Point Mutations Alter the Cellular Distribution of the Human Folate Receptor in Cultured Chinese Hamster Ovary Cells

Diminished cellular need for folate results in decreased function of the human folate receptor (FR) but increased expression of this protein in cells grown at different rates. Much of this FR is intracellular and not available for vitamin transport, raising the following question: what is the function of this excess receptor? In this study, we characterized the effects of three point mutations on FR regulation in Chinese hamster ovary cells stably transfected to express either wild-type receptor or FR containing mutations at positions 67(S-->P),144(E-->D), and/or 201(N-->D). The 201(N-->D) FR responded functionally like the wild-type receptor but was localized predominantly at the cell surface (>90% vs. <40% for wild-type). This mutation disrupted a N-linked glycosylation site and generated a partially deglycosylated receptor. The 67(S-->P) mutation also shifted the cellular distribution such that more FR was surface accessible ( approximately 80%) but did not affect glycosylation. Because previous results showed that these mutations influence the conformation of FR, our findings suggest that structural changes in the receptor facilitate its trafficking to the cell surface. FR containing the 67(S-->P) mutation with either a 144(E-->D) or 201(N-->D) change was not processed from the high-mannose to complex glycoform but was still transported to the cell surface and able to transport folates. Thus, conformational changes introduced by specific point mutations can influence FR processing and/or trafficking to the cell surface. Furthermore, the fact that mutated FR can be trafficked to the cell surface more efficiently suggests that the native receptor may be retained intracellularly to perform some function there.