Role of N-glycosylation in the expression of human SLC26A2 and A3 anion transport membrane glycoproteins.

The human solute carrier 26 (SLC26) gene family of anion transporters consists of 10 members (SLC26A1-A11, A10 being a pseudogene) that encode membrane glycoproteins with 14 transmembrane (TM) segments and a C-terminal cytoplasmic sulfate transporter anti-sigma antagonist (STAS) domain. Thus far, mutations in eight members of the SLC26 family (A1 - A6, A8 and A9) have been linked to diseases in humans. Our goal is to characterize the role of N-glycosylation and the effect of mutations in SLC26A2 and A3 proteins on their functional expression in transfected HEK-293 cells. We found that certain mutants were retained in the ER via an interaction with the lectin chaperone, calnexin. Some could escape protein quality control and traffic to the cell surface upon removal of the N-glycosylation sites. Furthermore, we found that loss of N-glycosylation reduced expression of SLC26A2 at the cell surface. Loss of N-glycosylation had no effect on the stability of SLC26A3, yet resulted in a profound decrease in transport activity. Thus, N-glycosylation plays three roles in the functional expression of SLC26 proteins: 1) to retain mis-folded proteins in the ER, 2) to stabilize the protein at the cell surface, and 3) to maintain the transport protein in a functional state.