Reversibility of the affinity labelled-biotin transport system in yeast cells

Selective labeling of sulfhydryls and disulfides on blot transfers using avidin-biotin technology: studies on purified proteins and erythrocyte membranes

Various strategies for the use of 3-(N-maleimido-propionyl) biocytin (MPB) as a general label for distinguishing between protein sulfhydryls and disulfides on blot transfers are presented. In the first approach, endogenous SH groups in proteins were labeled directly with MPB. For disulfide staining, endogenous sulfhydryls were blocked with N-ethylmaleimide, disulfides were then reduced with mercaptoethanol, and the newly formed SH groups were labeled with MPB. In this approach, all treatments were performed in vitro, and, between steps, excess reagent was removed by dialysis. The MPB-labeled proteins were then separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) (in the presence of mercaptoethanol), the labeled proteins were transferred to nitrocellulose, and the blotted proteins were detected by avidin-biotin technology. In the second approach, MPB treatment was performed directly on blot transfers. For SH labeling, proteins were subjected to SDS-PAGE in the absence of mercaptoethanol, thus retaining the status of endogenous sulfhydryl and disulfide groups. For S-S labeling, proteins were treated with N-ethylmaleimide in vitro and then subjected to SDS-PAGE in the presence of mercaptoethanol, such that endogenous sulfhydryls were blocked and endogenous disulfides were converted to SH groups. Subsequent treatments and washings were performed on blots. In the third approach, immobilized proteins (i.e., in artificial systems or in natural systems such as membrane preparations or intact cells) were treated essentially as described in the first approach, except that washings were carried out by centrifugation. In vitro treatments were performed before SDS-PAGE (carried out in the presence of mercaptoethanol) and subsequent blot transfer. The relative merits of the three strategies are discussed.

Biotin uptake: influx, efflux and countertransport in Escherichia coli K12

Biotin uptake by Escherichia coli K12 has been reinvestigated. The vitamin uptake is an active process depending on energy and inhibited by uncouplers. The kinetic parameters (Km = 0.27 microM, Vmax = 6.8 pmol/min per mg dry cells) are close to those previously determined for a biotin-dependent strain E. coli C162 (Piffeteau, A., Zamboni, M. and Gaudry, M. (1982) Biochim. Biophys. Acta 688, 29-36). By use of biotin p-nitrophenyl ester, an affinity label of the biotin transport system, it was shown, under conditions of steady state, that the efflux of biotin is not energy dependent and is mainly mediated by a diffusion mechanism. Reexamination of the regulation of the biotin transport by biotin, revealed that only 50% of the biotin uptake system is under control by the vitamin.

Irreversible inhibitors of methotrexate transport in L1210 cells. Characteristics of inhibition by an N-hydroxysuccinimide ester of methotrexate

Methotrexate, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide react to form an activated ester of methotrexate which is a potent irreversible inhibitor of methotrexate transport in L1210 cells. In cells treated with the reagent at 37 degrees C, inhibition was rapid (t1/2 less than 1 min), optimal at pH 6.8, half-maximal at an inhibitor concentration of 20 nM, and complete at high levels of the reagent. Specificity was indicated by the fact that excess methotrexate added during the pretreatment step protected the transport system against inactivation. Irreversible inhibition was also observed in cells exposed to the reagent at 4 degrees C. Inactivation in this case was qualitatively similar to the corresponding process at 37 degrees C; it appeared rapidly, was half-maximal at 20 nM, and could be prevented by the addition of high concentrations of the substrate. The extent of the inhibition, however, reached a maximum of only 75%, even in samples containing excess or multiple additions of reagent. The latter findings suggest that at 4 degrees C the transport protein exists in two forms, one (75% of the total) containing binding sites which are accessible to the active ester, and the other (25% of the total) with inaccessible sites. The identity of these sites is suggested to be transport proteins which have outward and inward orientations, respectively.

Biotin transport by a biotin-deficient strain of Escherichia coli

Biotin uptake has been investigated using an Escherichia coli biotin requiring auxotroph grown under biotin-deficient conditions. This strain accumulated biotin in the free and bound form. In agreement with a previous report by O. Prakash and M.A. Eisenberg (J. Bacteriol. 120 (1974) 785-791), the biotin entry proved to be an active process which depended on an energy source and was inhibited in the presence of uncouplers. The kinetic parameters have been determined (KM = 0.05 microM, Vmax = 7 pmol/min per mg dry weight). The pool of free biotin could be readily exchanged with external biotin and decreased to a very low level in the absence of an energy source. The use of several biotin analogues revealed that this transport system was quite specific for biotin: slight modifications, for instance in the valeric chain, lowered drastically the affinity for the carrier.