Characterization of two cDNAs encoding folate-binding proteins from L1210 murine leukemia cells: Increased expression associated with a genomic rearrangement

L1210 murine leukemic cells grown under conditions of continuous low folate concentrations acquire increased levels of a high affinity/low capacity folate-binding protein (FBP). Using an oligonucleotide probe complementary to the human FBP, we have cloned and sequenced two murine FBP cDNAs isolated from a library constructed using a L1210 subline adapted for growth on low levels of 5-formyltetrahydrofolate. The encoding proteins, designated FBP1 and FBP2, have predicted M_r values of 29,415 and 28,821, respectively. These proteins share significant sequence identity with each other (70%) and with the deduced sequences of the human- and bovine-encoded FBPs (68-79%). Southern blot analysis of the low folate-adapted cell line revealed that, while neither of the two FBP-encoding genes was amplified, the FBP1 genomic locus had undergone rearrangement. On Northern blot analysis, this rearrangement was reflected in the enhanced expression (>100-fold) of a FBP1-specific transcript which was 200 nucleotides shorter than the corresponding L1210 parental transcript. The increased expression of this transcript coincided with the increased expression of a membrane protein (M_r= 38,000) which could be affinity-labeled with a N-hydroxysuccinimide ester of [³H]folate. Accordingly, the FBP1 transcript appears to encode the high affinity/ low capacity FBP. Compared to parental L1210 cells, expression of the FBP2-encoding transcript was unchanged in this cell line and, while the exact nature of the protein is unclear, FBP2 may represent a fetal form of the FBP.