L1210 mouse
leukemia cells exhibit two distinct types of
nucleoside transport activity that have similar kinetic properties and substrate specificity, but differ markedly in their sensitivity to the inhibitor nitrobenzylthioinosine (
NBMPR) (Belt, J. A. (1983) Mol. Pharmacol. 24, 479-484). It is not known whether these two transport activities are mediated by a single
protein or by separate and distinct
nucleoside transport proteins. We have isolated a mutant from the L1210 cell line that has lost the
NBMPR-insensitive component of
nucleoside transport, but retains
NBMPR-sensitive transport. In the parental cell line 20-40% of the
nucleoside transport activity is insensitive to 1 microM
NBMPR. In the mutant, however,
uridine and
thymidine transport are almost completely inhibited by
NBMPR. Consistent with the loss of
NBMPR-insensitive transport, the mutant cells can be protected from the toxic effects of several
nucleoside analogs by
NBMPR. In contrast, the toxicity of the same analogs in the wild type cells is not significantly affected by
NBMPR, presumably due to uptake of the
nucleosides via the
NBMPR-insensitive transporter. On the other hand,
NBMPR-sensitive transport in the mutant appears to be unaltered. The mutant is not resistant to cytotoxic
nucleosides in the absence of
NBMPR and the cells retain the wild type
complement of high affinity binding sites for
NBMPR. Furthermore, the affinity of the binding site for the inhibitor is similar to that of parental L1210 cells. These results suggest that
NBMPR-sensitive and
NBMPR-insensitive
nucleoside transport in L1210 cells are mediated by genetically distinct
proteins. To our knowledge this is the first report of a mutant deficient in
NBMPR-insensitive
nucleoside transport.