Actin' like actin?

The most biologically significant property of actin is its ability to self-associate and form two-stranded polymeric microfilaments. In living cells, these micro filaments form the actin cytoskeleton, essential for maintenance of the shape, passive mechanical properties and active motility of eukaryotic cells. Recently discovered actin-related proteins (ARPs) appear to share a common ancestor with conventional actin. At present, six classes of ARPs have been discovered, three of which have representatives in diverse species across eukaryotic phyla and may share functional characteristics with conventional actin. The three most ubiquitous ARPs are predicted to share a common core structure with actin and contain all the residues required for ATP binding. Surface residues involved in protein protein interactions, however, have diverged. Models of these proteins based on the atomic structure of actin provide some clues about how ARPs interact with each other, with conventional actin and with conventional actin-binding proteins.     

Seasonal and long-term soil water regime in West African tropical forest

Abstract. Variation of soil matric potential of a Wet Evergreen and a Moist Semi-deciduous West African forest were compared. The two forest types differed strongly in their soil water regime. Wet Evergreen forest experienced matric potentials below ?100 kPa only occasionally, while in Moist Semi-deciduous forest matric potentials were less than ?2.5 MPa for periods of several weeks or more each season. A water balance equation was used to simulate the soil water regime at both sites and predict severity and length of the dry period. The predictions showed good agreement with the field measurements of soil water potential over a 2-yr period. The methodology was used to estimate the occurrence and severity of droughts over longer periods at the two sites. The balance calculations suggest that droughts occur occasionally in the Wet Evergreen forest under study. The potential impact of droughts on species distribution and vegetation disturbance in tropical forests is discussed.     

Caprine Arthritis-Encephalitis Virus Is Distinct from Visna and Progressive Pneumonia Viruses as Measured by Genome Sequence Homology

Competitive inhibition of hybridization between ¹²⁵I-labeled caprine arthritis-encephalitis viral RNA and homologous cDNA by heterologous viral RNA shows that the caprine retrovirus shares <20% genome sequence homology with visna and progressive pneumonia viruses. These viruses, however, are indistinguishable in immunodiffusion reactions involving the major structural protein (p28).     

Characterization of the zinc binding site of bacterial phosphotriesterase.

The bacterial phosphotriesterase has been found to require a divalent cation for enzymatic activity. This enzyme catalyzes the detoxification of organophosphorus insecticides and nerve agents. In an Escherichia coli expression system significantly higher concentrations of active enzyme could be produced when 1.0 mM concentrations of Mn2+, Co2+, Ni2+, and Cd2+ were included in the growth medium. The isolated enzymes contained up to 2 equivalents of these metal ions as determined by atomic absorption spectroscopy. The catalytic activity of the various metal enzyme derivatives was lost upon incubation with EDTA, 1,10-phenanthroline, and 8-hydroxyquinoline-5-sulfonic acid. Protection against inactivation by metal chelation was afforded by the binding of competitive inhibitors, suggesting that at least one metal is at or near the active site. Apoenzyme was prepared by incubation of the phosphotriesterase with beta-mercaptoethanol and EDTA for 2 days. Full recovery of enzymatic activity could be obtained by incubation of the apoenzyme with 2 equivalents of Zn2+, Co2+, Ni2+, Cd2+, or Mn2+. The 113Cd NMR spectrum of enzyme containing 2 equivalents of 113Cd2+ showed two resonances at 120 and 215 ppm downfield from Cd(ClO4)2. The NMR data are consistent with nitrogen (histidine) and oxygen ligands to the metal centers.     

A freeze-fracture study of hormone-induced branching in the fungus Achlya.

The induction of the male sexual organ primordia (antheridial hyphae) by the steriod hormone antheridiol in the water mold Achlya ambisexualis Raper requires the production and secretion of the enzyme cellulase. It is postulated that a localized secretion of cellulase produces a limited area of wall hydrolysis that is blown out into a lateral bleb by turgor pressure. Freeze-etch preparations show membrane profiles similar to those seen in other systems where exocytosis is occurring. Such a mechanism would provide the required localized secretion of cellulase. Water stress, imposed by polyethylene glycol, prevents the formation of antheridial hyphae, the secretion of cellulase and the expected membrane profiles. After a period of recovery from water stress antheridial hyphae are formed, cellulase secretion occurs and the expected membrane profiles are restored.     

Developmental progression of myosin gene expression in cultured muscle cells

Myosin heavy chains are encoded by distinct members of a multigene family at different stages of muscle development. Study of the underlying regulatory mechanisms has been hindered because transitions in myosin expression have not been readily attained in tissue culture. Here we show a transition from early (fetal) to late (perinatal/adult) myosins defined by two monoclonal antibodies, F1.652 and N3.36, in the myotubes of mouse C2C12 cells. On day 1 of differentiation, essentially all myosin was early myosin. By day 8, early myosin dropped to 25% of its day 1 value and was replaced by late myosin. The transition occurred without neural contact, connective tissue components, or complex substrates, suggesting that its regulation may be intrinsic to the muscle cell. Our results demonstrate that a developmental progression in myosin gene expression, which occurs rapidly, with high frequency, and under relatively simple conditions, is now amenable to molecular analysis in cultured muscle cells.     

D2O induced alterations of mitosis in PtK1 cells

Deuterium oxide (D2O) was applied to PtK1 cells to assess its effect on mammalian mitosis. Cells exposed to culture medium containing up to 50% D2O were able to enter and complete mitosis, but the duration of mitosis was increased proportionally to the concentration of D2O applied. Cells exposed to 50% D2O showed increases of more than 300% for the interval between nuclear envelope breakdown and anaphase onset, and approximately 65% for the interval between anaphase onset and initial furrowing. At a concentration of 80%, D2O acted as an inhibitor of mitosis; after 8 h exposure to this concentration, cultures showed an increase in the proportion of mulinucleate cells and an absence of mitotic figures. When applied early in anaphase, 80% D2O effectively slowed chromosome separation, prolonging anaphase for more than 60 min. Normal chromosome motion was restored when medium containing D2O was replaced with control medium. Mitotic chromosomes remained condensed throughout prolonged anaphase intervals. Immunofluoresence examination of spindles stained using a monoclonal anti-tubulin revealed no pronounced increase in microtubule polymerization after exposure of cells to 20-80% D2O.