Phosphate transport in Halobacterium halobium depends on cellular ATP levels.

Freshly harvested Halobacterium halobium cells grown in the presence of 0.5 mM Pi took up phosphate with a low apparent Km. Import depended on intracellular ATP levels; sodium and proton (electro)chemical gradients alone were not competent to drive Pi uptake. Although most of the phosphate accumulated as Pi in the cells, efflux of Pi was difficult to achieve.     

Signal transduction in erythropoiesis.

The polypeptide hormone erythropoietin (Ep) is a growth factor whose actions on the erythroid progenitor cell induce proliferation and differentiation. The signal transduction system activated by Ep to mediate these cellular processes remains largely uncharacterized despite many years of research devoted to its elucidation. It is clear that an Ep receptor-mediated activation of adenylate cyclase or guanylate cyclase does not occur, although cAMP and cGMP may play modulatory roles. The role of calcium in the action of Ep is less clear. Although the presence of extracellular calcium seems to be an absolute requirement for Ep-induced proliferation, the positive changes induced by Ep in intracellular calcium occur with a time course suggestive of influx through ion channels opening within the cell membrane rather than release of intracellular stores by inositol trisphosphate. There is good evidence for the involvement of phospholipases A2 and C in the actions of Ep, including an early rise in lipoxygenase metabolites of arachidonic acid. Activation of phospholipase C can also result in the activation of protein kinase C in response to Ep. We present a model for the signal transduction pathway of Ep that is consistent with current knowledge and provides a framework for the coordinate actions of several intracellular mechanisms in the mediation of Ep-induced proliferation.     

Signal transduction in photoreceptors.

The biochemical role of the visual-pigment protein, rhodopsin, is reviewed, with reference to vertebrate rods and cones and the microvillar photoreceptors of invertebrates. New results are presented on the structure of squid rhodopsin, which possesses an extensive proline-rich repeat at its C-terminus, using negative-stain electron microscopy.     

Signal transduction in thymus development.

Reciprocal interaction between bone marrow derived lymphoid precursor cells and the thymic environment leads, through a series of developmental events, to the generation of a diverse repertoire of functional T-cells. During thymopoiesis fetal liver or bone marrow derived precursors enter the thymus and develop into mature T-cells in response to cues derived from the environment. The thymic micro-environment provides signals to the lymphoid cells as a result of cell-cell interactions, locally produced cytokines, chemokines and hormones. Developing thymocytes, in turn, influence the thymic stroma to form a supportive micro-environment. Stage-specific signals provide an exquisite balance between cellular proliferation, differentiation, cell survival and death. The result of this intricate signaling concert is the production of the requisite numbers of well educated self-restricted T-cells. Mature T-cells are exported to the peripheral lymphoid organs, where, upon encountering antigen, naive T-cells further mature into effector cells that provide cytolytic or T helper functions. While there are extra-thymic locations for T-cell development, majority of T-cells in peripheral lymphoid organs are thymus derived. In mice and humans, T-cells develop throughout life although the efficacy declines significantly with age. It is not clear if this is a direct consequence of deterioration of the thymic environment by involution, a paucity of bone marrow derived precursors, or both. However, new data clearly shows that the involuted adult thymus retains the ability to generate new T-cells. Recent advances have revealed many components of an exquisitely balanced signaling cascades that regulate cell fate, cellular proliferation and cell death in the thymus. This article describes fundamental features of developing thymocytes and the thymic micro-environment as they relate to the signaling pathways.     

Signal transduction in plant cells.

In plants, unlike animals, signal transduction studies are in their infancy. While intracellular Ca2+ appears to have second messenger functions, attempts to show that protein kinases, inositol phosphates and cyclic AMP are involved in signal transduction in plants have run into considerable difficulty.     

Sensory rhodopsin-controlled release of the switch factor fumarate in Halobacterium salinarium

Halobacterium salinarium responds to blue light by reversing its swimming direction. Fumarate has been proposed as one of the molecular components of this sensory system and is involved in the switching process of the flagellar motor. In order to obtain chemical proof for this role of fumarate, cells were stimulated with a pulse of blue light and lysed by rapid mixing with distilled water. The lysate contained fumarate in free and bound form, which were separated by ultrafiltration. The fumarate concentration in the low-molecular-mass fraction (<5 kDa) of the lysate was assayed enzymatically and a light-induced increase was observed. Additionally, the total cellular fumarate content decreased in response to light, indicating that fumarate was released from a cellular pool rather than being formed by de novo synthesis. The light-induced release was not detected in a mutant defective in sensory rhodopsin-l and -II. Therefore it is concluded that photoreceptor activation rather than a direct effect of light on the activity of metabolic enzymes causes fumarate release. For each photoactivated sensory rhodopsin-II molecule at least 350 molecules of fumarate were liberated demonstrating efficient amplification. The rate of light-induced fumarate release is at least 10-times faster than the fumarate turnover number of the citric acid cycle which was estimated as approximately 4300 per cell and second. Therefore this metabolic process is not expected to be part of the signal transduction chain in the halobacterial cell.     

The proton pump bacteriorhodopsin is a photoreceptor for signal transduction in Halobacterium halobium.

Halobacterium halobium swims by rotating its polarly inserted flagellar bundle. The cells are attracted by green-to-orange light which they can use for photophosphorylation but flee damaging blue or ultraviolet light. It is generally believed that this kind of 'colour vision' is achieved by the combined action of two photoreceptor proteins, sensory rhodopsins-I and -II, that switch in the light the rotational sense of the bundle and in consequence the swimming direction of a cell. By expressing the bacteriorhodopsin gene in a photoreceptor-negative background we have now demonstrated the existence of a proton-motive force sensor (protometer) and the function of bacteriorhodopsin as an additional photoreceptor covering the high intensity range. When the bacteriorhodopsin-generated proton-motive force drops caused by a sudden decrease in light intensity, the cells respond by reversing their swimming direction. This response does not occur when the proton-motive force is saturated by respiration or fermentation.     

Signal transduction and regulation of neurotrophins.

Our understanding of the molecular nature of neurotrophic interactions has been greatly enhanced by the recent isolation and characterization of several new neurotrophic factors and their receptors. Neurotrophic factors have been found to be regulated by neuronal activity in the central nervous system, and may be involved in activity-dependent processes throughout development and maturity.