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Novel substrate analogs delineate an endocytotic mechanism for uptake of folate via the high-affinity, glycosylphosphatidylinositol-linked transport protein in L1210 mouse leukemia cells.

The mechanism of folate uptake, mediated by the high-affinity folate receptor linked via glycosylphosphatidylinositol to the plasma membrane of L1210 mouse leukemia cells, has been investigated with the use of two substrate analogs, a fluorescein derivative of folate and a gold-conjugated, streptavidin-biotin derivative of folate, in conjunction with fluorescence microscopy and electron microscopy. These substrate probes bind to receptors that are uniformly distributed rather than clustered on the membrane, and the receptor-probe complexes are internalized by an endocytotic mechanism in which vesicles containing the complexes migrate from the membrane toward the nucleus. Measurements using [3H]-folate demonstrate that receptors are recycled to the plasma membrane.