Structural basis for molecular recognition of folic acid by folate receptors

Folate receptors (FR alpha, FR beta and FR gamma) 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 FR alpha, are expressed at high levels in numerous cancers to meet the folate demand of rapidly dividing cells under low folate conditions(1-3). The folate dependency of many tumours has been therapeutically and diagnostically exploited by administration of anti-FR alpha antibodies, high-affinity antifolates(4,5), folate-based imaging agents and folate-conjugated drugs and toxins(6-8). To understand how folate binds its receptors, we determined the crystal structure of human FR alpha in complex with folic acid at 2.8 angstrom resolution. FR alpha 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 FR alpha 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.