Submitted on April 19, 2004
Accepted on September 17, 2004
Conserved residues F316 and G476 in the concentrative nucleoside transporter 1 (hCNT1) impact guanosine sensitivity and membrane expression, respectively
Yurong Lai1, Eun-Woo Lee1, Carl C Ton1, Shashi Vijay1, Huixia Zhang1, and Jashvant D Unadkat1*
1 Pharmaceutics, University of Washington, Seattle, WA, USA
* To whom correspondence should be addressed. E-mail: jash{at}u.washington.edu.
The functional significance of two highly conserved amino acid residues, F316 (putative TM7) and G476 (putative TM11), in the concentrative nucleoside transporter hCNT1 (SLC28A1) was examined by site-directed mutagenesis. Conservative mutations at these positions (F316A, F316Y and G476A, G476L) were generated and expressed in MDCK cells as fusion polypeptides with green fluorescent protein (GFP). Unlike wild-type hCNT1, G476A-GFP or G476L-GFP were not expressed in the plasma membrane in undifferentiated or differentiated MDCK cells and had no functional activity. Like wild-type hCNT1, F316A-GFP or F316Y-GFP were expressed in the plasma membrane of undifferentiated MDCK cells and in the apical membrane of differentiated MDCK cells. Remarkably, transport of 3H-uridine by F316Y-GFP or F316A-GFP was highly sensitive to inhibition by guanosine. Furthermore, genotyping of exon 11 of hCNT1 (TM7) in a panel of 260 anonymous human DNA samples revealed a novel F316H variant (TT>CA; 1/260). When expressed in MDCK cells, 3H-uridine transport by F316H was also found to be sensitive to inhibition by guanosine (IC50=148 µM). The effect of the F316H mutation resembles the "N4" type nucleoside transporter phenotype previously reported to be present in human kidneys. We suggest that the "N4 transport system" is a naturally occurring variant of hCNT1, perhaps at the F316 position. Collectively, our data shows that G476 is important for correct membrane targeting, folding and/or intracellular processing of hCNT1. In addition, we have discovered for the first time that hCNT1 displays natural variation at position F316 and that the variant F316H confers on the transporter an unusual sensitivity to inhibition by guanosine.
