondrial and cytosolic isozymes of aspartate aminotransferase, binding exclusively the mitochondrial form. By screening a library of synthetic peptides spanning the sequence of the mitochondrial enzyme, we have identified binding sites in this polypeptide that interact with Hsc70. These potential binding sites are scattered over the entire sequence and map to secondary structure elements, particularly the alpha-helix, that are partly exposed on the surface of the native protein. Several peptides corresponding to analogous positions in the cytosolic enzyme sequence do not bind to Hsc70. Phylogenetic analyses suggest that Hsc70 binding sequences have diverged as a consequence of biochemical specialization ensuring differential interaction of each isozyme with the cellular machinery in charge of protein folding and translocation.

Conformation of aspartate aminotransferase isozymes folding under different conditions probed by limited proteolysis

The partially homologous mitochondrial (mAAT) and cytosolic (cAAT) aspartate aminotransferase have nearly identical three-dimensional structures but differ in their folding rates in cell-free extracts and in their affinity for binding to molecular chaperones. In its native state, each isozyme is protease-resistant. Using limited proteolysis as an index of their conformational states, we have characterized these proteins (a) during the early stages of spontaneous refolding; (b) as species trapped in stable complexes with the chaperonin GroEL; or (c) as newly translated polypeptides in cell-free extracts. Treatment of the refolding proteins with trypsin generates reproducible patterns of large proteolytic fragments that are consistent with the formation of defined folding domains soon after initiating refolding. Binding to GroEL affords considerable protection to both isozymes against proteolysis. The tryptic fragments are similar in size for both isozymes, suggesting a common distribution of compact and flexible regions in their folding intermediates. cAAT synthesized in cell-free extracts becomes protease-resistant almost instantaneously, whereas trypsin digestion of the mAAT translation product produces a pattern of fragments qualitatively akin to that observed with the protein refolding in buffer. Analysis of the large tryptic peptides obtained with the GroEL-bound proteins reveals that the cleavage sites are located in analogous regions of the N-terminal portion of each isozyme. These results suggest that (a) binding to GroEL does not cause unfolding of AAT, at least to an extent detectable by proteolysis; (b) the compact folding domains identified in AAT bound to GroEL (or in mAAT fresh translation product) are already present at the early stages of refolding of the proteins in buffer alone; and (c) the two isozymes seem to bind in a similar fashion to GroEL, with the more compact C-terminal portion completely protected and the more flexible N-terminal first 100 residues still partially accessible to proteolysis.

Opposite behavior of two isozymes when refolding in the presence of non-ionic detergents

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.

Aminotransferase variants as probes for the role of the N-terminal region of a mature protein in mitochondrial precursor import and processing

Of the two homologous isozymes of aspartate aminotransferase that are also nearly identical in their folded structures, only the mitochondrial form (mAAT) is synthesized as a precursor (pmAAT). After its in vitro synthesis in rabbit reticulocyte lysate, it can also be efficiently imported into isolated rat liver mitochondria, where it is processed to its native form by removal of the N-terminal presequence. The homologous cytosolic isoenzyme (cAAT) is not imported into mitochondria, even after fusion of the mitochondrial presequence from pmAAT to its N-terminal end. Substitution of the 30-residue N-terminal peptide of the mature portion of pmAAT with the corresponding sequence from the homologous, import-incompetent cytosolic isozyme (pcmAAT) does not prevent import but reduces substantially its processing in the matrix. A detectable amount of the pcmAAT chimera is found associated with the inner mitochondrial membrane. Single and double substitution mutants of Trp-5 and Trp-6 at the N-terminal end of the mature protein are imported into mitochondria with efficiency similar to that of wild type. However, replacement of Trp-5 with proline, or of both tryptophans with either alanine (W5A/W6A mutant) or valine and alanine (W5V/W6A mutant), allows import but interferes with the correct processing of the imported protein despite the presence of an intact cleavage site for the processing peptidase. Similar cleavage results were obtained using newly synthesized proteins and mitochondrial matrix extracts. These results indicate that translocation and processing for a precursor are independent events and that sequences C-terminal to the cleavage site are indeed important for the correct maturation of pmAAT in the matrix, probably because of their contribution to the conformation and flexibility of the peptide region surrounding the cleavage site required for efficient processing. The same region from the mature component of the protein may play a role in the commitment of the passenger protein to complete its translocation into the matrix.

Insight into the conformation of protein folding intermediate(s) trapped by GroEL

Many aspects of the mechanism by which the GroEL/ES chaperonins mediate protein folding are still unclear, including the amount of structure present in the substrate bound to GroEL. To address this issue we have analyzed the susceptibility to limited proteolysis and to alkylation of cysteine residues of mitochondrial aspartate aminotransferase (mAAT) bound to GroEL. Several regions of the N-terminal portion of GroEL-bound mAAT are highly susceptible to proteolysis, whereas a large core of about 200 residues containing the C-terminal half of the polypeptide chain is protected in the complex. This protection does not extend to the mAAT sulfhydryl groups which in the GroEL-mAAT complex have similar reactivity as in fully unfolded mAAT. These results suggest that the mAAT species bound to GroEL represent folding intermediates with a conformation that is substantially more disorganized than that of the native state. The N-terminal half of the molecule is more flexible and lies exposed at the mouth of the central cavity of GroEL. The more compact C-terminal section of mAAT, which contains residues located at the subunit interface in the native dimer, appears to be hidden in the central cavity of GroEL. Thus, the bulk of the interactions in the GroEL.mAAT complex seems to involve residues from the more compact C-terminal section of the substrate.

Refolding intermediates of acid-unfolded mitochondrial aspartate aminotransferase bind to hsp70

The cytosolic (cAAT) and mitochondrial (mAAT) isozymes of eukaryotic aspartate aminotransferase share a high degree of sequence identity and almost identical three-dimensional structure. The rat liver proteins can be refolded and reassembled into active dimers after unfolding at low pH. However, refolding of the mitochondrial form after unfolding at pH 2.0 is arrested in the presence of hsp70, whereas this chaperone does not affect the refolding of the cytosolic isozyme unfolded under similar conditions. To elucidate the nature of the differential interaction between hsp70 and the two transaminase forms, we have characterized their refolding from their acid-unfolded states. The recovery of activity of the cytosolic enzyme is monophasic and can be adequately described by a single first-order reaction. By contrast, two sequential first-order rate-limiting steps can be detected for the refolding and reactivation of the mitochondrial protein. The overall refolding pathway of mAAT includes a very fast collapse to an intermediate with 80% of the secondary structure of the active dimer. This is followed by a slow isomerization to form assembly-competent monomers that rapidly associate to form an inactive dimer and a final structural rearrangement of the dimer to the native conformation. Analysis of the interaction of hsp70 with intermediates along the folding pathway of mAAT shows that the polypeptide loses its ability to bind to the chaperone after it has proceeded through the first isomerization/fast dimerization steps. Thus it appears that only the first collapsed intermediate states in the folding of mAAT bind hsp70. By contrast a faster refolding of cAAT from this collapsed state could explain, at least in part, the inability of hsp70 to bind this isozyme.

Uptake of long chain free fatty acids is selectively up-regulated in adipocytes of Zucker rats with genetic obesity and non-insulin-dependent diabetes mellitus

To examine whether fatty acid transport is abnormal in obesity, the kinetics of [3H]oleate uptake by hepatocytes, cardiac myocytes, and adipocytes from adult male Wistar (+/+), Zucker lean (fa/+) and fatty (fa/fa), and Zucker diabetic fatty (ZDF) rats were studied. A tissue-specific increase in oleate uptake was found in fa/fa and ZDF adipocytes, in which the Vmax was increased 9-fold (p < 0.005) and 13-fold (p < 0.001), respectively. This increase greatly exceeded the 2-fold increase in the surface area of adipocytes from obese animals, and did not result from trans-stimulation secondary to increased lipolysis. Adipocyte tumor necrosis factor-alpha mRNA levels, assayed by Northern hybridization, increased in the order +/+ < fa/fa < ZDF. Oleate uptake was also studied in adipocytes from 20-24-day-old male +/+, fa/+, and fa/fa weanlings. These animals were not obese, and had equivalent plasma fatty acid and glucose levels. Tumor necrosis factor-alpha mRNA levels in +/+ and fa/fa cells also were similar. Nevertheless, Vmax was increased 2.9-fold (p < 0.005) in fa/fa compared +/+ cells. These studies indicate 1) that regulation of fatty acid uptake is tissue-specific and 2) that up-regulation of adipocyte fatty acid uptake is an early event in Zucker fa/fa rats. These findings are independent of the role of any particular fatty acid transporter. Adipocyte mRNA levels of three putative transporters, mitochondrial aspartate aminotransferase, fatty acid translocase, and fatty acid transporting protein (FATP) were also determined; mitochondrial aspartate aminotransferase and FATP mRNAs correlated strongly with fatty acid uptake.

Structural features of the precursor to mitochondrial aspartate aminotransferase responsible for binding to hsp70

The precursor (pmAspAT) and mature (mAspAT) forms of mitochondrial aspartate aminotransferase interact with hsp70 very early during translation when synthesized in either rabbit reticulocyte lysate or wheat germ extract (Lain, B., Iriarte, A., and Martinez-Carrion. (1994) J. Biol. Chem. 269, 15588-15596). The nature of the structural elements responsible for recognition and binding of this protein to hsp70 has been studied by examining the folding and potential association with the chaperone of several engineered forms of this enzyme. Whereas pmAspAT and mAspAT bind hsp70 very early during translation, the cytosolic form of this enzyme (cAspAT) does not interact with hsp70. A fusion protein consisting of the mitochondrial presequence peptide attached to the amino terminus of cAspAT associates with hsp70 only after the protein has acquired its native-like conformation, apparently through binding to the presequence exposed on the surface of the folded protein. Deletion of the amino-terminal segment of mAspAT or its replacement with the corresponding domain from the cytosolic isozyme eliminates the cotranslational binding of hsp70 to the mitochondrial protein. We conclude that both the presequence and NH2-terminal region of pmAspAT represent recognition signals for binding of hsp70 to the newly synthesized mitochondrial precursor. Results from competition studies with synthetic peptides support this conclusion. The ability of hsp70 to discriminate between these two highly homologous proteins probably involves the recognition of specific sequence elements in the NH2-terminal portion of the mitochondrial protein and may relate to their separate localization in the cell. A slower folding rate and higher affinity for cytosolic chaperones may represent evolutionary adaptations of translocated mitochondrial proteins to ensure their efficient importation into the organelle.

3T3 fibroblasts transfected with a cDNA for mitochondrial aspartate aminotransferase express plasma membrane fatty acid-binding protein and saturable fatty acid uptake

To explore the relationship between mitochondrial aspartate aminotransferase (mAspAT; EC 2.6.1.1) and plasma membrane fatty acid-binding protein (FABPpm) and their role in cellular fatty acid uptake, 3T3 fibroblasts were cotransfected with plasmid pMAAT2, containing a full-length mAspAT cDNA downstream of a Zn(2+)-inducible metallothionein promoter, and pFR400, which conveys methotrexate resistance. Transfectants were selected in methotrexate, cloned, and exposed to increasing methotrexate concentrations to induce gene amplification. Stably transfected clones were characterized by Southern blotting; those with highest copy numbers of pFR400 alone (pFR400) or pFR400 and pMAAT2 (pFR400/pMAAT2) were expanded for further study. [3H]Oleate uptake was measured in medium containing 500 microM bovine serum albumin and 125-1000 microM total oleate (unbound oleate, 18-420 nM) and consisted of saturable and nonsaturable components. pFR400/pMAAT2 cells exhibited no increase in the rate constant for nonsaturable oleate uptake or in the uptake rate of [14C]octanoate under any conditions. By contrast, Vmax (fmol/sec per 50,000 cells) of the saturable oleate uptake component increased 3.5-fold in pFR400/pMAAT2 cells compared to pFR400, with a further 3.2-fold increase in the presence of Zn2+. Zn2+ had no effect in pFR400 controls (P > 0.5). The overall increase in Vmax between pFR400 and pFR400/pMAAT2 in the presence of Zn2+ was 10.4-fold (P < 0.01) and was highly correlated (r = 0.99) with expression of FABPpm in plasma membranes as determined by Western blotting. Neither untransfected 3T3 nor pFR400 cells expressed cell surface FABPpm detectable by immunofluorescence. By contrast, plasma membrane immunofluorescence was detected in pFR400/pMAAT2 cells, especially if cultured in 100 microM Zn2+. The data support the dual hypotheses that mAspAT and FABPpm are identical and mediate saturable long-chain free fatty acid uptake.

Regulation of citrate metabolism by androgen in the LNCaP human prostate carcinoma cell line

Citrate production and accumulation are characteristic physiological functions of the prostate gland that are regulated by testosterone and prolactin. Results reported here show that treatment of LNCaP cells with dihydrotestosterone (DHT) resulted in increased intracellular citrate and increased citrate accumulation in the medium. Moreover, DHT also caused an increase in both mitochondrial aspartate aminotransferase (mAAT) activity and the steady state level of pmAAT (precursor) mRNA. Androgen treatment increased the rate of citrate oxidation by LNCaP cells as it does in rat ventral prostate which suggests that DHT increased aconitase activity in LNCaP cells. The results reported here are consistent with the operation of the glutamate-aspartate-citrate pathway that we described for rat ventral prostate. In addition, these results provide the first evidence that androgen responsive functions associated with citrate metabolism are retained in LNCaP cells. In addition, and more important, these results suggest that the more aggressive PC-3 carcinoma cell line has a higher rate of citrate oxidation than the less aggressive LNCaP cell line. This could have significant implications for our understanding of the relationship between alterations in prostate citrate metabolism and expression of the malignant phenotype.

Androgen modulation of multiple transcription start sites of the mitochondrial aspartate aminotransferase gene in rat prostate

Mitochondrial aspartate aminotransferase (mAAT) is one of two key enzymes in the pathway of citrate production in prostate. Expression of mAAT is modulated by testosterone and prolactin in prostate. We cloned the promoter and 5'-flanking region of the rat mAAT gene and sequenced 2.0 kilobases of the DNA. This fragment contains the 5'-regulatory promoter region that lacks a TATA and a CCAAT box but is G+C rich. The 5'-upstream flanking region contains sequences that have high homology with the consensus glucocorticoid response element/androgen response element (ARE) and a reported ARE sequence that is different from the consensus sequence. Functional transcription studies showed that a 481-base region containing the two ARE sequences was sufficient for androgen-regulated gene expression. There are multiple transcription start sites that are regulated by testosterone in prostate. In liver, on the other hand, castration did not affect transcription from any of the start sites. Therefore, these data provide evidence that transcriptional regulation of the rat pmAAT gene occurs through an ARE located in the 5'-region. In addition, not only is gene expression modulated by testosterone, but the effect of testosterone on transcription is cell specific.

Nucleotide sequence of a cDNA coding for bovine mitochondrial aspartate aminotransferase

Aspartate aminotransferase is a pyridoxal-phosphate dependent enzyme which plays a key role in cell metabolism. We describe the cloning and sequence analysis of the cDNA encoding bovine mitochondrial aspartate aminotransferase and compare the sequence with those of isoenzymes from other mammalian species. An adult bovine heart cDNA library constructed in lambda lambda gt11 was screened using two 32P-end labeled synthetic oligonucleotides. From the screening of the cDNA library two positive clones were isolated. A subclone in pEMBL18, 6B2, generated from the longest recombinant phage was further analyzed. This clone contains an insert of 2500 bp with an Open Reading Frame of 1287 bp that encodes a protein of 430 amino acids. The deduced amino acid sequence confirms previous results obtained by mass spectrometric sequencing. We calculated the percentage of amino acid identity for each protein pair and for each comparison the average number of amino acid substitution per site (Kaa); the lowest Kaa values were obtained from the comparison between the bovine and pig enzymes. This study shows that the rate of evolution of mammalian mitochondrial AspAT is lower and more constant than the equivalent cytosolic enzyme and adds to the growing body of knowledge on the evolution of the aspartate aminotransferase.

Characterization of a soybean cDNA clone encoding the mitochondrial isozyme of aspartate aminotransferase, AAT4

A soybean leaf cDNA clone, pSAT2, was isolated by hybridization to a carrot aspartate aminotransferase (EC 2.6.1.1.; AAT) cDNA clone at low stringency. pSAT2 contained an open reading frame encoding a 47640 Da protein. The protein encoded by pSAT2 showed significant sequence similarity to AAT proteins from both plants and animals. It was most similar to two Panicum mitochondrial AATs, 81.5% and 82.0% identity. Alignment of the pSAT2-encoded protein with other mature AAT enzymes revealed a 25 amino acid N-terminal extension with characteristics of a mitochondrial transit peptide. A plasmid, pEXAT2, was constructed to encode the mature pSAT2 protein lacking the putative mitochondrial transit peptide. Escherichia coli containing the plasmid expressed a functional AAT isozyme which comigrated with the soybean AAT4 isozyme during agarose gel electrophoresis. Equilibrium sucrose gradient sedimentation of soybean extracts demonstrated that AAT4 specifically cofractionated with mitochondria. Antibodies raised against the pEXAT2-encoded AAT protein reacted with AAT4 of soybean and not with other AAT isozymes detected in soybean tissues, providing further evidence that clone pSAT2 encodes the soybean mitochondrial isozyme AAT4.

Genomic structure, expression and evolution of the alfalfa aspartate aminotransferase genes

Genomic clones encoding two isozymes of aspartate aminotransferase (AAT) were isolated from an alfalfa genomic library and their DNA sequences were determined. The AAT1 gene contains 12 exons that encode a cytosolic protein expressed at similar levels in roots, stems and nodules. In nodules, the amount of AAT1 mRNA was similar at all stages of development, and was slightly reduced in nodules incapable of fixing nitrogen. The AAT1 mRNA is polyadenylated at multiple sites differing by more than 250 bp. The AAT2 gene contains 11 exons, with 5 introns located in positions identical to those found in animal AAT genes, and encodes a plastid-localized isozyme. The AAT2 mRNA is polyadenylated at a very limited range of sites. The transit peptide of AAT2 is encoded by the first two and part of the third exon. AAT2 mRNA is much more abundant in nodules than in other organs, and increases dramatically during the course of nodule development. Unlike AAT1, expression of AAT2 is significantly reduced in nodules incapable of fixing nitrogen. Phylogenetic analysis of deduced AAT proteins revealed 4 separate but related groups of AAT proteins; the animal cytosolic AATs, the plant cytosolic AATs, the plant plastid AATs, and the mitochondrial AATs.

Isolation and characterization of a soybean cDNA clone encoding the plastid form of aspartate aminotransferase

Five aspartate aminotransferase (EC 2.6.1.1; AAT) isozymes were identified in soybean seedling extracts and designated AAT1 to AAT5 based on their rate of migration on non-denaturing electrophoretic gels. AAT1 was detected only in extracts of cotyledons from dark-grown seedlings. AAT3 and AAT4 were detected in crude extracts of leaves and in cotyledons of seedlings grown in the light. AAT2 and AAT5 were detected in all tissues examined. A soybean leaf cDNA clone, pSAT17, was identified by hybridization to a carrot AAT cDNA clone at low stringency. pSAT17 had an open reading frame which could encode a 50,581 Da protein. Alignment of the deduced amino acid sequence from the pSAT17 open reading frame with mature AAT protein sequences from rat disclosed a 60 amino acid N-terminal extension in the pSAT17 protein. This extension had characteristics of a plastid transit peptide. A plasmid, pEXAT17, was constructed which encoded the mature protein lacking the putative chloroplast transit polypeptide. Transformed Escherichia coli expressed a functional soybean AAT isozyme, which comigrated with the soybean AAT5 isozyme during agarose gel electrophoresis. Differential sucrose gradient sedimentation of soybean extracts indicated that AAT5 specifically cofractionated with chloroplasts. Antibodies raised against the pEXAT17-encoded AAT protein specifically reacted with the AAT5 isozyme of soybean and not with any of the other isozymes, indicating that the soybean cDNA clone, pSAT17, encodes the chloroplast isozyme, AAT5.

Testosterone regulates mitochondrial aspartate aminotransferase gene expression and mRNA stability in prostate

The effect of testosterone on the precursor mitochondrial aspartate aminotransferase (pmAAT) gene and on pmAAT-mRNA was studied in rat ventral prostate (VP) and pig prostate epithelial cells. Castration significantly decreased the level of nuclear pmAAT transcripts in VP; whereas testosterone treatment of castrated animals restored the level of pmAAT transcripts. Correspondingly, castration resulted in a marked decrease in the transcription rate of the pmAAT gene; whereas testosterone treatment markedly increased the transcription rate. In vitro studies with isolated pig prostate epithelial cells demonstrated that testosterone directly and rapidly induced a transient increase in the transcription rate of the pmAAT gene. The increase in transcription was associated with an increase in the steady-state level of pmAAT-mRNA. Similar in vitro effects were observed with isolated VP epithelial cells. In addition to its stimulatory effect on transcription of the pmAAT gene, testosterone also increased the half-life of pmAAT-mRNA from 2 h in the absence of hormone to 16 h in its presence. Consequently, testosterone appears to stabilize the pmAAT-mRNA. The combination of its immediate effect on stimulating the transcription of the pmAAT gene and its stabilizing effect on pmAAT-mRNA would account for the increase in the steady-state level of pmAAT-mRNA by testosterone. These studies support our proposal that, through these effects, testosterone increases the biosynthesis of mAAT thereby increasing the transamination of aspartate to oxaloacetate and ultimately increasing the synthesis of citrate. This appears to provide at least one of the mechanisms by which testosterone regulates prostate citrate production.

Prolactin stimulates transcription of aspartate aminotransferase in prostate cells

Prolactin (PRL) has been reported to stimulate citrate production and the activity of mitochondrial aspartate aminotransferase (mAAT) and its precursor form pmAAT in prostate epithelial cells. The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) caused the same result as PRL, which suggests that the PRL effect on mAAT activity might be mediated by protein kinase C (PKC) stimulation of pmAAT gene transcription. Both PRL and TPA increased the level of pmAAT mRNA by 2.5- to 3-fold in pig prostate cells. The PKC inhibitor gossypol completely inhibited the PRL and TPA induced increases. In addition, the effects of both PRL and TPA were inhibited by down-regulation of prostate PKC. Nuclear run-off assays indicated that PRL and TPA induction of pmAAT occurred primarily at the transcriptional level. The stimulation of pmAAT transcription by TPA suggests that the pmAAT gene contains a TPA response element. Thus, these results are consistent with our previous observation that PRL directly induces pmAAT and that the mechanism of this PRL effect might involve stimulation of PKC.

Cloning and sequence analysis of cDNA encoding aspartate aminotransferase isozymes from Panicum miliaceum L., a C4 plant

The cytosolic and mitochondrial isozymes of aspartate aminotransferase (AspAT) function in the C4 dicarboxylate cycle of photosynthesis. We constructed a cDNA library from leaf tissues of Panicum miliaceum, an NAD-malic-enzyme-type C4 plant and screened the library for AspAT isozymes. A full-length cDNA clone for cytosolic AspAT was isolated. This clone contains an open reading frame that encodes 409 amino acids. We also isolated two cDNA clones for different precursors of mitochondrial AspAT. Comparing these two sequences in the coding regions, we found 12 amino acid substitutions out of 28 base substitutions. The encoded amino acid sequences predict that mitochondrial AspAT are synthesized as precursor proteins of 428 amino acid residues, which each consist of a mature enzyme of 400 amino acid residues and a 28-amino-acid presequence. This prediction coincides with the observation that the in vitro translation product of the mRNA for mitochondrial AspAT was substantially larger than the mature form. A comparison of the amino acid sequences of the AspAT isozymes from P. miliaceum with the published sequences for the enzymes from various animals and microorganisms reveals that functionally and/or structurally important residues are almost entirely conserved in all AspAT species.

Analysis of the polyadenylation consensus sequence context in the genes of nuclear encoded mitochondrial proteins

A compilation of the pre-mRNA ends of the genes of nuclear encoded mitochondrial proteins resulted in a consensus sequence of the type (T/A)NTTNNNNNTTTNAATAAA. Nucleotide positions +8, +13, +14, +16 and +17 downstream of the AATAAA sequence show also a predominance of nucleotide T. This consensus sequence suggests the importance of the immediate surroundings of the cannonical polyadenylation signal sequence AATAAA on the efficiency of the cleavage and polyadenylation of this specific group of pre-mRNAs.

Concepts of citrate production and secretion by prostate: 2. Hormonal relationships in normal and neoplastic prostate

A unique and major function of prostate secretory epithelial cells is to synthesize, accumulate, and secrete extraordinarily high levels of citrate. This function is regulated by testosterone and by prolactin. Concepts of the mechanisms of hormonal regulation are presented. The relationship of testosterone and prolactin to the origin and homologies of different prostate cell lines is described. The metabolic differentiation of citrate and non-citrate producing prostate secretory epithelial cells is discussed. Concepts of the pathogenesis of prostatic neoplasms are presented based on hormonal, metabolic, and homologous relationships associated with citrate production. Characterization of normal and neoplastic secretory epithelial cells by their citrate function is emphasized. The urgency and necessity for research relating to all aspects of prostate citrate production in normal and pathological prostate are emphasized.

Characterization and sequencing of the active site of 1-aminocyclopropane-1-carboxylate synthase

The pyridoxal phosphate (PLP)-dependent 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (S-adenosyl-L-methionine methylthioadenosine-lyase, EC 4.4.1.14), the key enzyme in ethylene biosynthesis, is inactivated by its substrate S-adenosylmethionine (AdoMet). Apple ACC synthase was purified with an immunoaffinity gel, and its active site was probed with NaB3H4 or Ado[14C]Met. HPLC separation of the trypsin digest yielded a single radioactive peptide. Peptide sequencing of both 3H- and 14C-labeled peptides revealed a common dodecapeptide of Ser-Leu-Ser-Xaa-Asp-Leu-Gly-Leu-Pro-Gly-Phe-Arg, where Xaa was the modified, radioactive residue in each case. Acid hydrolysis of the 3H-labeled enzyme released radioactive N-pyridoxyllysine, indicating that the active-site peptide contained lysine at position 4. Mass spectrometry of the 14C-labeled peptide indicated a protonated molecular ion at m/z 1390.6, from which the mass of Xaa was calculated to be 229, a number that is equivalent to the mass of a lysine residue alkylated by the 2-aminobutyrate portion of AdoMet, as we previously proposed. These results indicate that the same active-site lysine binds the PLP and convalently links to the 2-aminobutyrate portion of AdoMet during inactivation. The active site of tomato ACC synthase was probed in the same manner with Ado[14C]Met. Sequencing of the tomato active-site peptide revealed two highly conserved dodecapeptides; the minor peptide possessed a sequence identical to that of the apple enzyme, whereas the major peptide differed from the minor peptide in that methionine replaced leucine at position 6.

Purification of rat liver mitochondrial aspartate aminotransferase and separation of its isoforms utilizing high-performance liquid chromatography

A rapid method for purification of mitochondrial aspartate aminotransferase from rat liver employing high-performance liquid chromatography is reported. The product is purified 80-fold with a recovery greater than or equal to 50% in a single day. The amino acid composition, N-terminal amino acid sequence, specific activity, and spectral characteristics of the isolated enzyme are similar to those previously reported for this protein. The protein is homogeneous by standard electrophoretic and chromatographic criteria, but can be resolved into at least five isoforms by a carboxymethylated resin column using high-performance liquid chromatography. The principal isoform initially isolated is converted into two additional isoforms with lower specific activity upon storage at 4 degrees C.

Structure of the genes of two homologous intracellularly heterotopic isoenzymes. Cytosolic and mitochondrial aspartate aminotransferase of chicken

The genes of mitochondrial and cytosolic aspartate aminotransferase of chicken were cloned and sequenced. In both genes nine exons encode the mature enzyme. The additional exon for the N-terminal presequence that directs mitochondrial aspartate aminotransferase into the mitochondria is separated by the largest intron from the rest of the gene. A comparison of the two genes of chicken with the aspartate aminotransferase genes of mouse [Tsuzuki, T., Obaru, K., Setoyama, C. & Shimada, K. (1987) J. Mol. Biol. 198, 21-31; Obaru, K., Tsuzuki, T., Setoyama, C. & Shimada, K. (1988) J. Mol. Biol. 200, 13-22] reveals closely similar structures: in the gene of both the mitochondrial and the cytosolic isoenzyme all but one intron positions are conserved in the two species and five introns out of nine are placed at the same positions in all four genes indicating that the introns were in place before the genes of the two isoenzymes diverged. The variant consensus sequence (T/C)11 T(C/T)AG at the 3' splice site of the introns of the genes for nuclear-encoded mitochondrial proteins, which had been deduced from a total of 34 introns [Juretić, N., Jaussi, R., Mattes, U. & Christen, P. (1987) Nucleic Acids Res. 15, 10,083-10,086], was confirmed by including an additional 22 introns into the comparison. The position -4 at the 3' splice site is occupied by base T in 43% of the total 56 introns and appears to be subject to a special evolutionary constraint in this particular group of genes. The following course of evolution of the aspartate aminotransferase genes is proposed. Originating from a common ancestor, the genes of the two isoenzymes intermediarily evolved in separate lineages, i.e. the ancestor eukaryotic and ancestor endosymbiontic cells. When endosymbiosis was established, part of the endosymbiontic genome, including the aspartate aminotransferase gene, was transferred to the nucleus. This process probably led to the conservation of certain splicing factors specific for nuclear-encoded mitochondrial proteins. The presequence for the mitochondrial isoenzyme was acquired by DNA rearrangement. In the eukaryotic lineage, the mitochondrial isoenzyme evolved more slowly than its cytosolic counterpart.

Plasma membrane fatty acid-binding protein and mitochondrial glutamic-oxaloacetic transaminase of rat liver are related

The hepatic plasma membrane fatty acid-binding protein (h-FABPPM) and the mitochondrial isoenzyme of glutamic-oxaloacetic transaminase (mGOT) of rat liver have similar amino acid compositions and identical amino acid sequences for residues 3-24. Both proteins migrate with an apparent molecular mass of 43 kDa on SDS/polyacrylamide gel electrophoresis, have a similar pattern of basic charge isomers on isoelectric focusing, are eluted similarly from four different high-performance liquid chromatographic columns, have absorption maxima at 435 nm under acid conditions and 354 nm at pH 8.3, and bind oleate with a Ka approximately 1.2-1.4 x 10(7) M-1. Sinusoidally enriched liver plasma membranes and purified h-FABPPM have GOT enzymatic activity; the relative specific activities (units/mg) of the membranes and purified protein suggest that h-FABPPM constitutes 1-2% of plasma membrane protein in the rat hepatocyte. Monospecific rabbit antiserum against h-FABPPM reacts on Western blotting with mGOT, and vice versa. Antisera against both proteins produce plasma membrane immunofluorescence in rat hepatocytes and selectively inhibit the hepatocellular uptake of [3H]oleate but not that of [35S]sulfobromophthalein or [14C]taurocholate. The inhibition of oleate uptake produced by anti-h-FABPPM can be eliminated by preincubation of the antiserum with mGOT; similarly, the plasma membrane immunofluorescence produced by either antiserum can be eliminated by preincubation with the other antigen. These data suggest that h-FABPPM and mGOT are closely related.

Regulation of aspartate aminotransferase messenger ribonucleic acid level by testosterone

The effect of testosterone on precursor mitochondrial aspartate aminotransferase (pmAAT) mRNA was studied in rat ventral prostate and primary cell cultures of mini-pig prostate. Testosterone induced a 2-3-fold increase in pmAAT mRNA level in both rat ventral prostate and mini-pig prostate cultures. The pmAAT mRNA induction occurred 30 min after testosterone treatment and was maximal by 1.5 h. Prostatic mAAT activity was also induced by testosterone with a 1-2 h lag period. The time-course of induction of pmAAT mRNA, pmAAT activity and mAAT activity was consistent with stimulation of mRNA synthesis followed by increased synthesis and import of pmAAT into mitochondria. The effect of testosterone on pmAAT mRNA was specific because the increase in pmAAT mRNA was at least 2-fold greater than the increase in poly (A+) RNA. These results suggest that testosterone stimulated mAAT activity by induction of pmAAT mRNA. This continues to support our proposal that a major physiological effect of testosterone is increased pmAAT mRNA steady-state levels which result in increased pmAAT synthesis and increased mAAT activity. These changes ultimately result in increased citrate production by prostate epithelial cells.