Folate receptor alpha
external side of plasma membrane
UMSCC38 cells amplified at 11q13 for the folate receptor synthesize a mutant nonfunctional folate receptor.
Some cells accumulate folate via a receptor-coupled process termed potocytosis. The folate receptor, a glycosyl phosphatidylinositol anchored M(r) 38,000-39,000 glycoprotein, is coded for by at least two genes (FR alpha and FR beta) at 11q13. The karyotype of UMSCC38, a human squamous cell carcinoma cell line, suggests that it may contain multiple copies of the folate receptor gene(s). Southern blot analysis confirms the presence of four to six copies. Using polymerase chain reaction methodology, Northern blot analysis, immunoblotting, and immunofluorescence, we find relatively limited expression of FR alpha and no FR beta in UMSCC38 cells when compared to nonamplified lines. Antigen is observed on the cell surface in a punctate pattern, and the protein is anchored via a glycosyl phosphatidylinositol anchor. Transport of 5-[methyl-3H]tetrahydrofolic acid is blocked by 5-methyltetrahydrofolic acid and probenecid, which suggests anion transport. Monensin, an inhibitor of potocytosis, and folic acid, a high-affinity ligand for the receptor, do not effectively block 5-methyltetrahydrofolic acid transport. Taken together, the results suggest that UMCSCC38 cells, although gene amplified, synthesize surprisingly small amounts of receptor and that receptor is nonfunctional. In order to establish further the nature of the receptor, 16 clones were obtained, and the complementary DNA was sequenced. Three mutations were found.