Structural basis for molecular recognition of folic acid by folate receptors

Folate receptor-α (FRα) is overexpressed in many cancer cells and is therefore an important therapeutic target: here the X-ray crystal structure of folate-bound FRα is presented, revealing details of the ligand-binding pocket that may be useful in the development of small-molecule inhibitors for anticancer therapy. Folic acid, or folate, is an essential vitamin that is needed for many biological processes, including DNA synthesis, DNA repair and cell division. 'Normal' cells express relatively low amounts of the three folate receptors α, β and γ, but they are commonly overexpressed in cancer cell lines; for this reason, they are potential targets for new chemotherapeutics and cancer-imaging reagents. In this manuscript, the authors solve the X-ray crystal structure of the folate-bound form of human folate receptor α, which mediates folate uptake into cells. The authors map the ligand-binding pocket, providing data that should be useful for the development of new small molecules to target the receptor. Folate receptors (FRα, FRβ and FRγ) are cysteine-rich cell-surface glycoproteins that bind folate with high affinity to mediate cellular uptake of folate. Although expressed at very low levels in most tissues, folate receptors, especially FRα, are expressed at high levels in numerous cancers to meet the folate demand of rapidly dividing cells under low folate conditions1,2,3. The folate dependency of many tumours has been therapeutically and diagnostically exploited by administration of anti-FRα antibodies, high-affinity antifolates4,5, folate-based imaging agents and folate-conjugated drugs and toxins6,7,8. To understand how folate binds its receptors, we determined the crystal structure of human FRα in complex with folic acid at 2.8 Å resolution. FRα has a globular structure stabilized by eight disulphide bonds and contains a deep open folate-binding pocket comprised of residues that are conserved in all receptor subtypes. The folate pteroate moiety is buried inside the receptor, whereas its glutamate moiety is solvent-exposed and sticks out of the pocket entrance, allowing it to be conjugated to drugs without adversely affecting FRα binding. The extensive interactions between the receptor and ligand readily explain the high folate-binding affinity of folate receptors and provide a template for designing more specific drugs targeting the folate receptor system.