32 research outputs found
Independent Ion Migration in Suspensions of Strongly Interacting Charged Colloidal Spheres
We report on sytematic measurements of the low frequency conductivity in
aequous supensions of highly charged colloidal spheres. System preparation in a
closed tubing system results in precisely controlled number densities between
1E16/m3 and 1E19/m^3 (packing fractions between 1E-7 and 1E-2) and electrolyte
concentrations between 1E-7 and 1E-3 mol/l. Due to long ranged Coulomb
repulsion some of the systems show a pronounced fluid or crystalline order.
Under deionized conditions we find s to depend linearily on the packing
fraction with no detectable influence of the phase transitions. Further at
constant packing fraction s increases sublinearily with increasing number of
dissociable surface groups N. As a function of c the conductivity shows
pronounced differences depending on the kind of electrolyte used. We propose a
simple yet powerful model based on independent migration of all species present
and additivity of the respective conductivity contributions. It takes account
of small ion macro-ion interactions in terms of an effectivly transported
charge. The model successfully describes our qualitatively complex experimental
observations. It further facilitates quantitative estimates of conductivity
over a wide range of particle and experimental parameters.Comment: 32 pages, 17 figures, 2 tables, Accepted by Physical Review
Expression of Actin-interacting Protein 1 Suppresses Impaired Chemotaxis of Dictyostelium Cells Lacking the Na+-H+ Exchanger NHE1
Dictyostelium cells lacking the intracellular pH regulator NHE1 have defective chemotaxis. A modifier screen and reconstitution studies show expression of recombinant actin interacting protein 1 (Aip1) suppresses the Ddnhe1-phenotype. Aip1 promotes cofilin-dependent actin remodeling, which is likely a major determinant in pH-dependent chemotaxis
The PPP-Family Protein Phosphatases PrpA and PrpB of Salmonella enterica Serovar Typhimurium Possess Distinct Biochemical Properties
Salmonella enterica serovar Typhimurium requires Mn(2+), but only a few Mn(2+)-dependent enzymes have been identified from bacteria. To characterize Mn(2+)-dependent enzymes from serovar Typhimurium, two putative PPP-family protein phosphatase genes were cloned from serovar Typhimurium and named prpA and prpB. Their DNA-derived amino acid sequences showed 61% identity to the corresponding Escherichia coli proteins and 41% identity to each other. Each phosphatase was expressed in E. coli and purified to near electrophoretic homogeneity. Both PrpA and PrpB absolutely required a divalent metal for activity. As with other phosphatases of this class, Mn(2+) had the highest affinity and stimulated the greatest activity. The apparent K(a) of PrpA for Mn(2+) of 65 Ī¼M was comparable to that for other bacterial phosphatases, but PrpB had a much higher affinity for Mn(2+) (1.3 Ī¼M). The pH optima were pH 6.5 for PrpA and pH 8 for PrpB, while the optimal temperatures were 45 to 55Ā°C for PrpA and 30 to 37Ā°C for PrpB. Each phosphatase could hydrolyze phosphorylated serine, threonine, or tyrosine residues, but their relative specific activities varied with the specific substrate tested. These differences suggest that each phosphatase is used by serovar Typhimurium under different growth or environmental conditions such as temperature or acidity