Identification of the nucleoside transporter in cultured mouse lymphoma cells. Photoaffinity labeling of plasma membrane-enriched fractions from nucleoside transport-competent (S49) and nucleoside transport-deficient (AE1) cells with [3H]nitrobenzylthioinosine.

Plasma membrane-enriched fractions from disrupted S49 lymphoma cells contained high affinity sites for [3H]nitrobenzylthioinosine, a potent and specific inhibitor of nucleoside transport. These sites were absent from similar preparations from AE1 cells, a nucleoside-transport deficient clone derived from the S49 cell line. Reversible binding of [3H]nitrobenzylthioinosine to the S49 membrane preparations was inhibited by adenosine, nitrobenzylthioguanosine, and dipyridamole. Exposure of S49 membrane preparations to UV light in the presence of [3H]nitrobenzylthioinosine resulted in the covalent radiolabeling of a membrane protein(s) which migrated on sodium dodecyl sulfate-polyacrylamide gels with an apparent Mr of 45,000 to 66,000. Labeling of this protein was abolished in the presence of nitrobenzylthioguanosine and markedly reduced in the presence of adenosine and dipyridamole. AE1 membrane proteins were not covalently labeled under these conditions.