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KB cells express a folate-binding protein that is anchored to the plasma membrane by a glycosylated phosphatidylinositol (GPI) tail and these cells can grow in medium containing a very low folate concentration (1 nM). In contrast, mouse 3T3 cells do not express a membrane-associated folate-binding protein and cannot grow under similar low folate conditions. In these studies, 3T3 cells were transfected with a vector containing the cDNA that codes for the KB cell folate-binding protein. In contrast to the wild-type 3T3 cells, the transfected 3T3 cells express a level of folate-binding protein similar to KB cells, 1 and 1.4 ng/,ug protein, respectively. The capacity for binding 13HIfolate to the surface of transfected 3T3 cells cultured in folatedeficient medium is 7.7 pmol/ 106 cells, and this is -50% of the surface binding capacity of KB cells under similar culture conditions. Moreover, after treatment of the transfected 3T3 cells with phospholipase C specific for phosphatidylinositol, the binding of 13HI folate to the surface of these cells is reduced by 90%, indicating that, like the KB cells, the folate-binding protein is anchored to the plasma membrane by a GPI tail. Although the doubling time of wild-type 3T3 cells markedly increases after 13 d of culture in folate-deficient medium, the doubling time of both the transfected 3T3 cells and KB cells do not change. The results of these experiments indicate that the GPI-anchored folate-binding protein provides a mechanism to maintain a level of folate that permits the folate-dependent metabolic functions necessary for cell survival under low folate conditions. (J. Clin. Invest. 1992. 90:840-847.)

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A folate-binding protein (FBP) anchored to cell membranes by a glycosyl phosphatidylinositol (GPI) adduct is constitutively expressed in some transformed and cultured cell lines. Its expression is upregulated when these cells are grown in medium containing low folate, but whether this occurs in vivo with nutritional folate deficiency is unknown. To address this question, the GPI-FBP in the liver, kidney, and brain of rats on control and folate-deficient (FD) diets was measured. The GPI-FBP in the kidney of FD rats decreased significantly in contrast to the upregulation of this protein in cultured cells. Northern blot analysis and nuclear run-on assays indicated that transcription of the GPI-FBP gene in the kidney was not reduced by folate deficiency. This decrease of the GPI-FBP appears to result from its proteolysis, similar to the enzymatic degradation of the apoprotein that occurs in vitro. Because the GPI-FBP is on the brush borders of the proximal renal tubules and provides for the reabsorption of folate, this function diminishes when the protein decreases in folate deficiency.