N-linked glycosylation is not required for Na+/glucose symport activity in LLC-PK1 cells

Cys351 and Cys361 of the Na+/glucose cotransporter are important for both function and cell-surface expression

Here, we identify Cys351 and Cys361 as novel residues critical for the function and plasma membrane targeting of the Na+/glucose transporter-1 (SGLT1). HEK-293 cells expressing the C351A and C361A mutants showed no detectable Na(+)-coupled uptake for alpha-methyl glucoside (AMG). Cell-surface biotinylation and Western blot revealed that the two mutants were overexpressed in 293 cells; however, none of them exhibited normal cell-surface expression. When reconstituted in proteoliposomes, mutant SGLT1s demonstrated significantly lower affinity for AMG compared with the wild-type transporter. Incubation with the reducing agent dithiothreitol did not alter the catalytic activity of wild-type protein, but surprisingly, it nearly restored the ability of SGLT1-C351A and -C361A to bind and translocate AMG. Thus, the C351A and C361A mutations might cause a global reorganization of the disulfide bonds of SGLT1. Furthermore, we showed that a double mutation (C351A/C361A) restored the cell-surface expression of the single C-to-A mutants (C351A and C361A).

The apical localization of SGLT1 glucose transporter is determined by the short amino acid sequence in its N-terminal domain

SGLT1, an isoform of Na+-dependent glucose cotransporters, is localized at the apical plasma membrane in the epithelial cells of the small intestine and the kidney, where it plays a pivotal role in the absorption and reabsorption of sugars, respectively. To search the domain responsible for the apical localization of SGLT1, we constructed an N-terminal deletion clone series of rat SGLT1 and analyzed the localization of the respective products in Madin-Darby canine kidney (MDCK) cells. The products of N-terminal deletion clones up to the 19th amino acid were localized at the apical plasma membrane, whereas the products of N-terminal 20- and 23-amino-acid deletion clones were localized along the entire plasma membrane. Since single-amino-acid mutations of either D28N or D28G in the N-terminal domain give rise to glucose/galactose malabsorption disease, we examined the localization of these mutants. The products of D28N and D28G clones were localized in the cytoplasm, showing that the aspartic acid-28 may be essential for the delivery of SGLT1 to the plasma membrane. These results suggest that a short amino acid sequence of the N-terminal domain of SGLT1 plays important roles in plasma membrane targeting and specific apical localization of the protein.

Roles of individual N-glycans for ATP potency and expression of the rat P2X1 receptor

P2X(1) receptor subunits assemble in the ER of Xenopus oocytes to homotrimers that appear as ATP-gated cation channels at the cell surface. Here we address the extent to which N-glycosylation contributes to assembly, surface appearance, and ligand recognition of P2X(1) receptors. SDS-polyacrylamide gel electrophoresis (PAGE) analysis of glycan minus mutants carrying Gln instead of Asn at five individual NXT/S sequons reveals that Asn(284) remains unused because of a proline in the +4 position. The four other sites (Asn(153), Asn(184), Asn(210), and Asn(300)) carry N-glycans, but solely Asn(300) located only eight residues upstream of the predicted reentry loop of P2X(1) acquires complex-type carbohydrates. Like parent P2X(1), glycan minus mutants migrate as homotrimers when resolved by blue native PAGE. Recording of ATP-gated currents reveals that elimination of Asn(153) or Asn(210) diminishes or increases functional expression levels, respectively. In addition, elimination of Asn(210) causes a 3-fold reduction of the potency for ATP. If three or all four N-glycosylation sites are simultaneously eliminated, formation of P2X(1) receptors is severely impaired or abolished, respectively. We conclude that at least one N-glycan per subunit of either position is absolutely required for the formation of P2X(1) receptors and that individual N-glycans possess marked positional effects on expression levels (Asn(154), Asn(210)) and ATP potency (Asn(210)).

N-linked glycosylation of the thyroid Na+/I- symporter (NIS). Implications for its secondary structure model

The Na+/I- symporter (NIS), a 618-amino acid membrane glycoprotein that catalyzes the active accumulation of I- into thyroid cells, was identified and characterized at the molecular level in our laboratory (Dai, G., Levy, O., and Carrasco, N. (1996) Nature 379, 458-460). Because mature NIS is highly glycosylated, it migrates in SDS-polyacrylamide gel electrophoresis as a broad polypeptide of higher molecular mass (approximately 90-110 kDa) than nonglycosylated NIS (approximately 50 kDa). Using site-directed mutagenesis, we substituted both separately and simultaneously the asparagine residues in all three putative N-linked glycosylation consensus sequences of NIS with glutamine and assessed the effects of the mutations on function and stability of NIS in COS cells. All mutants were active and displayed 50-90% of wild-type NIS activity, including the completely nonglycosylated triple mutant. This demonstrates that to a considerable extent, function and stability of NIS are preserved in the partial or even total absence of N-linked glycosylation. We also found that Asn225 is glycosylated, thus proving that the hydrophilic loop that contains this amino acid residue faces the extracellular milieu rather than the cytosol as previously suggested. We demonstrated that the NH2 terminus faces extracellularly as well. A new secondary structure model consistent with these findings is proposed.