Producing membranous proteins in large quantities isn't difficult, but producing them as part of a membrane is. That's a problem, since the proteins must be part of a membrane to be functional. But if you want a membrane protein expressed, "we'd be the ones to do it," says Harry Meade, senior vice president of research and development at GTC Biotherapeutics in Framingham, Mass. GTC was recently awarded US patent 6,743,966 for a method to do so.

The method, explains Meade, is based on the natural secretion process of fat from the mammary glands. Each fat droplet has a membrane around it. In GTC's method, a DNA sequence encoding a membrane protein and a mammary gland-specific promoter are inserted into the mammal's genome. "You can direct these membrane proteins to the cell surface, and then when the fat droplets bud out of there, they carry the protein with it," he says....

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