The association of glycosomal enzymes and microtubules: a physiological phenomenon or an experimental artifact?

Postnatal growth velocity modulates alterations of proteins involved in metabolism and neuronal plasticity in neonatal hypothalamus in rats born with intrauterine growth restriction

Intrauterine growth restriction (IUGR) due to maternal protein restriction is associated in rats with an alteration in hypothalamic centers involved in feeding behaviour. In order to gain insight into the mechanism of perinatal maternal undernutrition in the brain, we used proteomics approach to identify hypothalamic proteins that are altered in their expression following protein restriction in utero. We used an animal model in which restriction of the protein intake of pregnant rats (8% vs. 20%) produces IUGR pups which were randomized to a nursing regimen leading to either rapid or slow catch-up growth. We identified several proteins which allowed, by multivariate analysis, a very good discrimination of the three groups according to their perinatal nutrition. These proteins were related to energy-sensing pathways (Eno 1, E(2)PDH, Acot 1 and Fabp5), redox status (Bcs 1L, PrdX3 and 14-3-3 protein) or amino acid pathway (Acy1) as well as neurodevelopment (DRPs, MAP2, Snca). In addition, the differential expressions of several key proteins suggested possible shunts towards ketone-body metabolism and lipid oxidation, providing the energy and carbon skeletons necessary to lipogenesis. Our results show that maternal protein deprivation during pregnancy only (IUGR with rapid catch-up growth) or pregnancy and lactation (IUGR with slow postnatal growth) modulates numerous metabolic pathways resulting in alterations of hypothalamic energy supply. As several of these pathways are involved in signalling, it remains to be determined whether hypothalamic proteome adaptation of IUGR rats in response to different postnatal growth rates could also interfere with cerebral plasticity or neuronal maturation.

Large-scale identification of tubulin-binding proteins provides insight on subcellular trafficking, metabolic channeling, and signaling in plant cells

Microtubules play an essential role in the growth and development of plants and are known to be involved in regulating many cellular processes ranging from translation to signaling. In this article, we describe the proteomic characterization of Arabidopsis tubulin-binding proteins that were purified using tubulin affinity chromatography. Microtubule co-sedimentation assays indicated that most, if not all, of the proteins in the tubulin-binding protein fraction possessed microtubule-binding activity. Two-dimensional gel electrophoresis of the tubulin-binding protein fraction was performed, and 86 protein spots were excised and analyzed for protein identification. A total of 122 proteins were identified with high confidence using LC-MS/MS. These proteins were grouped into six categories based on their predicted functions: microtubule-associated proteins, translation factors, RNA-binding proteins, signaling proteins, metabolic enzymes, and proteins with other functions. Almost one-half of the proteins identified in this fraction were related to proteins that have previously been reported to interact with microtubules. This study represents the first large-scale proteomic identification of eukaryotic cytoskeleton-binding proteins, and provides insight on subcellular trafficking, metabolic channeling, and signaling in plant cells.

The Wnt signaling inhibitor dickkopf-1 is required for reentry into the cell cycle of human adult stem cells from bone marrow

Adult human mesenchymal stem cells from bone marrow stroma (hMSCs) differentiate into numerous mesenchymal tissue lineages and are attractive candidates for cell and gene therapy. When early passage hMSCs are plated or replated at low density, the cultures display a lag phase of 3-5 days, a phase of rapid exponential growth, and then enter a stationary phase without the cultures reaching confluence. We found that as the cultures leave the lag phase, they secrete high levels of dickkopf-1 (Dkk-1), an inhibitor of the canonical Wnt signaling pathway. The addition of recombinant Dkk-1 toward the end of the lag period increased proliferation and decreased the cellular concentration of beta-catenin. The addition of antibodies to Dkk-1 in the early log phase decreased proliferation. Also, expression of Dkk-1 in hMSCs decreased during cell cycle arrest induced by serum starvation. The results indicated that high levels of Dkk-1 allow the cells to reenter the cell cycle by inhibiting the canonical Wnt/beta-catenin signaling pathway. Since antibodies to Dkk-1 also increased the lag phase of an osteosarcoma line that expressed the gene, Dkk-1 may have a similar role in some other cell systems.

Identification of a rice RNA- and microtubule-binding protein as the multifunctional protein, a peroxisomal enzyme involved in the beta -oxidation of fatty acids

The control of subcellular mRNA localization and translation is often mediated by protein factors that are directly or indirectly associated with the cytoskeleton. We report the identification and characterization of a rice seed protein that possesses both RNA and microtubule binding activities. In vitro UV cross-linking assays indicated that this protein binds to all mRNA sequences tested, although there was evidence for preferential binding to RNAs that contained A-C nucleotide sequence motifs. The protein was purified to homogeneity using a two-step procedure, and amino acid sequencing identified it as the multifunctional protein (MFP), a peroxisomal enzyme known to possess a number of activities involved in the beta-oxidation of fatty acids. The recombinant version of this rice MFP binds to RNA in UV cross-linking and gel mobility shift experiments, co-sediments specifically with microtubules, and possesses at least two enzymatic activities involved in peroxisomal fatty acid beta-oxidation. Taken together these data suggest that MFP has an important role in mRNA physiology in the cytoplasm, perhaps in regulating the localization or translation of mRNAs through an interaction with microtubules, in addition to its peroxisomal function.

Purification of a dichlorophenol-indophenol oxidoreductase from rat and bovine synaptic membranes: tight complex association of a glyceraldehyde-3-phosphate dehydrogenase isoform, TOAD64, enolase-gamma and aldolase C

NADH-dichlorophenol-indophenol oxidoreductases (PMOs) were purified from synaptic plasma membranes or synaptic vesicles (small recycling vesicles) from both bovine and rat brains and from a neuroblastoma cell line, NB41A3. Several isoforms could be identified in purified plasma membranes and vesicles. Purification of the enzyme activity involved protein extraction with detergents, (NH4)2SO4 precipitation, chromatography under stringent conditions and native PAGE. PMO activity could be attributed to a very tight complex of several proteins that could not be separated except by SDS/PAGE. SDS/PAGE resolved the purified complex into at least five proteins, which could be micro-sequenced and identified unambiguously as hsc70, TOAD64 and glyceraldehyde-3-phosphate dehydrogenase tightly associated with the brain-specific proteins aldolase C and enolase-gamma. Enzyme activity could be purified from both synaptic plasma membranes and recycling vesicles, yields being much greater from the latter source. Highly purified plasma membranes (prepared from a neuroblastoma cell line NB41A3 by iminobiotinylation of intact cells and affinity purification with avidin and anti-avidin antibodies under very stringent conditions) also displayed PMO activity tightly associated with TOAD64. The association of PMO in a tight complex was confirmed by its immunoprecipitation from cellular and membrane extracts of NB41A3 using antibodies directed against any component protein of the complex followed by immunodetection with antibodies directed against the other members. Antibodies also inhibited the enzyme activity synergistically. In addition, induction of the different components of the complex during dichlorophenol-indophenol stress was demonstrated by the S1 RNase-protection assay in synchronized NB41A3 cells. The role of the complex in membrane fusion and cellular response to extracellular oxidative stress during growth and development is discussed.

Structure within eukaryotic cytoplasm and its relationship to glycolytic metabolism

Taken together, the results reviewed here indicate that both structural proteins and enzymes exist in a relatively mobile, uncomplexed form and in a relatively immobile form, complexed with the matrix. The relative amounts of free and complexed forms of each protein are dependent upon the local concentrations of both small molecules and other macromolecules and hence may vary in time and space throughout the cell. Free and cytomatrix-bound enzymes exchange rapidly, while free and cytomatrix-bound structural proteins exchange more slowly. These two distinct time scales suggest that the slowly exchanging structural proteins form the core of fibrous structural elements--having many stabilizing intermolecular contacts with near neighbours--whereas the more rapidly exchanging enzymes adsorb to the surface of the structural elements and have fewer near neighbour contacts. The hierarchical nature of these associations is depicted schematically in Figure 3. Metabolism is proposed to proceed primarily via transport of small metabolites rather than by transport of enzymes, which may be organized in functional clusters to facilitate, metabolic regulation.

There are multiple forms of glyceraldehyde-3-phosphate dehydrogenase in prostate cancer cells and normal prostate tissue

We analyzed glyceraldehyde-3-phosphate dehydrogenase (GAPDH) expression in normal and malignant human prostate tissues, normal rat prostate, and Dunning R-3327 rat prostate cancer cell lines. We detected multiple forms of GAPDH in Dunning cell lines by two-dimensional protein electrophoresis and Western analysis. Five forms of GAPDH that differed by isoelectric point were detected for each of the two metastatic Dunning cell lines, while four or fewer forms were detected for Dunning cell lines with low metastatic ability. We also detected multiple forms of GAPDH in normal and malignant human prostate specimens by two dimensional protein electrophoresis and immunohistochemical analysis. GAPDH was undetectable in normal human prostate secretory epithelium by immunohistochemistry, but was abundant in nuclei of normal basal cells and stromal cells. In human prostate cancer specimens, there was a rough correlation between cytoplasmic staining for GAPDH and tumor grade, but GAPDH staining was extremely heterogeneous. GAPDH was abundant in nuclei of some high-grade human prostate tumors. Both of the human prostate cancer bone metastases analyzed with immunohistochemistry had markedly elevated cytoplasmic GAPDH expression. We conclude that multiple forms of GAPDH may play diverse roles in normal prostate tissue and in prostate cancer.

Glycolytic enzymes and assembly of microtubule networks

The ability of glycolytic enzymes, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), aldolase, pyruvate kinase (PK), and lactate dehydrogenase muscle-type (LDH(M)), to generate interactive microtubule networks was investigated. Bundles have previously been defined as the parallel alignment of several microtubules and are one form of microtubule networks. Utilizing transmission electron microscopy, interactive networks of microtubules as well as bundles were readily observed in the presence of GAPDH, aldolase, or PK. These networks appear morphologically as cross-linked microtubules, oriented in many different ways. Light scattering indicated that the muscle forms of GAPDH, aldolase, PK and LDH(m) caused formation of the microtubule networks. Triose phosphate isomerase (TPI) and lactate dehydrogenase heart-type (LDH(H)), glycolytic enzymes which do not interact with tubulin or microtubules, did not produce bundles, or interactive networks. Sedimentation experiments confirmed that the enzymes that cross-link also co-pellet with the microtubules. Such cross-linking of microtubules indicate that the enzymes are multivalent with the capability of simultaneous binding to more than one microtubule.

Microtubule organization by cross-linking and bundling proteins

To understand microtubule function the factors regulating their spatial organization and their interaction with cellular organelles, including other microtubules, must be elucidated. Many proteins are implicated in these organizational events and the known consequences of their actions within the cell are increasing. For example, the function of microtubule bundles at the surfaces of polarized cells has recently received attention, as has the action in cortical rotation of a transient arrangement of microtubules found beneath the vegetal surface of fertilized frog eggs. The in vivo association of microtubules during early Xenopus oogenesis has added interest as microtubules bundled in cell-free extracts are protected against the action of a severing protein found in this animal. A 52 kDa F-actin bundling protein purified from Physarum polycephalum organizes microtubules and causes the cobundling of microtubules and microfilaments. These observations, in concert with others that are presented, emphasize the diversity within the family of microtubule cross-linking proteins. The challenge is to determine which proteins are relevant from a physiological perspective, to ascertain their molecular mechanisms of action and to describe how they affect cytoplasmic organization and cell function. To realize this objective, the proteins which cross-link and bundle microtubules must be investigated by techniques which reveal different but related aspects of their properties. Cloning and sequencing of genes for cross-linking proteins, their subcellular localization especially as microtubule-related changes in cell morphology are occurring and the application of genetic studies are necessary. Study of the neural MAP provides the best example of just how powerful current experimental approaches are and at the same time shows their limits. The neural MAP have long been noted for their enhancement of tubulin assembly and microtubule stability. Their spatial distribution has been studied during the morphogenesis of neural cells. Sequencing of cloned genes has revealed the functional domains of neural MAP including carboxy-terminal microtubule-binding sites. Similarities to microtubule binding proteins from other cell types stimulate interest in the neural MAP and further suggest their importance in microtubule organization. For example, MAP4 enjoys a wide cellular distribution and has microtubule-binding sequences very similar to those in the neural MAP. Moreover, the nontubulin proteins of marginal bands are immunologically related to neural MAP, indicating shared structural/functional domains. Even with these findings the mechanism by which neural MAP cross-link microtubules remains uncertain. Indeed, some researchers express doubt that microtubule cross-linking is actually a function of neural MAP in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation and characterization of a unique 15 kilodalton trypanosome subpellicular microtubule-associated protein

A protein of 15 kDa (p15) was isolated from Trypanosoma brucei subpellicular microtubules by tubulin affinity chromatography. The protein bound tubulin specifically both in its native form and after SDS-PAGE in tubulin overlay experiments. p15 promoted both the in vitro polymerization of purified calf brain tubulin and the bundling of preformed mammalian microtubules. Immunolabeling identified p15 at multiple sites along microtubule polymers comprising calf brain tubulin and p15 as well as on the subpellicular microtubules of cryosectioned trypanosomes. Antibodies directed against p15 did not cross react with mammalian microtubules. It is suggested that p15 is a trypanosome-specific microtubule-associated protein (MAP) that contributes to the unique organization of the subpellicular microtubules.