• Biomedical Science Letters
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• v.14 no.4
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• pp.203-210
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• 2008

One of cell surface receptor proteins, human folate receptor (hFR) involves in the uptake of folates through cell membrane into cytoplasm, and is anchored to the plasma membrane by a fatty acid linkage, which has been identified in some cells as a glycosylphosphatidylinositol (GPI)-tailed protein with a molecular mass of about 40 kDa. The hFR is released by phosphatidylinositol phospholipase C (PI-PLC) because it contains fatty acids and inositol on the GPI tail. Caveolin decorates the cytoplasmic surface of caveolae and has been proposed to have a structural role in maintaining caveolae. It is unknown whether caveolin is involved in targeting, and is necessary for the function of GPI-tailed proteins. To compare the ability of folic acid binding, internalization and expression of hFR, and the effect of caveolin at the both apical and basolateral side of cell surfaces in Chinese hamster ovary (CHO) clone cells overexpressed the hFR and/or caveolin. Our present results suggest a possibility that the overexpression of caveolin does not be involved in expression of hFR, but plays a role as a factor in PI-PLC releasing kinetics, and for a regulation of formation, processing and function of hFR in CHO clone cells overexpressed cavcolin.

• BMB Reports
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• v.37 no.3
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• pp.362-369
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• 2004

The human folate receptor (hFR) is a glycosylphosphatidylinositol (GPI) linked plasma membrane protein that mediates delivery of folates into cells. We studied the sorting of the hFR using transfection of the hFR cDNA into MDCK cells. MDCK cells are polarized epithelial cells that preferentially sort GPI-linked proteins to their apical membrane. Unlike other GPI-tailed proteins, we found that in MDCK cells, hFR is functional on both the apical and basolateral surfaces. We verified that the same hFR cDNA that transfected into CHO cells produces the hFR protein that is GPI-linked. We also measured the hFR expression on the plasma membrane of type III paroxysmal nocturnal hemoglobinuria (PNH) human erythrocytes. PNH is a disease that is characterized by the inability of cells to express membrane proteins requiring a GPI anchor. Despite this defect, and different from other GPI-tailed proteins, we found similar levels of hFR in normal and type III PNH human erythrocytes. The results suggest the hypothesis that there may be multiple mechanisms for targeting hFR to the plasma membrane.

• BMB Reports
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• v.35 no.4
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• pp.395-402
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• 2002

Caveolae are small, flask-shaped, non-clathrin coated invaginations of the plasma membrane of many mammalian cells. Caveolae have a coat that includes caveolin. They have been implicated in numerous cellular processes, including potocytosis. Since the human folate receptor (hFR) and other glycosyl-phosphatidylinositol (GPI)-tailed proteins have been co-localized to caveolae, we studied the caveolin role in the hFR function by transfecting hFR and/or caveolin cDNA into Fischer rat thyroid epithelial (FRT) cells that normally do not express detectable levels of either protein. We isolated and characterized stable clones as follows: they express (1) high levels of caveolin alone, (2) hFR and caveolin, or (3) hFR alone. We discovered that hFR is correctly processed, sorted, and anchored by a GPI tail to the plasma membrane in FRT cells. No difference in the total folic acid binding or cell surface folic acid binding activity were found between the FRT cells that were transfected with hFR, or cells that were transfected with hFR and caveolin. The hFR that was expressed on the cell surface of clones that were transfected with hFR was also sensitive to phosphatidylinositol-specific phospholipase C (PI-PLC) release, and incorporated radiolabeled ethanolamine that supports the attachment of a GPI-tail on hFR. We conclude that the processing, sorting, and function of hFR is independent on the caveolin expression in FRT cells.