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The periodic synthesis of tubulin in the Physarum cell cycle. Characterization of Physarum tubulins by affinity for monoclonal antibodies and by peptide mapping.

Polypeptides preferentially labeled in the G2 phase of the synchronous nuclear replication cycle of Physarum macroplasmodia were compared in electrophoretic mobility and peptide map with the tubulins enriched from Physarum myxamoebae. One major and one minor fluorographic species match the myxamoebal alpha and beta chains, respectively. Thus, tubulins are among the proteins of Physarum selectively synthesized before nuclear division. A third species P, prominently labeled in premitotic plasmodia, is distinct from the two myxamoebal tubulins even though it co-polymerizes with microtubules. The nature of P remains unknown. Two rat monoclonal antibodies directed against yeast tubulin were found to bind selectively to the alpha tubulin of porcine brain. These served to confirm the assignment of the 50,000-dalton Physarum myxamoebal tubulin as an alpha-like polypeptide.

Antibodies, Monoclonal

The F-actin capping proteins of Physarum polycephalum: cap42(a) is very similar, if not identical, to fragmin and is structurally and functionally very homologous to gelsolin; cap42(b) is Physarum actin.

We have carried out a primary structure analysis of the F-actin capping proteins of Physarum polycephalum. Cap42(b) was completely sequenced and was found to be identical with Physarum actin. Approximately 88% of the sequence of cap42(a) was determined. Cap42(a) and fragmin were found to be identical by amino acid composition, isoelectric point, mol. wt, elution time on reversed-phase chromatography and amino acid sequence of their tryptic peptides. The available sequence of cap42(a) is greater than 36% homologous with the NH2-terminal 42-kd domain of human gelsolin. A highly homologous region of 16 amino acids is also shared between cap42(a), gelsolin and the Acanthamoeba profilins. Cap42(a) binds two actin molecules in a similar way to gelsolin suggesting a mechanism of F-actin modulation that has been conserved during evolution.

Actin Depolymerizing Factors

A method for separation of pigments from plasmodia of the true slime molds, Physarum polycephalum and Physarum nudum.

Plasmodia of the true slime molds Physarum polycephalum and Physarum nudum were grown on agar semidefined medium (J. W. Daniel and H. Baldwin (1964) in Methods in Cell Physiology (Precsott, D. M., Ed.), Vol. 1, pp. 9-44, Academic Press, New York.) and pigments were extracted from 8-day-old plasmodia with the same solvent mixture that was used later as a mobile phase in the HPLC analysis. The separation was carried out on a strong anion exchanger column in a methanol/borate buffer solution. Under these conditions eight pigments from P. polycephalum and eight from P. nudum were revealed. This method can be used for detailed studies of plasmodial pigments in these species.

Chromatography, High Pressure Liquid

Growth of Physarum flavicomum and Physarum rigidum in chemically defined minimal media.

Physarum flavicomum, P. polycephalum, and P. rigidum grew at pH 4.2 in a medium composed of mineral salts, glucose, biotin, thiamine, hematin, and four amino acids. Important differences in pH tolerance were noted among the species. The minimal medium of P. flavicomum and P. polycephalum contained the amino acids methionine, glycine, and arginine, but valine was also required by P. rigidum. Starting with an inoculum of about 0.3 mg of protein per 25 ml of minimal medium, P. flavicomum and P. polycephalum grew to 23 mg and P. rigidum to 12 mg of protein per 25 ml in 3, 2, and 4 weeks, respectively. P. flavicomum and P. polycephalum grew with valine or leucine replacing arginine in the minimal medium but the growth yields and growth rates were decreased. All three species utilized homocysteine thiolactone in the minimal media in place of methionine. Serine adequately replaced glycine for P. rigidum but was inhibitory in the minimal medium of P. flavicomum or P. polycephalum unless homocysteine thiolactone also replaced methionine. Growth rates of all three organisms were increased in the presence of seven amino acids (original four plus leucine, lysine, and isoleucine).

Amino Acids

Some properties of Physarum actinin. A regulatory protein of actin polymerization.

A factor termed Physarum actinin was isolated and partially purified from plasmodia of a myxomycete, Physarum polycephalum. When Physarum actinin was mixed with purified Physarum or rabbit striated muscle G-actin in a weight ratio of about 1 actinin to 9 actin and then the polymerization of G-actin induced, G-actin polymerized to the ordinary F-actin on addition of 0.1 M KCl. However, it polymerized to Mg-polymer on addition of 2 mM MgCl2. The reduced viscosity (etasp/C) of the Mg-polymer was 1.2 dl/g, about one-seventh of that of the F-actin (7.4 dl/g). The sedimentation coefficient of the Mg-polymer was 22.8 S, almost the same as that of the F-actin (29.4 S). The Mg-polymer showed the specific ATPase activity of the order of 1 . 10(-3) mumol ATP/mg actin per min. It was shown that Physarum actinin copolymerized with G-actin to form Mg-polymer on addition of 2 mM MgCl2. The molecular weights of Physarum actinin were about 90 000 in salt-free or slat solutions and 43 000 in a dodecyl sulfate solution. The range of salting out with ammonium sulfate was 50--65% saturation, which was different from that of Physarum actin (15--35% saturation). Physarum actinin did not interact with Physarum myosin or muscle heavy meromyosin. When the weight ratio of actinin to actin increased, the flow birefringence of the formed Mg-polymer decreased, and it became almost zero at the weight ratio of 1 actinin to 5 actin. ATPase activity reached the maximum level (2.2 . 10(-3) mumol ATP/mg actin per min) at the same ratio. On the addition of Physarum actinin to purified Physarum F-actin which had been polymerized on addition of 2 mM MgCl2 the viscosity decreased rapidly, suggesting that the F-actin filaments were broken in the smaller fragments or that they transformed to Mg-polymers. A factor with properties similar to Physarum actinin was isolated from acetone powder of sea urchin eggs.

Actinin

Physarum myosin light chain binds calcium.

Myosin from the slime mold Physarum polycephalum contains three sizes of polypeptides: a heavy chain and two light chains, LC-1 and LC-2. Using a simple qualitative test for calcium binding by comparing electrophoretic migration of the polypeptides in sodium dodecyl sulfate (SDS) acrylamide gels in the presence and absence of calcium, we have found that Physarum myosin light chain LC-2 migrates with an apparent molecular weight of 16,900 daltons in the presence of the metal ion chelator ethylene glycol bis (B-aminoethyl ether) N,N'-tetraacetic acid (EGTA). However, if calcium chloride is added to the sample prior to electrophoresis, the apparent molecular weight decreases to 16,100. Lanthanide and cadmium ions, but not magnesium, can substitute for calcium. Because the ionic radii of Ca2+, La3+, and Cd2+ are almost identical, we conclude that Physarum myosin LC-2 possesses a very size-specific binding site for calcium. Physarum myosin LC-1 and the heavy chain give no evidence for binding calcium by this test. Since cytoplasmic streaming in the plasmodium of Physarum requires calcium, our evidence indicates that the calcium-binding property of Physarum myosin LC-2 may be important in regulating the production of force by actomyosin in the ectoplasm. Unexpectedly, the myosin light chain in Physarum capable of binding calcium, LC-2, is the essential light chain, while LC-1 is a member of the regulatory class of myosin light chains [V. T. Nachmias, personal communication]. Until now, essential myosin light chains have not been shown to have high affinity divalent cation binding sites. This means a new version of the myosin-based model for actomyosin regulation by calcium may be required to explain cytoplasmic movement in Physarum, and perhaps in other motile systems involving cytoplasmic myosins as well.

Actomyosin

Isolation, identification, and characterization of histones from plasmodia of the true slime mold Physarum polycephalum using extraction with guanidine hydrochloride.

Histones from plasmodia of the true slime mold Physarum polycephalum have been prepared free of slime by an approach to histone isolation that uses extraction of nuclei with 40% guanidine hydrochloride and chromatography of the extract on Bio-Rex 70. This procedure followed by chromatography or electrophoresis has been used to obtain pure fractions of histones from Physarum microplasmodia. Physarum microplasmodia have five major histone fractions, and we show by amino acid analysis, apparent molecular weight on three gel systems containing sodium dodecyl sulfate, mobility on gels containing Triton X-100, and other characterizations that these fractions are analogous to mammalian histones H1, H2A, H2B, H3, and H4. Significant differences between Physarum and mammalian histones are noted, with histone H1 showing by far the greatest variation. Histones H1 and H4 from Physarum microplasmodia have similar, but not identical, products of partial chymotryptic digestion compared with those of calf thymus histones H1 and H4. Labeling experiments, in vivo, showed that histone H1 is the major phosphorylated histone and approximately 15 separate phosphopeptides are present in a tryptic digest of Physarum histone H1. The core histones from Physarum, histones H2A, H2B, H3, and H4, are rapidly acetylated; histone H4 shows five subfractions, analogous to the five subfractions of mammalian histone H4 (containing zero to four acetyllysine residues per molecule); histone H3 has a more complex pattern that we interpret as zero to four acetyllysine residues on each of two sequence variants of histone H3; histones H2A and H2B show less heterogeneity. Overall, the data show that Physarum microplasmodia have a set of histones that is closely analogous to mammalian histones.

Chemical Phenomena

Calcium sensitivity of hybrid complexes of muscle myosin and Physarum proteins.

UNLABELLED: 1. A myosin-actin hybrid complex was used to study actin-associated calcium sensitivity of a "cytoplasmic" actomyosin. The approach should be generally applicable. 2. Low salt extracts of Physarum polycephalum contain actin which remains in solution after centrifugation at 46 000 times g or at 100 000 times g for 1 h. The actin was precipitated by the addition of muscle myosin to the supernatants and detected in the hybrid complex by electron microscopy, sodium dodecyl sulfate gel analysis, super-precipitation and activation of the myosin ATPase activity. Actin was also precipitable from high speed supernatants of brain tissue or platelets. 3. The hybrid complexes from Physarum possessed 1.5-5-fold calcium dependency which could be removed by washing. Reincubation of the washed complex with concentrated wash solution resulted in high calcium sensitivity. On sodium dodecyl sulfate gels, unwashed complexes from Physarum contained high molecular weight material in addition to bands of molecular weights less than actin. The bands in the size range of 39 000 to 18 000 were primarily lost from the Physarum complex concomitantly with loss of calcium dependence. 4. When the Physarum supernatants were made 40 mM in MgCl2, precipitates were formed containing actin which possessed calcium sensitivity which was also lost on washing with low ionic strength solutions. This calcium dependency was partially reversed by the addition of desensitized rabbit actin to the precipitate before assay. 5. CONCLUSION: calcium regulation of actomyosin in Physarum is mediated primarily by factors that are bound to the actin component. The regulatory factors are soluble in low salt buffers. The molecular weights of the polypeptide chains of several of these factors are similar to those of the troponin polypeptides of striated muscle. In Physarum but not in platelet or brain a prominent polypeptide chain of approx. 55 000 molecular weight also occurs which coprecipitates with the hybrid complex and which is not easily removed.

Adenosine Triphosphatases

Pteridine biosynthesis and nitric oxide synthase in Physarum polycephalum.

Physarum polycephalum, an acellular slime mould, serves as a model system to study cell-cycle-dependent events since nuclear division is naturally synchronous. This organism was shown to release isoxanthopterin which is structurally related to tetrahydrobiopterin, a cofactor of aromatic amino acid hydroxylases and of nitric oxide synthases (NOSs) (EC 1.14.13.39). Here, we studied Physarum pteridine biosynthesis in more detail and found that high amounts of tetrahydrobiopterin are produced and NOS activity is expressed. Physarum pteridine biosynthesis is peculiar in as much as 7,8-dihydroneopterin aldolase (EC 4.1.2.25), an enzyme of folic acid biosynthesis usually not found in organisms producing tetrahydrobiopterin, is detected in parallel. NOS purified from Physarum depends on NADPH, tetrahydrobiopterin and flavins. Enzyme activity is independent of exogenous Ca2+ and is inhibited by arginine analogues. The purified enzyme (with a molecular mass of 130 kDa) contains tightly bound tetrahydrobiopterin and flavins. During the synchronous cell cycle of Physarum, pteridine biosynthesis increases during S-phase whereas NOS activity peaks during mitosis, drops at telophase and peaks again during early S-phase. Our results characterize Physarum pteridine biosynthesis and NOS and suggest a possible link between NOS activity and mitosis.

Amino Acid Oxidoreductases

Acetylation and methylation sites in histone H4 from Physarum polycephalum.

Histone H4 has been isolated and purified from plasmodia of Physarum polycephalum. The four major fragments produced by hydrolysis of H4 by acetic acid were separated and the complete amino acid sequence of two of them was determined. By analogy with calf H4, these peptides are at the C-terminus and give the sequence from residue 68 to the C-terminus (residue 102). In this 35 residue sequence there are two minor differences from calf H4: (i) residue 77 is arginine in Physarum H4 and lysine in calf H4; (ii) lysine-79 is partially methylated in Physarum. Arginine occurs at position 77 in pea H4 but the occurrence of methylated lysine at position 79 has not been reported for other species. In the N-terminal region, amino acid compositions of acetic acid, tryptic and chymotryptic peptides indicate that Physarum H4 and calf H4 have identical sequences from the N-terminus to residue 47. There may be minor differences in the region from residue 46 to residue 67. The sites of acetylation were determined by Edman degradation of acetate-labelled peptide 4-17 of Physarum H4. Acetylation was observed at positions 5, 8, 12, and 16. The only other labelled peptide was the N-terminal peptide, which is not susceptible to Edman degradation and is thus probably alpha-N-acetylated as in most other organisms. The results confirm the conservation of H4 sequence and place Physarum H4 in an intermediate position between lower eukaryote H4, such as yeast or Tetrahymena H4, and higher eukaryote H4, such as mammalian H4 or pea H4.

Acetylation

Incorporation of tubulin from an evolutionarily diverse source, Physarum polycephalum, into the microtubules of a mammalian cell.

Physarum myxamoebal tubulin was injected into PtK2 cells to determine whether tubulin from this eukaryotic microbe could act as a reporter for microtubule growth and dynamics in a mammalian cell. The distribution of Physarum tubulin was determined by the use of a monoclonal antibody specific for Physarum tubulin and unable to detect mammalian tubulin. Physarum tubulin was incorporated into the microtubules of both interphase arrays and the mitotic spindle. Measurements of microtubule turnover kinetics were found to be similar to those of other studies in which chemically modified brain tubulin has been used. Results using this heterologous system demonstrate that tubulin from an evolutionarily diverse organism can be used as a marker for microtubule growth in mammalian cells. Furthermore, the Physarum tubulin was able to endow the injected cells with novel properties. Resistance to colchicine-induced microtubule disassembly, a characteristic of Physarum tubulin, was conferred on the injected PtK2 cells. Use of this heterologous reporter tubulin system has also revealed features of variation in microtubule dynamics both within individual cells and between cells.

Animals

Physarum vitronectin-like protein has extensive homology to dihydrolipoamide acetyltransferase.

Physarum vitronectin-like protein with a molecular mass of 70 kDa cross-reacts with anti-bovine vitronectin and promotes cell-spreading (Miyazaki, K. et al. 1992. Exp. Cell Res., 199: 106-110.). The amino-terminal sequence of Physarum vitronectin-like protein is, however, distinct from those of animal vitronectins but shows significant sequence homology with dihydrolipoamide acetyltransferase, a component of pyruvate dehydrogenase complex. We have investigated the structural relationships between Physarum vitronectin-like protein and dihydrolipoamide acetyltransferase by using both antibody and protein-chemical methods. The vitronectin-like protein reacted with both anti-bovine vitronectin IgG and anti-rat pyruvate dehydrogenase complex IgG, indicating that it shares common antigenic determinant(s) with rat pyruvate dehydrogenase complex. Furthermore, sequencing studies of peptides obtained by lysylendopeptidase digestion indicated that internal sequences of Physarum vitronectin-like protein show significant homology with dihydrolipoamide acetyltransferase, but do not show any homology with the primary structures of authentic vitronectins. Immunocytochemistry revealed that the protein is widely localized in cytoplasm and nuclei of Physarum polycephalum, but is not present in the central area of vacuoles. Our results indicate that Physarum vitronectin-like protein is a molecule structurally and immunologically related to dihydrolipoamide acetyltransferase but functionally similar to animal vitronectin, although its localization is unique.

Acetyltransferases

Electron microscope observations on myosin from Physarum polycephalum.

Myosin has been separated from Physarum polycephalum actomyosin in confirmation of the results of Hatano and Tazawa. In an intermediate step, myosin-enriched actomyosin has also been obtained. The mean yield of free myosin was 4.4 mg from 100 g of mold. It was obtained as water-clear solutions at micro = 0.055 with calcium ATPase activity of up to 0.5 microM P(i)/min per mg. Negatively stained preparations were examined by electron microscopy. Physarum myosin in 0.5 M KCl interacted with actin from rabbit skeletal muscle to form polarized arrowhead complexes similar to but less regular than those of natural actomyosin from muscle or myosin-enriched Physarum actomyosin. The Physarum myosin-enriched actomyosin at low ionic strength displayed evidence of head-to-tail and tail-to-tail aggregation attributable to the myosin component. Yet Physarum myosin alone did not produce detectable filaments at micro = 0.055 at pH 7, 6.5, or 5.8, nor when dialyzed against 0.01 M ammonium acetate, nor when the dielectric constant of the medium was reduced. However, aggregation approaching the extent of 'thick filaments' up to 0.3 micro long was found in some preparations of myosin-enriched actomyosin put into solutions containing adenosine triphosphate. Myosin alone in such solutions did not form filaments. The results are compatible with the idea that head-to-tail aggregations are favored by actin-myosin interactions in Physarum, possibly due to alignment of the extended or tail portions of this myosin molecule.

Actomyosin

Sequence organisation in nuclear DNA from Physarum polycephalum. Genomic organisation of DNA segments containing foldback sequences.

DNA clones containing foldback sequences, derived from Physarum polycephalum nuclear DNA, can be classified according to their pattern of hybridisation to Southern blots of genomic DNA. One group of DNA clones map to unique DNA loci when used as a probe to restriction digests of Physarum nuclear DNA. These cloned segments appear to contain dispersed repetitive sequence elements located at many hundreds of sites in the genome. Similar patterns of hybridisation are generated when these cloned DNA probes are annealed to DNA restriction fragments of genomic DNA obtained from a number of different Physarum strains, indicating that no detectable alteration has occurred at these genomic loci subsequent to the divergence of the strains as a result of the introduction or deletion of mobile genetic elements. However, deletion of segments of some cloned DNA fragments occurs following their propagation in Escherichia coli. A second, distinct group of clones are shown to be derived from highly methylated segments of Physarum DNA which contain very abundant repetitive sequences with regular, though complex, arrangements of restriction sites at their various genomic locations. It is suggested that these DNA segments contain clustered repetitive sequence elements. The results lead to the conclusion that foldback elements in Physarum DNA are located in segments of the genome which display markedly different patterns of sequence organisation and degree of DNA methylation.

Base Sequence

Sequence organisation in nuclear DNA from Physarum polycephalum. Arrangement of highly-repeated sequences.

Recombinant plasmids containing highly repetitive Physarum DNA segments were identified by colony hybridisation using a radioactively-labelled total Physarum DNA probe. A large number of these clones also hybridised to a foldback DNA probe purified from Physarum nuclear DNA. The foldback DNA probe was characterised by reassociation kinetic analysis. About one-half of this component was shown to consist of highly repeated sequences with a kinetic complexity of 1100 bp and an average repetition frequency of 5200. Direct screening of 67 recombinant plasmids for foldback sequences using the electron microscope revealed that about one-half were located in segments of DNA containing highly repetitive sequences; the remainder were present in clones containing low-copy number repeated elements. Analysis of two DNA clones showed that they contained repetitive elements located in over half of all DNA segments containing highly repetitive DNA and that the foci containing these highly repetitive sequences had different sequence arrangements. The results are consistent with the hypothesis that the most highly repeated DNA sequence families in the Physarum genome are few in number and are clustered together in different arrangements in about one-sixth of the genome. Over one-half of the foldback DNA complement in the Physarum genome is derived from these segments of DNA.

DNA, Fungal

A library of trimethylguanosine-capped small RNAs in Physarum polycephalum.

We have constructed a cDNA library for the trimethylguanosine-capped small RNAs (sRNAs) in the acellular slime mold Physarum polycephalum. Capped sRNAs were purified from total cellular RNA of vegetative microplasmodia by preparative immunoprecipitation with anti-trimethylguanosine antibody. The purified RNA was analyzed by polyacrylamide gel electrophoresis. Approx. eleven different capped sRNAs were observed with a size range of 70-204 nucleotides (nt). Based on their approximate sizes, the presence of trimethylguanosine cap, and the presence of a lupus type-Sm antigen, molecules U1-U7 (excluding U3) were identified. Further confirmation of the identity of molecule U1a was established by Northern hybridization, U4a by colony hybridization, and U6 and U7a by direct chemical sequence analysis. Purified capped sRNAs were tailed with oligo(A), and inserted into oligo(dT)-tailed plasmid pCDV1. The cDNAs were used to transform Escherichia coli strain HB101. Approx. 1.9 X 10(5) ampicillin-resistant (ApR) transformants were obtained per microgram of tailed sRNA. Dot-blot hybridization, using Physarum RNA precipitated with anti-cap antibody as a probe, indicated that approx. 94% of the ApR colonies contained recombinant DNAs. The library was screened by colony hybridization using heterologous sRNA probes. Clones hybridizing with heterologous sRNAs U1, U2, U4 and U7 were each represented in the library in approximately the same frequency as their relative abundance in the Physarum sRNA population they were derived from. The insert of one Physarum U4 clone was sequenced and was found to have 57.1% homology with nt 1-91 of the published sequence for rat U4 RNA. A 12-nt 'functional' subdomain of the rat U4 molecule was 83.3% conserved in Physarum U4.

Autoantigens

Stable, selectable, integrative DNA transformation in Physarum.

The Physarum polycephalum actin promoter, PardC, can drive transient expression of heterologous genes in Physarum amoebae. The hph gene, encoding hygromycin (Hy) phosphotransferase, can confer resistance to Hy on a broad spectrum of organisms. When PardC is translationally fused to hph and transformed into yeasts on high-copy-number vectors, the yeasts become Hy resistant (HyR), showing that PardC-hph is a functional, selectable genetic element. To establish a stable transformation system for Physarum, we electroporated plasmids bearing PardC-hph into Physarum amoebae and then selected for HyR transformants. We show that HyR amoebae arise upon the stable integration of PardC-hph into the nuclear genome in single copy. These results establish a transformation system that can be used to add plasmid-borne genetic information to Physarum.

Actins

Amphibian oocyte maturation induced by extracts of Physarum polycephalum in mitosis.

The orderly progression of eukaryotic cells from interphase to mitosis requires the close coordination of various nuclear and cytoplasmic events. Studies from our laboratory and others on animal cells indicate that two activities, one present mainly in mitotic cells and the other exclusively in G1-phase cells, play a pivotal role in the regulation of initiation and completion of mitosis, respectively. The purpose of this study was to investigate whether these activities are expressed in the slime mold Physarum polycephalum in which all the nuclei traverse the cell cycle in natural synchrony. Extracts were prepared from plasmodia in various phases of the cell cycle and tested for their ability to induce germinal vesicle breakdown and chromosome condensation after microinjection into Xenopus laevis oocytes. We found that extract of cells at 10-20 min before metaphase consistently induced germinal vesicle breakdown in oocytes. Preliminary characterization, including purification on a DNA-cellulose affinity column, indicated that the mitotic factors from Physarum were functionally very similar to HeLa mitotic factors. We also identified a number of mitosis-specific antigens in extracts from Physarum plasmodia, similar to those of HeLa cells, using the mitosis-specific monoclonal antibodies MPM-2 and MPM-7. Interestingly, we also observed an activity in Physarum at 45 min after metaphase (i.e., in early S phase since it has no G1) that is usually present in HeLa cells only during the G1 phase of the cell cycle. These are the first studies to show that maturation-promoting factor activity is present in Physarum during mitosis and is replaced by the G1 factor (or anti-maturation-promoting factor) activity in a postmitotic stage. A comparative study of these factors in this slime mold and in mammalian cells would be extremely valuable in further understanding their function in the regulation of eukaryotic cell cycle and their evolutionary relationship to one another.

Animals