INCREASED EXPRESSION AND CHARACTERIZATION OF 2 DISTINCT FOLATE BINDING-PROTEINS IN MURINE ERYTHROLEUKEMIA-CELLS

We previously identified two membrane-bound folate binding proteins, F BP1 and FBP2, in murine L1210 leukemia cells. We now report on the dev elopment of two variant murine erythroleukemia cell lines that were us ed for direct comparison and biochemical characterization of the two m urine folate binding proteins. Based on the results of northern analys is and the mobilities of affinity-labeled proteins on polyacrylamide g els, these cell lines exhibit specific up-regulated expression of FBP1 or FBP2. The affinities of the folate binding proteins for various (a nti)folates were determined based upon the ability of the compounds to inhibit binding of [H-3]folic acid. The two proteins exhibited consid erably different affinities and stereospecificities and, in general, F BP2 consistently bound each test compound with lesser affinity than FB P1. Both proteins displayed greatest affinity for folic acid, 5-methyl tetrahydrofolate, and the antifolates CB3717 and 5,10-dideazatetrahydr ofolate (DDATHF). Conversely, the proteins exhibited poor affinity for the dihydrofolate reductase inhibitors methotrexate and aminopterin. For 5-formyltetrahydrofolate, FBP1 had high affinity for the (6S) dias tereoisomer, whereas FBP2 showed preference for the non-physiologic (6 R) diastereoisomer. The binding properties of FBP1 and FBP2 overexpres sed in these cell lines closely paralleled those of their respective h uman homologs. These lines provide a model system in which to examine the biochemical characteristics of the individual folate binding prote ins without the potential problems associated with expression of prote ins in dissimilar cell lines.