Carboxy terminus of glucose transporter 3 contains an apical membrane targeting domain

We previously demonstrated that distinct facilitative glucose transporter isoforms display differential sorting in polarized epithelial cells. In Madin-Darby canine kidney (MDCK) cells, glucose transporter 1 and 2 (GLUT1 and GLUT2) are localized to the basolateral cell surface whereas GLUTs 3 and 5 are targeted to the apical membrane. To explore the molecular mechanisms underlying this asymmetric distribution, we analyzed the targeting of chimeric glucose transporter proteins in MDCK cells. Replacement of the carboxy-terminal cytosolic tail of GLUT1, GLUT2, or GLUT4 with that from GLUT3 resulted in apical targeting. Conversely, a GLUT3 chimera containing the cytosolic carboxy terminus of GLUT2 was sorted to the basolateral membrane. These findings are not attributable to the presence of a basolateral signal in the tails of GLUTs 1, 2, and 4 because the basolateral targeting of GLUT1 was retained in a GLUT1 chimera containing the carboxy terminus of GLUT5. In addition, we were unable to demonstrate the presence of an autonomous basolateral sorting signal in the GLUT1 tail using the low-density lipoprotein receptor as a reporter. By examining the targeting of a series of more defined GLUT1/3 chimeras, we found evidence of an apical targeting signal involving residues 473 - 484 (DRSGKDGVMEMN) in the carboxy tail. We conclude that the targeting of GLUT3 to the apical cell surface in MDCK cells is regulated by a unique cytosolic sorting motif.

Transient production of recombinant proteins by chinese hamster ovary cells using polyethyleneimine/DNA complexes in combination with microtubule disrupting anti-mitotic agents

We have developed a simple and robust transient expression system utilizing the 25 kDa branched cationic polymer polyethylenimine (PEI) as a vehicle to deliver plasmid DNA into suspension-adapted Chinese hamster ovary cells synchronized in G2/M phase of the cell cycle by anti-mitotic microtubule disrupting agents. The PEI-mediated transfection process was optimized with respect to PEI nitrogen to DNA phosphate molar ratio and the plasmid DNA mass to cell ratio using a reporter construct encoding firefly luciferase. Optimal production of luciferase was observed at a PEI N to DNA P ratio of 10:1 and 5 mug DNA 10(6) cells(-1). To manipulate transgene expression at mitosis, we arrested cells in G2/M phase of the cell cycle using the microtubule depolymerizing agent nocodazole. Using secreted human alkaline phosphatase (SEAP) and enhanced green fluorescent protein (eGFP) as reporters we showed that continued inclusion of nocodazole in cell culture medium significantly increased both transfection efficiency and reporter protein production. In the presence of nocodazole, greater than 90% of cells were eGFP positive 24 h post-transfection and qSEAP was increased almost fivefold, doubling total SEAP production. Under optimal conditions for PEI-mediated transfection, transient production of a recombinant chimeric IgG(4) encoded on a single vector was enhanced twofold by nocodazole, a final yield of approximately 5 mug mL(-1) achieved at an initial viable cell density of 1 x 10(6) cells mL(-1). The glycosylation of the recombinant antibody at Asn(297) was not significantly affected by nocodazole during transient production by this method. (C) 2004 Wiley Periodicals, Inc.