THE INFLUENCE OF THE BACKWARD REACTION IN THE PEPTIC HYDROLYSIS OF ALBUMIN

1. No destruction of pepsin by heat is demonstrable at pH 1.6 until a temperature of 40°C. is exceeded. 2. The influence of the backward reaction in peptic hydrolysis is shown in the diminishing rate at which increasing concentrations of protein are hydrolyzed. 3. The backward reaction causes the optimum for the hydrolysis of higher concentrations of protein to be attained at a lower temperature than with more dilute solutions. 4. The proteose and peptone associated with commercial pepsin retard hydrolysis in the same sense as the products due to the action of the enzyme.     

Mechanisms of Self-Organization of Cortical Microtubules in Plants Revealed by Computational Simulations

Microtubules confined to the two-dimensional cortex of elongating plant cells must form a parallel yet dispersed array transverse to the elongation axis for proper cell wall expansion. Some of these microtubules exhibit free minus-ends, leading to migration at the cortex by hybrid treadmilling. Collisions between microtubules can result in plus-end entrainment (“zippering”) or rapid depolymerization. Here, we present a computational model of cortical microtubule organization. We find that plus-end entrainment leads to self-organization of microtubules into parallel arrays, whereas catastrophe-inducing collisions do not. Catastrophe-inducing boundaries (e.g., upper and lower cross-walls) can tune the orientation of an ordered array to a direction transverse to elongation. We also find that changes in dynamic instability parameters, such as in mor1-1 mutants, can impede self-organization, in agreement with experimental data. Increased entrainment, as seen in clasp-1 mutants, conserves self-organization, but delays its onset and fails to demonstrate increased ordering. We find that branched nucleation at acute angles off existing microtubules results in distinctive sparse arrays and infer either that microtubule-independent or coparallel nucleation must dominate. Our simulations lead to several testable predictions, including the effects of reduced microtubule severing in katanin mutants.     

Transient exposure to ethylene stimulates cell division and alters the fate and polarity of hypocotyl epidermal cells

After transient exposure to the gaseous hormone ethylene, dark-grown cucumber (Cucumis sativus) hypocotyls developed unusual features. Upon ethylene's removal, the developing epidermis showed significant increases in cell division rates, producing an abundance of guard cells and trichomes. These responses to ethylene depended on the stage of development at the time of ethylene exposure. In the upper region of the hypocotyl, where cells were least differentiated at the onset of ethylene treatment, complex, multicellular protuberances formed. Further down the hypocotyl, where stomata and trichomes were beginning to develop at the onset of ethylene exposure, an increase in the number of stomata and trichomes was observed. Stomatal complexes developing after the ethylene treatment had a significant increase in the number of stomatal subsidiary cells and the number of cells per trichome increased. Analysis of division patterns in stomatal complexes indicated that exposure to ethylene either suspended or altered cell fate. Ethylene also altered cell division polarity, resulting in aberrant stomatal complexes and branched trichomes. To our knowledge, the results of this study demonstrate for the first time that transient treatment with physiological concentrations of ethylene can alter cell fate and increase the propensity of cells to divide.     

Nuclear crystals, lampbrush-chromosome-like structures, and perinuclear cytoskeletal elements associated with nuclear fragmentation in characean internodal cells

Summary This article gives a survey of nucleus-associated structures and inclusions in a diverse range of characean algae includingChara braunii Gm.,Chara corallina Klein ex Willd., em. R.D.W.,Nitella cristata A.Br., em. R.D.W.,Nitella flexilis (L.) Ag.,Nitella furcata (Roxb. ex Bruz.) Ag. em. R.D.W.,Nitella hyalina (DC.) Ag.,Nitella pseudoflabellata A.Br., em. R.D.W.,Nitella pseudoflabellata var.imperialis T.F.A.,Nitella translucens var.axillaris (A.Br.) R.D.W. andNitellopsis obtusa (Desv. in Lois.) J.Gr. Lampbrushchromosome-like structures were found in nuclei ofNitella flexilis andNitellopsis obtusa and seem to be involved in the distribution of genetic material during nuclear fragmentation. Intranuclear tubular crystals of unknown protein composition were present in all species, especially in young, elongating cells, and could be important for establishing the main axis of the nuclei. Spindle-shaped protein crystals that originate in the nucleus and are released into the cytoplasm upon nuclear degeneration were observed in branchlet internodal cells of one population ofNitella flexilis. Perinuclear microtubules were present in all species, but perinuclear actin fibrils were hitherto only found in mostNitella species and inNitellopsis obtusa. None of these nucleus-associated structures seems to be responsible for the formation of constrictions leading to nuclear fragmentation. These constrictions were perpendicular to the main axis of the nucleus and symmetrical in theNitella species but asymmetric inC. braunii, C. corallina, and inNitellopsis obtusa. Statistical analysis of nuclear size, number and constriction sites indicate that fragmentation is a nonsynchronous process independent of the light-dark cycle.Abbreviations CLSM confocal laser scanning microscopy - DAPI 4,6-diamidino-2-phenylindole - DIC Nomarski differential interference contrast - LCLS lampbrush chromosome-like structure(s) Dedicated to Professor Walter Gustav Url on the occasion of his 70th birthday     

Cytochalasin Rearranges Cortical Actin of the Alga Nitella into Short, Stable Rods

The cortical cytoplasm of the alga Nitella contains reticulateactin that does not survive perfusion fixation with glutaraldehydeunless prestabilized with the cross-linker 3-maleimidobenzoyl-N-hydroxysuccinimidester (MBS). Cytochalasin D remodels thiscortical actin into short rods which are more stable, survivingaldehyde fixation without MBS pre-treatment. The overall alignmentof these actin rods correlates with that of cortical microtubules(transverse in young cells, random in old cells) but probablydoes not involve one-to-one correspondence. The time course,dose dependence and reversibility of these structural changesbroadly resemble those for streaming inhibition by cytochalasinbut the cortical actin responds to concentrations that do notslow streaming. Because the structural changes concern the corticaland not the subcortical actin, they seem unlikely to directlyinhibit streaming. Formation of cortical rods is not a responseto streaming inhibition per se since it does not occur whentwo other inhibitors of streaming (2,4-dinitrophenol (DNP) andNethyl maleimide (NEM)) are used. NEM, however, resembles MBSin stabilizing the reticulate form of cortical actin even thoughit cannot cross link. ¹Address from July 1995; Department of Biology, Faculty of Science,Osaka University, Machikaneyama 1-1, Tayonaka, Osaka, 560 Japan.     

Effect of neighbour visibility on nest attendance patterns of Barn Swallows Hirundo rustica in loose colonies

To assess the benefits of nesting at a site hidden from neighbours in a loosely colonial species, the Barn Swallow Hirundo rustica, we carried out two field experiments, obstruction removal and mirror placement, both replicating a situation in which a nest is made visible from another nest. Under manipulated conditions in both experiments, females increased the length of time they stayed at their nests during the egg‐laying and late incubation stages, while males extended their duration of stay during the egg‐laying and early nestling stages in mirror placement experiments only. The results suggest that Barn Swallows conceal their nests to reduce fitness costs imposed by neighbours nesting in view and that hiding the nest can reduce the amount of time spent guarding the nest during certain stages of the breeding period.     

Gibberellin-induced changes in growth anisotropy precede gibberellin-dependent changes in cortical microtubule orientation in developing epidermal cells of barley leaves. Kinematic and cytological studies on a gibberellin-responsive dwarf mutant, M489

We conducted kinematic and cytological studies on "between vein" epidermal cells of the gibberellin (GA)-deficient M489 dwarf mutant of barley (Hordeum vulgare L. Himalaya). GAs affect radial and axial components of cell expansion and cortical microtubule orientation. Adaxial cells in particular expand radially after leaving the elongation zone (EZ), probably as part of leaf unrolling. Exogenous gibberellic acid corrects the mutant's short, wide blades, short EZ, and slow elongation rate. Cell production rates increase more on the adaxial than on the abaxial surface. Cells spend equal periods of time elongating in dwarf and tall plants, but relative elemental growth rates start to decline sooner in the dwarf. GA increased the rate at which longitudinal wall area increased because the increased axial growth more than compensated for reduced radial growth. In dwarf leaves, increased radial expansion was detected in basal parts of the EZ before cortical microtubules lost transverse orientation in the distal elongation zone. We conclude that loss of microtubule orientation is not required for low GA levels to reduce growth anisotropy.     

Failure of catecholamines to shift T-cell cytokine responses toward a Th2 profile in patients with rheumatoid arthritis

To further understand the role of neuro-immunological interactions in the pathogenesis of rheumatoid arthritis (RA), we studied the influence of sympathetic neurotransmitters on cytokine production of T cells in patients with RA. T cells were isolated from peripheral blood of RA patients or healthy donors (HDs), and stimulated via CD3 and CD28. Co-incubation was carried out with epinephrine or norepinephrine in concentrations ranging from 10(-5) M to 10(-11) M. Interferon (IFN)-gamma, tumour necrosis factor (TNF)-alpha, interleukin (IL)-4, and IL-10 were determined in the culture supernatant with enzyme-linked immunosorbent assay. In addition, IFN-gamma and IL-10 were evaluated with intracellular cytokine staining. Furthermore, basal and agonist-induced cAMP levels and catecholamine-induced apoptosis of T cells were measured. Catecholamines inhibited the synthesis of IFN-gamma, TNF-alpha, and IL-10 at a concentration of 10(-5) M. In addition, IFN-gamma release was suppressed by 10(-7) M epinephrine. Lower catecholamine concentrations exerted no significant effect. A reduced IL-4 production upon co-incubation with 10(-5) M epinephrine was observed in RA patients only. The inhibitory effect of catecholamines on IFN-gamma production was lower in RA patients as compared with HDs. In RA patients, a catecholamine-induced shift toward a Th2 (type 2) polarised cytokine profile was abrogated. Evaluation of intracellular cytokines revealed that CD8-positive T cells were accountable for the impaired catecholaminergic control of IFN-gamma production. The highly significant negative correlation between age and catecholamine effects in HDs was not found in RA patients. Basal and stimulated cAMP levels in T-cell subsets and catecholamine-induced apoptosis did not differ between RA patients and HDs. RA patients demonstrate an impaired inhibitory effect of catecholamines on IFN-gamma production together with a failure to induce a shift of T-cell cytokine responses toward a Th2-like profile. Such an unfavorable situation is a perpetuating factor for inflammation.     

The cytoskeleton and growth polarity

We are currently witnessing the discovery of many novel proteins that are associated with cytoskeletal activity. Integrated analyses of growth, cytoskeletal and cell-wall patterns are yielding surprising results, which demand reflection on the current model for wall construction. Meanwhile, research on actin filament and microtubule activity during gravitropic bending and trichome morphogenesis is stimulating new ideas about the establishment and maintenance of polarity.