Identification of Amino Acid Residues that Determine the Differential Ligand Specificities of Folate Receptors and 
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- 作者：Feng Shen ; Xuan Zheng ; Jianyong Wang ; and Manohar Ratnam
- 刊名：Biochemistry
- 出版年：1997
- 出版时间：May 20, 1997
- 年：1997
- 卷：36
- 期：20
- 页码：6157 - 6163
- 全文大小：168K
- 年卷期：v.36,no.20(May 20, 1997)
- ISSN：1520-4995
文摘
The homologous folate receptor (FR) types 
and 
from both human and murine sourceshave opposite stereospecificities for reduced folate coenzymes anddifferent affinities for a variety of(anti)folate compounds. The present study identifies thecritical amino acid sequence divergence underlyingfunctional differences between FR- and FR-. Chimericconstructs of the cDNAs encoding humanFR- and FR- were expressed in human 293 fibroblasts. Theresulting membrane associated proteinswere characterized in terms of their ability to bind[3H]folic acid and their relative affinities for the(6S)and (6R) diastereoisomers ofN5-methyltetrahydrofolate. Substitution ofthe amino-terminal portion (residues1-92) in the mature FR- polypeptide with the corresponding segmentof FR- resulted in folate bindingcharacteristics similar to FR-. Next, a series of chimericconstructs were generated, involving substitutionof progressively shorter segments within residues 1-92 in FR- withthe corresponding peptides of FR-. In this fashion, it was determined that the alanine residue atposition 49 in FR- was critical for itsfunctional divergence from FR-, since substitution at this positionwith Leu (the corresponding residuein FR-) resulted in the folate binding characteristics of FR-.Reciprocal substitution in FR- withpeptide 1-92 of FR- resulted in poor expression of a[3H]folic acid binding protein. By analysisofchimeric constructs, the poor [3H]folic acid bindingof the FR-1-92/93-237 chimera couldbe attributedto interference of a short segment from FR- in the vicinity of Ala49 (peptide 39-59) with properfolding of the chimera. Conversion of the ligand bindingproperties of FR- to those of FR- requiredthe reciprocal mutation of Leu 49 to Ala, but in addition, substitutionof one or more residues downstreamof amino acid 92 of FR- with the corresponding residues in FR-was essential. The homologous murineFR types and , which are functionally analogous to the humanreceptor isoforms, also contain a similarAla vs Leu substitution. These results indicate thatsteric/hydrophobic effects of the side chains of Leuvs Ala at position 49 will critically modulate the affinities andstereospecificities of FR isoforms forfolate compounds. Furthermore, additional amino acid sequencedivergence at one or more positionsdownstream of residue 92 in FR- is also an essential determinant ofthe unique functional characteristicsof this receptor isoform.
and from both human and murine sourceshave opposite stereospecificities for reduced folate coenzymes anddifferent affinities for a variety of(anti)folate compounds. The present study identifies thecritical amino acid sequence divergence underlyingfunctional differences between FR- and FR-. Chimericconstructs of the cDNAs encoding humanFR- and FR- were expressed in human 293 fibroblasts. Theresulting membrane associated proteinswere characterized in terms of their ability to bind[3H]folic acid and their relative affinities for the(6S)and (6R) diastereoisomers ofN5-methyltetrahydrofolate. Substitution ofthe amino-terminal portion (residues1-92) in the mature FR- polypeptide with the corresponding segmentof FR- resulted in folate bindingcharacteristics similar to FR-. Next, a series of chimericconstructs were generated, involving substitutionof progressively shorter segments within residues 1-92 in FR- withthe corresponding peptides of FR-. In this fashion, it was determined that the alanine residue atposition 49 in FR- was critical for itsfunctional divergence from FR-, since substitution at this positionwith Leu (the corresponding residuein FR-) resulted in the folate binding characteristics of FR-.Reciprocal substitution in FR- withpeptide 1-92 of FR- resulted in poor expression of a[3H]folic acid binding protein. By analysisofchimeric constructs, the poor [3H]folic acid bindingof the FR-1-92/93-237 chimera couldbe attributedto interference of a short segment from FR- in the vicinity of Ala49 (peptide 39-59) with properfolding of the chimera. Conversion of the ligand bindingproperties of FR- to those of FR- requiredthe reciprocal mutation of Leu 49 to Ala, but in addition, substitutionof one or more residues downstreamof amino acid 92 of FR- with the corresponding residues in FR-was essential. The homologous murineFR types and , which are functionally analogous to the humanreceptor isoforms, also contain a similarAla vs Leu substitution. These results indicate thatsteric/hydrophobic effects of the side chains of Leuvs Ala at position 49 will critically modulate the affinities andstereospecificities of FR isoforms forfolate compounds. Furthermore, additional amino acid sequencedivergence at one or more positionsdownstream of residue 92 in FR- is also an essential determinant ofthe unique functional characteristicsof this receptor isoform.