Functional Characterization of Glycosylation-Deficient Human P-Glycoprotein Using A Vaccinia Virus Expression System

P-glycoprotein (P-gp), the product of human MDR1 gene, which functions as an ATP-dependent drug efflux pump, is N-linked glycosylated at asparagine residues 91, 94, and 99 located within the first extracellular loop. We report here the biochemical characterization of glycosylation-deficient (Gly−) P-gp using a vaccinia virus based transient expression system. The staining of HeLa cells expressing Gly− P-gp (91, 94, and 99N→Q), with P-gp specific monoclonal antibodies, MRK-16, UIC2 and 4E3 revealed a 40 to 50% lower cell-surface expression of mutant P-gp compared to the wild-type protein. The transport function of Gly− P-gp, assessed using a variety of fluorescent compounds indicated that the substrate specificity of the pump was not affected by the lack of glycosylation. Additional mutants, Gly− D (91, 94, 99N→D) and Gly−Δ (91, 94, 99 N deleted) were generated to verify that the reduced cell surface expression, as well as total expression, were not a result of the glutamine substitutions. Gly− D and Gly−Δ Pgps were also expressed to the same level as the Gly− mutant protein. 35S-Methionine/cysteine pulse-chase studies revealed a reduced incorporation of 35S-methionine/cysteine in full length Gly− P-gp compared to wild-type protein, but the half-life (∼3 hr) of mutant P-gp was essentially unaltered. Since treatment with proteasome inhibitors (MG-132, lactacystin) increased only the intracellular level of nascent, mutant P-gp, the decreased incorporation of 35S-methionine/cysteine in Gly− P-gp appears to be due to degradation of improperly folded mutant protein by the proteasome and endoplasmic reticulum-associated proteases. These results demonstrate that the unglycosylated protein, although expressed at lower levels at the cell surface, is functional and suitable for structural studies.