No abstract

The toxicity of Cd^^ in vivo during the early phases of radish {Raphanus sativus L.) seed germination and the in vitro Cd^* effect on radish caimodulin (CaM) were studied. Cd^"^ was taken up in the emhryo axes of radish seeds; the increase in fresh weight of emhryo axes after 24 h of incubation was inhihited significantly in the presence of 10 mmol m~^^ Cd^* in the external medium, when the Cd^* content in the embryo axes was c. 1-1 |imol g~' FW. The reabsorption of K*, which characterizes germination, was inhibited by Cd^*, suggesting that Cd^* affected metabolic reactivation. The slight effect of Cd^^ on the transmembrane electric potential of the cortical cells of the emhryo axes excluded a generalized toxicity of Cd^* at the plasma membrane level. After 24 h of incubation, Cd^* induced no increase in total acid-soluble thiols and Cd^*-binding peptides able to reduce Cd^^ toxicity. Ca^* added to the incubation medium partially reversed the Cd^'^-induced inhibition of the increase in fresh weight of embryo axes and concomitantly reduced Cd^* uptake. Equilibrium dialysis experiments indicated that Cd^^ bound to CaM and competed with Ca^^ in this binding. Cd^* inhibited the activation of Ca^^-CaM-dependent calf-brain phosphodiesterase, inhibiting the Ca^^-CaM active complex. Cd^r educed the binding of CaM to the Ca'^^-CaM binding enzymes present in the soluble fraction of the embryo axes of radish seeds. The possibility that Cd^* toxicity in radish seed germination is mediated by the action of Cd^* on Ca *-CaM is discussed in relation to the in vivo and in vitro effects of Cd^K

No abstract

The TRK proteins-Trk1p and Trk2p- are the main agents responsible for "active" accumulation of potassium by the yeast Saccharomyces cerevisiae. In previous studies, inward currents measured through those proteins by whole-cell patch-clamping proved very unresponsive to changes of extracellular potassium concentration, although they did increase with extracellular proton concentration-qualitatively as expected for H(+) coupling to K(+) uptake. These puzzling observations have now been explored in greater detail, with the following major findings: a) the large inward TRK currents are not carried by influx of either K(+) or H(+), but rather by an efflux of chloride ions; b) with normal expression levels for Trk1p and Trk2p in potassium-replete cells, the inward TRK currents are contributed approximately half by Trk1p and half by Trk2p; but c) strain background strongly influences the absolute magnitude of these currents, which are nearly twice as large in W303-derived spheroplasts as in S288c-derived cells (same cell-size and identical recording conditions); d) incorporation of mutations that increase cell size (deletion of the Golgi calcium pump, Pmr1p) or that upregulate the TRK2 promoter, can further substantially increase the TRK currents; e) removal of intracellular chloride (e.g., replacement by sulfate or gluconate) reveals small inward currents that are K(+)-dependent and can be enhanced by K(+) starvation; and f) finally, the latter currents display two saturating kinetic components, with preliminary estimates of K(0.5) at 46 micro M [K(+)](out) and 6.8 m M [K(+)](out), and saturating fluxes of approximately 5 m M/min and approximately 10 m M/min (referred to intracellular water). These numbers are compatible with the normal K(+)-transport properties of Trk1p and Trk2p, respectively.

Background: Fluoride is broadly toxic, and organisms use fluoride export (FEX) proteins to expel it. Results: FEX is a constitutively expressed fluoride channel, and mutations to the C-and N-terminal domains have asymmetric effects. Conclusion: Protection from fluoride is constantly needed, and a positive residue in the membrane is required. Significance: Understanding FEX furthers our knowledge of fluoride resistance mechanisms.