An affinity labeling technique has been developed to identify the folate-methotrexate transporter of Leishmania donovani promastigotes using “activated” derivatives of the ligands. These “activated” derivatives were synthesized by incubating folate and methotrexate with a 10-fold excess of 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) for 10 min at ambient temperature in dimethyl sulfoxide. Preincubation of intact cells with nonradioactive “activated” folate or methotrexate at a concentration of 40 μM inhibited the capacity of wild-type cells to transport submicromolar concentrations of unmodified ligand. When intact wild-type (DI700) Leishmania donovani or preparations of their membranes were incubated with a 0.4 μM concentration of either “activated” [3H]folate or “activated” [3H]methotrexate, the radiolabeled ligands were covalently incorporated into a polypeptide with a molecular weight of approximately 46 000, as demonstrated by SDS-polyacrylamide gel electrophoresis. No affinity labeling of a 46 000-dalton protein was observed when equimolar concentrations of “activated” radiolabeled ligands were incubated with intact cells or membranes prepared from a methotrexate-resistant mutant clone of Leishmania donovani, MTXA5, that is genetically defective in folate-methotrexate transport capability [Kaur, K., Coons, T., Emmett, K., & Ullman, B. (1988) J. Biol. Chem. 263, 7020-7028]. However, some labeling of a 46 000-dalton protein was observed when MTXA5 cells were incubated with higher concentrations of “activated” ligands. Time course studies indicated that maximal labeling of the 46 000-dalton protein occurred within 5-10 min of incubation of intact cells with “activated” ligand. Inhibitors of folate-methotrexate transport, including dihydrofolate, tetrahydrofolate, 5-methyltetrahydrofolate, 5-formyltetrahydrofolate, and (p-aminobenzoyl)glutamate, blocked the incorporation of radiolabeled ligand into the 46 000-dalton polypeptide, while biopterin and pteroic acid, two pterins that did not block folate-methotrexate entry into wild-type Leishmania donovani, did not interfere with the labeling of the 46 000-dalton moiety. These studies provide biochemical evidence that the folate-methotrexate transporter of Leishmania donovani can be identified in crude extracts by an affinity labeling technique and serve as a prerequisite to further analysis of the transport protein by providing a vehicle for subsequent purification of this membrane carrier. Moreover, these investigations suggest that the affinity labeling technique using EDC-activated ligands may be exploitable to analyze other cell surface binding proteins in Leishmania donovani, as well as in other organisms.
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