HUMAN DRA FUNCTIONS AS A SULFATE TRANSPORTER IN SF9 INSECT CELLS

DRA is a gene that is down-regulated in colon adenomas and adenocarcin omas in humans, We have previously shown that DRA proteins are found a s various forms in tissue due to differential glycosylation. This stud y has focused on the function of DRA related to its subcellular locali zation. We used the baculovirus expression system and overexpressed a nearly full-length DRA driven by a polyhedrin promoter in Sf9 insect c ells. DRA protein expressed in this cell was underglycosylated relativ e to normal colon mucosa, but uniformly targeted to the cell membrane. It also appears to undergo posttranslational cleavage, removing about 100 amino acids from its amino terminus. This membrane localization i s similar to what we observed in the colon mucosa. An ion transport as say demonstrated that DRA functions as a sulfate transporter, When DRA was expressed, sulfate import was increased more than threefold compa red to the control, Sulfate import was inhibitable by the anion transp orter inhibitor, DIDS, in a dose dependent fashion. Given that (1) DRA has high similarity to other identified sulfate transporters and the proposed structure of DRA polypeptide is characteristic of those trans porters, (2) DRA localization is limited to the cell membrane, and (3) DRA expression correlates with intestinal differentiation in mouse, w e suggest that DRA represents a tissue-specific member of the sulfate transporter family. (C) 1998 Academic Press.