ATPase kinetics of the Dictyostelium discoideum myosin II motor domain

Structural characterization of the mode of interaction of nucleotides bound to myosin has relied upon the crystal structure of the Dictyostelium discoideum myosin II motor domain. This fragment, denoted S1dC, lacks the regulatory domain and light chain subunits and may therefore fail to display the normal ATPase activity of the intact myosin molecule. Here we show that the elementary steps of the S1dC ATPase pathway and the effects of actin are similar to those of the complete myosin head fragment. This indicates that truncation at residue E759, with the removal of the light chain binding sites, is not crucial to catalytic activity. In particular, S1dC does not show the anomolous tight binding of ADP displayed by slightly shorter M754 construct reported elsewhere. We also show that the fluorescent analogue Cy3-EDA-ATP is a good substrate for S1dC and demonstrate the use of fluorescence correlation spectroscopy to determine the affinity of Cy3-EDA-ADP using microgram quantities of proteins.     

Nucleotide and actin binding properties of the isolated motor domain from Dictyostelium discoideum myosin

Nucleotide and actin binding properties of the truncated myosin head (S1dC) from Dictyostelium myosin II were studied in solution using rabbit skeletal myosin subfragment 1 as a reference material. S1dC and subfragment 1 had similar affinities for ADP analogues, epsilon ADP and TNP-ADP. The complexes of epsilon ADP and BeFx or AIF4- were less stable with S1dC than with subfragment 1. Stern-Volmer constants for acrylamide quenching of S1dC complexes with epsilon ADP, epsilon ADP.AIF4- and epsilon ADP.BeFx were 2.6, 2.9 and 2.2 M-1, respectively. The corresponding values for subfragment 1 were 2.6, 1.5 and 1.1 M-1. The environment of the nucleotide binding site was probed by using a hydrophobic fluorescent probe, PPBA. PPBA was a competitive inhibitor of S1dC Ca(2+)-ATPase (Ki = 1.6 microM). The binding of nucleotides to subfragment 1 enhanced PPBA fluorescence and caused blue shifts in the wavelength of its maximum emission in the order: ATP approximately ADP.AIF4- approximately ADP.BeFx > ATP gamma S > ADP > PPi. In the case of S1dC, the effects of different nucleotides were smaller and indistinguishable from each other. S1dC bound actin tighter than S1 (Kd = 7 nM and 60 nM, respectively). The actin activated MgATPase activity of S1dC varied between preparations, and the Vmax and K(m) values ranged between 3 and 7 s-1 and 60 and 190 microM, respectively. S1dC showed lower structural stability than S1 as revealed by their thermal inactivations at 35 degrees C. These results show that the nucleotide and actin binding of S1dC and subfragment 1 are similar but there are some differences in nucleotide and phosphate analogue-induced changes and the communication between the nucleotide and actin binding sites in these proteins.     

Force-generating domain of myosin motor

To understand the underlying mechanism of force generation by myosin motor, it is crucial to know which part of the molecule is essential for the process. Recent structure determination of myosin motor domain at atomic resolution has revealed that the domain comprises two smaller domains, the "ATPase domain" consisting of only an N-terminal segment of the heavy chain and the "neck domain" consisting of a long alpha-helix of the heavy chain and two light chains. This atomic structure begs the question of whether both domains are required for force generation. To answer it, we genetically truncated the head to generate a recombinant fragment composed of the "ATPase domain" alone. The truncated head drove sliding movement of actin filaments and generated force in a novel in vitro assay system, which allows us to hold a specific site of the head on a glass surface. These results indicate that the compact ATPase domain functions as a force-generating machinery of the myosin motor.     

DNA polymerase of Dictyostelium discoideum

Only one molecular weight species of DNA polymerase was found in different developmental stages of the eukaryotic microorganism Dictyostelium discoideum. The molecular weight of this DNA polymerase is estimated to be about 127 000 by sucrose gradient centrifugation. The enzyme is present in all stages of growth and development, including dormant spores. All DNA polymerase activity is lost upon incubation of the crude extract with N-ethylmaleimide. The reaction properties, molecular weight and N-ethylmaleimide sensitivity of the D. discoideum DNA polymerase are similar to those of the DNA polymerase-alpha from mammalian sources.     

Refined X-ray structure of Dictyostelium discoideum nucleoside diphosphate kinase at 1.8 A resolution

The X-ray structure of the nucleoside diphosphate kinase (NDP kinase) from Dictyostelium discoideum has been refined at 1.8 A resolution from a hexagonal crystal form with a 17 kDa monomer in its asymmetric unit. The atomic model was derived from the previously determined structure of a point mutant of the protein. It contains 150 amino acid residues out of 155, and 95 solvent molecules. The R-factor is 0.196 and the estimated accuracy of the average atomic position, 0.25 A. The Dictyostelium structure is described in detail and compared to those of Drosophila and Myxococcus xanthus NDP kinases. The protein is a hexamer with D3 symmetry. Residues 8 to 138 of each subunit form a globular alpha/beta domain. The four-stranded beta-sheet is antiparallel; its topology is different from other phosphate transfer enzymes, and also from the HPr protein which, like NDP kinase, carries a phosphorylated histidine. The same topology is nevertheless found in several other proteins that bind mononucleotides, RNA or DNA. Strand connections in NDP kinase involve alpha-helices and a 20-residue segment called the Kpn loop. The beta-sheet is regular except for a beta-bulge in edge strand beta 2 and a gamma-turn at residue Ile120 just preceding strand beta 4. The latter may induce strain in the main chain near the active site His122. The alpha 1 beta 2 motif participates in forming dimers within the hexamer, helices alpha 1 and alpha 3, the Kpn loop and C terminus, in forming trimers. The subunit fold and dimer interactions found in Dictyostelium are conserved in other NDP kinases. Trimer interactions probably occur in all eukaryotic enzymes. They are absent in the bacterial Myxococcus xanthus enzyme which is a tetramer, even though the subunit structure is very similar. In Dictyostelium, contacts between Kpn loops near the 3-fold axis block access to a central cavity lined with polar residues and filled with well-defined solvent molecules. Biochemical data on point mutants highlight the contribution of the Kpn loop to protein stability. In Myxococcus, the Kpn loops are on the tetramer surface and their sequence is poorly conserved. Yet, their conformation is maintained and they make a similar contribution to the substrate binding site.     

Nitric oxide, an endogenous regulator of Dictyostelium discoideum differentiation

We have previously demonstrated that nitric oxide (NO)-generating compounds inhibit D. discoideum differentiation by preventing the initiation of cAMP pulses (Tao, Y., Howlett, A. and Klein, C. (1996) Cell. Signal. 8, 37-43). In the present study, we demonstrate that cells produce NO at a relatively constant rate during the initial phase of their developmental cycle. The addition of oxyhemoglobin, an NO scavenger, stimulates cell aggregation, suggesting that NO has a negative effect on the development of aggregation competence. Starvation of cells in the presence of glucose, which has been shown to prevent the initiation of cAMP pulses (Darmon, M. and Klein, C. (1978) Dev. Biol. 63, 377-389), results in an increased production of NO. The inhibition of cell aggregation by glucose treatment can be reversed by oxyhemoglobin. These findings indicate that NO is a signaling molecule for D. discoideum cells and that physiological or environmental conditions that enhance external NO levels will delay the initiation of cAMP pulses, which are essential for cell differentiation.     

Occupational asthma to the slime mold Dictyostelium discoideum

Dictyostelium discoideum is a slime mold that exists in a unicellular amoeboid form under certain nutritional conditions. In this form, it produces unique lysosomal enzymes that are valuable in studying cell-to-cell signaling systems. We report on a research microbiologist who developed rhinoconjunctivitis and asthma after release of D. discoideum from a pressurized canister. Immediate skin test reactivity was demonstrated to whole and lysed organisms. Enzyme-linked immunosorbent assay results revealed IgE antibody against D. discoideum whole organism, lysed organism, and lysosomal enzymes with the strongest response being directed toward lysosomal enzymes. Pulmonary function testing showed a decline in forced expiratory volume in 1 second and forced expiratory flow after modified laboratory exposure to D. discoideum. This case represents the first report of occupational rhinoconjunctivitis and asthma from slime mild.     

Crystal structure of the kinesin motor domain reveals a structural similarity to myosin

Kinesin is the founding member of a superfamily of microtubule based motor proteins that perform force-generating tasks such as organelle transport and chromosome segregation. It has two identical approximately 960-amino-acid chains containing an amino-terminal globular motor domain, a central alpha-helical region that enables dimer formation through a coiled-coil, and a carboxy-terminal tail domain that binds light chains and possibly an organelle receptor. The kinesin motor domain of approximately 340 amino acids, which can produce movement in vitro, is much smaller than that of myosin (approximately 850 amino acids) and dynein (1,000 amino acids), and is the smallest known molecular motor. Here, we report the crystal structure of the human kinesin motor domain with bound ADP determined to 1.8-A resolution by X-ray crystallography. The motor consists primarily of a single alpha/beta arrowhead-shaped domain with dimensions of 70 x 45 x 45 A. Unexpectedly, it has a striking structural similarity to the core of the catalytic domain of the actin-based motor myosin. Although kinesin and myosin have virtually no amino-acid sequence++ identity, and exhibit distinct enzymatic and motile properties, our results suggest that these two classes of mechanochemical enzymes evolved from a common ancestor and share a similar force-generating strategy.     

Isolation and characterization of glycogen synthase in Dictyostelium discoideum

We have partially purified the protein and isolated the glcS gene for glycogen synthase in Dictyostelium. glcS mRNA is present throughout development and is the product of a single gene coding for 775 amino acids, with a predicted molecular mass of 87 kD. The sequence is highly similar to glycogen synthase from human muscle, yeast, and rat liver, diverging significantly only at the amino and carboxy termini. Phosphorylation and UDPG binding sites are conserved, with K(m) values for UDPG being comparable to those determined for other organisms, but in vitro phosphorylation failing to convert between the G6P-dependent (D) and -independent (I) forms. Enzyme activity is relatively constant throughout the life cycle: the I form of the enzyme isolates with the soluble fraction in amoebae, switches to the D form, becomes pellet-associated during early development, and finally reverts during late development to the I form, which again localizes to the soluble fraction. Deletion analysis of the promoter reveals a GC-rich element which, when deleted, abolishes expression of glcS.     

Production and secretion of recombinant proteins in Dictyostelium discoideum

We have expressed useful amounts of three recombinant proteins in a new eukaryotic host/vector system. The cellular slime mold Dictyostelium discoideum efficiently secreted two recombinant products, a soluble form of the normally cell surface associated D. discoideum glycoprotein (PsA) and the heterologous protein glutathione-S-transferase (GST) from Schistosoma japonicum, while the enzyme beta-glucuronidase (GUS) from Escherichia coli was cell associated. Up to 20mg/l of recombinant PsA and 1mg/l of GST were obtained after purification from a standard, peptone based growth medium. The secretion signal peptide was correctly cleaved from the recombinant GST- and PsA-proteins and the expression of recombinant PsA was shown to be stable for at least one hundred generations in the absence of selection.     

Kinetic analysis of biochemical differentiation in Dictyostelium discoideum

The metabolic network leading to accumulation of cellulose, trehalose, and mucopolysaccharide during development of Dictyostelium discoideum was simulated on a computer. The program consists of a metabolic map, the measured specific activity of the enzymes involved at each stage in development, and the substrate and inhibitor affinities. The Km values of four enzymes, amylase, UDP-galactose polysaccharide transferase, UDP-galactose epimerase, and cellulose synthetase, were determined for this study. At each iteration (1 min) during the period simulated (1500 min), the in vivo activity was calculated for each enzyme using Michaelis-Menten equations and new values for metabolites and end products were generated. The computed values for the concentration of both metabolites and polysaccharides were in close agreement with the measured values at all stages of development. We conclude that the in vitro measured values correlate well with the measured in vivo rates when treated in this manner. The program was modified to simulate the alterations in carbohydrate metabolism which might be expected in mutant strains with reduced activity of various enzymes. Trehalose was found to overaccumulate when either the peak value of the developmentally controlled increase in the specific activity of UDPGlc pyrophosphorylase was reduced. Trehalose accumulation was decreased in simulations of mutants lacking glycogen phosphorylase or glycogen synthetase. The interaction of these metabolic pathways is discussed.     

Glycoproteins of the plasma membrane of Dictyostelium discoideum during development

Polysomal RNA from cultured sublines of baby hamster kidney (BHK) cells directed protein synthesis in an in vitro system derived from wheat germ extract. One product of the in vitro synthesis was dihydrofolate reductase (DHFR), as confirmed by methotrexate-substituted Sepharose affinity chromatography followed by SDS-polyacrylamide slab gel electrophoresis and autoradiography of the proteins labeled with 35S-methionine. The DHFR synthesized in vitro comigrates in the gel with authentic BHK DHFR, indicating that the molecular weights and structures of the in vivo and in vitro enzymes are probably the same. Polysomal RNA obtained from the methotrexate-resistant BHK subline (A5), which possesses some 140 times higher DHFR levels than the methotrexate-sensitive parents subline (B1), directed the synthesis of approximately 70 times more DHFR per unit of total in vitro synthesized protein than did B1 polysomal RNA. Assuming then that the rates of translation of A5 and B1 DHFR mRNAs in the wheat germ cell-free system are the same, our results show that a major part of the high DHFR levels observed in A5 cells is due to the presence of elevated quantities of DHFR mRNA.     

Structural roles of the spore coat proteins in Dictyostelium discoideum

The integrity of spores formed by mutant strains of Dictyostelium discoideum lacking the major spore coat proteins, SP96, SP70, or SP60, was compared to that of wild-type strains. Single, double, and triple knock-out strains developed normally and produced spores which were indistinguishable from wild-type spores by light or electron microscopy. However, the mutant strains were susceptable to staining with the lectin, ricin A, which recognizes a galactose-rich polysaccharide that is normally hidden by overlying spore coat proteins. The intensity of staining with fluorescently labeled ricinA increased as the spore coat proteins were incrementally lost. While these results indicate that the major outer spore coat proteins are not essential for the construction of a multi-layered spore coat in Dictyostelium, they show that the spores are more porous which might make them at risk to predators before germination.     

Cloning and characterization of a rhoGAP homolog from Dictyostelium discoideum

Small GTPases interact with a variety of proteins that affect nucleotide binding and cleavage. GTPase activating proteins (GAPs) are one class of these proteins that act by accelerating the intrinsic GTPase rate resulting in the formation of the biologically inactive GDP-bound form of the GTPase. For the Rho subfamily of GTPases, there is a growing number of proteins with rhoGAP activity that are identifiable by a homologous region of about 150 amino acids. We have exploited this homology using the polymerase chain reaction to clone the first rhoGAP homolog, called DdRacGAP, from the slime mold Dictyostelium discoideum. The GAP domain of DdRacGAP (amino acids 1-212), when expressed and purified from Escherichia coli, is active on both Dictyostelium and human Rho family GTPases but not human Ras. The full-length protein is 1356 amino acids in length and has several interesting homologies in addition to the GAP domain, including an SH3 domain, a dbl homology domain, and a pleckstrin homology domain.     

Modulation of actin affinity and actomyosin adenosine triphosphatase by charge changes in the myosin motor domain

The effects of mutations in an actin-binding surface loop of myosin (loop 2) are described. Part of loop 2, the segment between myosin residues 618 and 622, was replaced with sequences enlarged by the introduction of positively charged GKK or neutral GNN motifs. Constructs with loops carrying up to 20 additional amino acids and charge variations from -1 to +12 were produced. Steady-state and transient kinetics were used to characterize the enzymatic behavior of the mutant motor domains. Binding of nucleotide was not affected by any of the alterations in loop 2. In regard to their interaction with actin, constructs with moderate charge changes (-1 to +2) displayed wild-type-like behavior. Introduction of more than one GKK motif led to stronger coupling between the actin- and nucleotide-binding sites of myosin and an up to 1000-fold increased affinity for actin in the absence of ATP and at zero ionic strength. In comparison to the wild-type construct M765, constructs with 4-12 extra charges displayed an increased dependence on ionic strength in their interaction with actin, a 2-3-fold increase in kcat, a more than 10-fold reduction in Kapp for actin, and a 34-70-fold increase in catalytic efficiency.     

Nucleotide sequence of alkyl-dihydroxyacetonephosphate synthase cDNA from Dictyostelium discoideum

OBJECTIVE AND IMPORTANCE: Congenital anomalies of the posterior arch of the atlas (C1) are uncommon. They range from partial clefts to total agenesis of the posterior arch. Developmental cervical canal stenosis is a congenital anomaly that may cause cervical myelopathy. Myelopathy caused by cervical stenosis at the level of the atlas has been reported in only three cases. We present two cases of nontraumatic cervical myelopathy caused by spinal stenosis at the level of the atlas associated with a hypoplastic but complete posterior arch of C1. CLINICAL PRESENTATION: Two elderly Chinese men developed cervical myelopathy gradually during months to years, without preceding trauma. Imaging revealed a hypoplastic but complete posterior C1 arch associated with changes of spondylosis in both patients, producing severe spinal stenosis and spinal cord compression. Posterior decompression was achieved in both by the removal of the posterior arch of C1 with its surrounding thickened posterior ligaments. Symptoms and clinical findings improved in the two patients during the follow-up period. CONCLUSION: The anomaly presented in our two cases differs from the established classification of congenital abnormalities of the posterior arch of the atlas, suggesting a different embryological defect. The hypoplastic posterior C1 arch created a congenitally narrowed spinal canal in our patients, rendering the spinal cord more susceptible to compression related to degenerative changes of the spine. Surgical removal of the shortened posterior C1 arch and surrounding degenerative ligaments is an effective treatment for symptomatic patients with this condition.     

A continuum analysis of the chemotactic signal seen by Dictyostelium discoideum

We developed a mathematical model of cell-to-cell-signalling in Dictyostelium discoideum that predicts the cAMP signal seen by individual cells in early aggregation. The model employs two cells on a plane and is designed to predict the space-time characteristics of both the extracellular cAMP signal seen by one cell when a nearby cell relays, and the intracellular cAMP response produced by the stimulus in the receiving cell. The effect of membrane bound phosphodiesterase is studied and it is shown that cells can orient effectively even in its absence. Our results give a detailed picture of how the spatio-temporal characteristics of the extracellular signal can be transduced into a time- and space-dependent intracellular gradient, and they suggest a plausible mechanism for orientation in a natural chemotactic wave.     

Disruption of a dynamin homologue affects endocytosis, organelle morphology, and cytokinesis in Dictyostelium discoideum

The identification and functional characterization of Dictyostelium discoideum dynamin A, a protein composed of 853 amino acids that shares up to 44% sequence identity with other dynamin-related proteins, is described. Dynamin A is present during all stages of D. discoideum development and is found predominantly in the cytosolic fraction and in association with endosomal and postlysosomal vacuoles. Overexpression of the protein has no adverse effect on the cells, whereas depletion of dynamin A by gene-targeting techniques leads to multiple and complex phenotypic changes. Cells lacking a functional copy of dymA show alterations of mitochondrial, nuclear, and endosomal morphology and a defect in fluid-phase uptake. They also become multinucleated due to a failure to complete normal cytokinesis. These pleiotropic effects of dynamin A depletion can be rescued by complementation with the cloned gene. Morphological studies using cells producing green fluorescent protein-dynamin A revealed that dynamin A associates with punctate cytoplasmic vesicles. Double labeling with vacuolin, a marker of a postlysosomal compartment in D. discoideum, showed an almost complete colocalization of vacuolin and dynamin A. Our results suggest that that dynamin A is likely to function in membrane trafficking processes along the endo-lysosomal pathway of D. discoideum but not at the plasma membrane.