A novel loss-of-function mutation in the proton-coupled folate transporter from a patient with hereditary folate malabsorption reveals that Arg 113 is crucial for function.
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Lasry I, Berman B, Straussberg R, Sofer Y, Bessler H, Sharkia M, Glaser F, Jansen G, Drori S, Assaraf YG
A novel loss-of-function mutation in the proton-coupled folate transporter from a patient with hereditary folate malabsorption reveals that Arg 113 is crucial for function.
Blood. 2008 Sep 1;112(5):2055-61. doi: 10.1182/blood-2008-04-150276. Epub 2008 Jun 17.
- PubMed ID
- 18559978 [ View in PubMed]
- Abstract
Hereditary folate malabsorption (HFM) patients harbor inactivating mutations including R113S in the proton-coupled folate transporter (PCFT), an intestinal folate transporter with optimal activity at acidic pH. Here we identified and characterized a novel R113C mutation residing in the highly conserved first intracellular loop of PCFT. Stable transfectants overexpressing a Myc-tagged wild-type (WT) and mutant R113C PCFT displayed similar transporter targeting to the plasma membrane. However, whereas WT PCFT transfectants showed a 22-fold increase in [(3)H]folic acid influx at pH 5.5, R113C or mock transfectants showed no increase. Moreover, WT PCFT transfectants displayed a 50% folic acid growth requirement concentration of 7 nM, whereas mock and R113C transfectants revealed 24- to 27-fold higher values. Consistently, upon fluorescein-methotrexate labeling, WT PCFT transfectants displayed a 50% methotrexate displacement concentration of 50 nM, whereas mock and R113C transfectants exhibited 12- to 14-fold higher values. Based on the crystal structure of the homologous Escherichia coli glycerol-3-phosphate transporter, we propose that the cationic R113 residue of PCFT is embedded in a hydrophobic pocket formed by several transmembrane helices that may be part of a folate translocation pore. These findings establish a novel loss of function mutation in HFM residing in an intracellular loop of PCFT crucial for folate transport.