A novel isoform of the human folate receptor (FR, type gamma) was recently identified in hematopoietic tissues [Shen et al. (1994) Biochemistry 33, 1209-1215]. In that report, Cos-1 cells, transiently transfected with the cDNA for FR-gamma, produced relatively poor expression of the receptor on the cell surface. In this study, several recombinant Chinese hamster ovary (CHO) cell lines were produced by stable transfection with the cDNA for FR-gamma followed by amplification. Similar recombinant CHO cell lines were produced that expressed the glycosylphosphatidylinositol- (GPI-) anchored FR type beta and a truncated form of FR type beta (FR-beta Delta), in which the normal carboxyl-terminal signal for GPI anchor attachment was deleted. Both FR-gamma- and FR-beta Delta-expressing CHO cells produced a [H-3]folic acid binding protein in the medium with a similar time course over a 24-h period; in contrast to intact FR-beta, relatively insignificant amounts of either FR-gamma or FR-beta Delta were associated with the CHO cell surface and this was unaltered by the absence of serum in the medium. The FR-gamma- and FR-beta Delta-producing CHO cells did not differ significantly in intracellular FR levels. Furthermore, the mRNA level for FR-gamma did not exceed that for FR-beta Delta. When deglycosylated with hydrogen fluoride, both FR-gamma and FR-beta Delta showed similar apparent molecular weights on Western blots as predicted for the intact polypeptides. Chimeric constructs of FR-gamma and FR-beta cDNAs resulting in interchanging of the unconserved, carboxyl-terminal segments of the two proteins were expressed in human 293 fibroblasts; the results indicate that FR-gamma is primarily a secretory protein due to a divergence of its carboxyl-terminal amino acid sequence from the other members of the FR gene family, resulting in an inefficient signal for GPI modification. The relative affinities of the secreted FR-gamma for folic acid and the diastereoisomers of reduced folate compounds were in the order folic acid (K-D = 0.42 x 10(-)9 M) > (6S)/(6R)-N-5-methyltetrahydrofolate > (6S)/(6R)-N-5-formyltetrahydrofolate. Evidence is presented that expression of the secreted full-length FR-gamma is restricted to certain hematopoietic cell types. FR-gamma is, therefore, a potential serum marker for certain malignancies of hematopoietic tissues and also a candidate protein for the high-affinity serum folate binder that is elevated during folate deficiency.
