IDENTIFICATION OF AMINO-ACID-RESIDUES THAT DETERMINE THE DIFFERENTIALLIGAND SPECIFICITIES OF FOLATE RECEPTOR-ALPHA AND RECEPTOR-BETA

The homologous folate receptor (FR) types alpha and beta from both hum an and murine sources have opposite stereospecificities for reduced fo late coenzymes and different affinities for a variety of (anti)folate compounds. The present study identifies the critical amino acid sequen ce divergence underlying functional differences between FR-alpha and F R-beta. Chimeric constructs of the cDNAs encoding human FR-alpha and F R-beta were expressed in human 293 fibroblasts. The resulting membrane associated proteins were characterized in terms of their ability to b ind [H-3]folic acid and their relative affinities for the (6S) and (6R ) diastereoisomers of N-5-methyltetrahydrofolate. Substitution of the amino-terminal portion (residues 1-92) in the mature FR-alpha polypept ide with the corresponding segment of FR-beta resulted in folate bindi ng characteristics similar to FR-beta. Next, a series of chimeric cons tructs were generated, involving substitution of progressively shorter segments within residues 1-92 in FR-alpha with the corresponding pept ides of FR-beta. In this fashion, it was determined that the alanine r esidue at position 49 in FR-alpha was critical for its functional dive rgence from FR-beta, since substitution at this position with Leu (the corresponding residue in FR-beta) resulted in the folate binding char acteristics of FR-beta. Reciprocal substitution in FR-beta with peptid e 1-92 of FR-alpha resulted in poor expression of a [3H]folic acid bin ding protein. By analysis of chimeric constructs, the poor [H-3]folic acid binding of the FR-alpha(1-92)/beta(93-237) chimera could be attri buted to interference of a short segment from FR-alpha in the vicinity of Ala 49 (peptide 39-59) with proper folding of the chimera. Convers ion of the ligand binding properties of FR-beta to those of FR-alpha r equired the reciprocal mutation of Leu 49 to Ala, but in addition, sub stitution of one or more residues downstream of amino acid 92 of FR-be ta with the corresponding residues in FR-alpha was essential. The homo logous murine FR types alpha and beta, which are functionally analogou s to the human receptor isoforms, also contain a similar Ala vs Leu su bstitution. These results indicate that steric/hydrophobic effects of the side chains of Leu vs Ala at position 49 will critically modulate the affinities and stereospecificities of FR isoforms for folate compo unds. Furthermore, additional amino acid sequence divergence at one or more positions downstream of residue 92 in FR-alpha is also an essent ial determinant of the unique functional characteristics of this recep tor isoform.