Effect of N-glycosylation on the transport activity of the peptide transporter PEPT1

The intestinal peptide transporter PEPT1 provides bulk quantities of amino acids to epithelial cells. PEPT1 is a high-capacity and low-affinity solute carrier of the SLC15 family found in apical membranes of enterocytes in small intestine and distal colon. Surprisingly, murine PEPT1 (mPEPT1) has an apparent molecular mass of similar to 95 kDa in the small intestine but similar to 105 kDa in the large intestine. Here we describe studies on mPEPT1 protein glycosylation and how glycans affect transport function. Putative N-glycosylation sites of mPEPT1 were altered by site-directed mutagenesis followed by expression in Xenopus laevis oocytes. Replacement of six asparagine residues (N) at positions N50, N406, N439, N510, N515, and N532 by glutamine (Q) resulted in a decrease of the mPEPT1 mass by around 35 kDa. Electrophysiology revealed all glycosylation-deficient transporters to be functional with comparable expression levels in oocyte membranes. Strikingly, the mutant protein with N50Q exhibited a twofold decreased affinity for Gly-Sar but a 2.5-fold rise in the maximal inward currents compared with the wild-type protein. Elevated maximal transport currents were also recorded for cefadroxil and tri-L-alanine. Tracer flux studies performed with [C-14]-Gly-Sar confirmed the reduction in substrate affinity and showed twofold increased maximal transport rates for the N50Q transporter. Elimination of individual N-glycosylation sites did not alter membrane expression in oocytes or overall transport characteristics except for the mutant protein N50Q. Because transporter surface density was not altered in N50Q, removal of the glycan at this location appears to accelerate the substrate turnover rate.