A glycolytic enzyme binding domain on tubulin

Sorting of metabolic pathway flux by the plasma membrane in cerebrovascular smooth muscle cells

We used beta-escin-permeabilized pig cerebral microvessels (PCMV) to study the organization of carbohydrate metabolism in the cytoplasm of vascular smooth muscle (VSM) cells. We have previously demonstrated (Lloyd PG and Hardin CD. Am J Physiol Cell Physiol 277: C1250-C1262, 1999) that intact PCMV metabolize the glycolytic intermediate [1-(13)C]fructose 1,6-bisphosphate (FBP) to [1-(13)C]glucose with negligible production of [3-(13)C]lactate, while simultaneously metabolizing [2-(13)C]glucose to [2-(13)C]lactate. Thus gluconeogenic and glycolytic intermediates do not mix freely in intact VSM cells (compartmentation). Permeabilized PCMV retained the ability to metabolize [2-(13)C]glucose to [2-(13)C]lactate and to metabolize [1-(13)C]FBP to [1-(13)C]glucose. The continued existence of glycolytic and gluconeogenic activity in permeabilized cells suggests that the intermediates of these pathways are channeled (directly transferred) between enzymes. Both glycolytic and gluconeogenic flux in permeabilized PCMV were sensitive to the presence of exogenous ATP and NAD. It was most interesting that a major product of [1-(13)C]FBP metabolism in permeabilized PCMV was [3-(13)C]lactate, in direct contrast to our previous findings in intact PCMV. Thus disruption of the plasma membrane altered the distribution of substrates between the glycolytic and gluconeogenic pathways. These data suggest that organization of the plasma membrane into distinct microdomains plays an important role in sorting intermediates between the glycolytic and gluconeogenic pathways in intact cells.

The glycolytic enzyme enolase is present in sperm tail and displays nucleotide-dependent association with microtubules

We examined the expression and localisation of enolase (2-phospho-D-glycerate hydrolase) in differentiating rat spermatogenic cells. We found that enolase is most abundant in mature spermatozoa and in residual cytoplasmic bodies detached from elongating spermatids with little to no enolase detected in meiotic primary spermatocytes and round spermatids. We localised enolase mostly to the tail of mature spermatozoa by immunoblotting and by immunofluorescence. RT-PCR analysis of differentiating spermatogenic cells detected only the alpha isoform of enolase. As several glycolytic enzymes are known to associate with microtubules prepared from brain, we investigated the association of enolase with brain and testis microtubules. We found that only a small fraction of testis and brain-derived cytosolic enolase (4.9% and 11.2%, respectively) co-sediments with microtubules stabilised in the presence of taxol. In the presence of certain nucleotides in excess (3 mM ATP, CTP, GTP and ITP) the association of enolase with microtubules was disrupted, however, this was not the case for UTP. This observation is consistent with the finding that in the presence of 0.5 mM AMP-PNP, a nonhydrolysable analogue of ATP, there is an increased association of enolase with microtubules. We propose that the nucleotide-dependent association of enolase with microtubules regulates enzyme activity by linking energy production to utilisation.

Involvement of a cellular glycolytic enzyme, phosphoglycerate kinase, in Sendai virus transcription

In vitro mRNA synthesis of Sendai virus is almost entirely dependent on the addition of cellular proteins (host factors). Previous studies indicated that the host factor activity from bovine brain was resolved into at least two complementary fractions, one of which may be tubulin. In this study, the host factor activity that stimulates the transcription in the presence of tubulin was further purified from bovine brain. This fraction was found to contain at least two complementary factors, and one of them was purified to a single polypeptide chain with an apparent M(r) of 46,000 (p46). From the amino acid sequence, biochemical, and immunological analyses, p46 was identified as a glycolytic enzyme, phosphoglycerate kinase (PGK). Purified native PGK from rabbit and yeast, and a recombinant human PGK substituted for p46. Although, as previously suggested, tubulin was involved in the transcription initiation complex formation by being integrated into the complex, p46 and its complementary factor had little effect on the complex formation. On the other hand, when p46 and the complementary factor were added to the RNA chain elongation reaction from the isolated initiation complex formed with tubulin, mRNA synthesis was dramatically stimulated. The enzymatic activity per se of PGK did not seem to be required for its activity. West-Western blot analysis showed that PGK could directly interact with tubulin. These data suggest that PGK stimulates Sendai virus mRNA synthesis at the elongation step, probably through its interaction with tubulin in the initiation complex.

Role of microtubules in the regulation of metabolism in isolated cerebral microvessels

We used (13)C-labeled substrates and nuclear magnetic resonance spectroscopy to examine carbohydrate metabolism in vascular smooth muscle of freshly isolated pig cerebral microvessels (PCMV). PCMV utilized [2-(13)C]glucose mainly for glycolysis, producing [2-(13)C]lactate. Simultaneously, PCMV utilized the glycolytic intermediate [1-(13)C]fructose 1,6-bisphosphate (FBP) mainly for gluconeogenesis, producing [1-(13)C]glucose with only minor [3-(13)C]lactate production. The dissimilarity in metabolism of [2-(13)C]FBP derived from [2-(13)C]glucose breakdown and metabolism of exogenous [1-(13)C]FBP demonstrates that carbohydrate metabolism is compartmented in PCMV. Because glycolytic enzymes interact with microtubules, we disrupted microtubules with vinblastine. Vinblastine treatment significantly decreased [2-(13)C]lactate peak intensity (87.8 +/- 3.7% of control). The microtubule-stabilizing agent taxol also reduced [2-(13)C]lactate peak intensity (90.0 +/- 2. 4% of control). Treatment with both agents further decreased [2-(13)C]lactate production (73.3 +/- 4.0% of control). Neither vinblastine, taxol, or the combined drugs affected [1-(13)C]glucose peak intensity (gluconeogenesis) or disrupted the compartmentation of carbohydrate metabolism. The similar effects of taxol and vinblastine, drugs that have opposite effects on microtubule assembly, suggest that they produce their effects on glycolytic rate by competing with glycolytic enzymes for binding, not by affecting the overall assembly state of the microtubule network. Glycolysis, but not gluconeogenesis, may be regulated in part by glycolytic enzyme-microtubule interactions.

Chromatographic methods to study protein-protein interactions

Proteins and enzymes are now generally thought to be organized within the cell to form clusters in a dynamic and versatile way, and heterologous protein-protein interactions are believed to be involved in virtually all cellular events. Therefore we need appropriate tools to detect and study such interactions. Chromatographic techniques prove to be well suited for this kind of investigation. Real complexes formed between proteins can be studied by classic gel filtration. When enzymes are studied, active enzyme gel chromatography is a useful alternative. A variant of classic gel filtration is gel filtration equilibrium analysis, which is similar to equilibrium dialysis. When the association formed is only dynamic and equilibrates very rapidly, either the Hummel-Dryer method of equilibrium gel filtration or large-zone equilibrium filtration sometimes allows the interactions to be analyzed, both qualitatively and quantitatively. Very often, however, interactions between enzymes and proteins can only be evidenced in vitro in media that mimic the intracellular situation. Immobilized proteins are excellent tools for this type of research. Several examples are indeed known where the immobilization of an enzyme on a solid support does not affect its real properties, but rather changes its environment in such a way that the diffusion becomes limiting. Affinity chromatography using immobilized proteins allows the analysis of heterologous protein-protein interactions, both qualitatively and quantitatively. A useful alternative appears to be affinity electrophoresis. The latter technique, however, is exclusively qualitative. All these techniques are described and illustrated with examples taken from the literature.

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.

Pyruvate kinase as a microtubule destabilizing factor in vitro

Endogenous control of microtubule dynamism is essential in many cell types. Numerous microtubule-adhering proteins stabilize the polymer status, while very few protein factors are described with opposite effects. The brain- and muscle-specific M1 isoform of the enzyme pyruvate kinase is investigated here in this respect. Three pieces of evidence indicate antimicrotubular effects of this protein. (1) Pyruvate kinase inhibits taxol-induced tubulin polymerization into microtubules as revealed by turbidimetry. (2) Pelleting experiments show that pyruvate kinase partially disassembles taxol-stabilized microtubules into less sedimentable oligomers leading to the appearance of tubulin in the supernatant fractions. (3) Electron microscopy reveals the kinase-induced formation of great amounts of thread-like tubulin oligomers which tend to accumulate in a light/less sedimentable fraction. Immunoelectron micrographs using labeled antibody against pyruvate kinase provide evidence for the binding of pyruvate kinase to the thread-like oligomeric forms. The present data allow the assumption that pyruvate kinase may display multiple regulatory functions as a glycolytic control enzyme and as a modulator of microtubule dynamism.

Alternative binding of two sequential glycolytic enzymes to microtubules. Molecular studies in the phosphofructokinase/aldolase/microtubule system

Simultaneous binding of two sequential glycolytic enzymes, phosphofructokinase and aldolase, to a microtubular network was investigated. The binding of the phosphofructokinase to microtubules and its bundling activity has been previously characterized (Lehotzky, A., Telegdi, M., Liliom, K., and Ovádi, J. (1993) J. Biol. Chem. 268, 10888-10894). Aldolase binding to microtubules at near physiological ionic strength is weak (Kd = 20 microM) as compared with that of the kinase (Kd = 1 microM). The interactions of both enzymes with microtubules are modulated by their common intermediate, fructose-1,6-bisphosphate. Pelleting and electron microscopic measurements have revealed that the aldolase binding interferes with that of phosphofructokinase, although they have distinct binding domains on microtubules. The underlying molecular mechanism responsible for this finding is that in the solution phase aldolase and phosphofructokinase form a bienzyme complex that does not bind to the microtubule. The bienzyme complex formation does not influence the catalytic activity of aldolase, however, it inhibits the dissociation-induced inactivation of the kinase by stabilizing a catalytically active molecular form. The present data suggest the first experimental evidence that two sequential glycolytic enzymes do not associate simultaneously to microtubules, but their complexation in solution provides kinetic advantage for glycolysis.