TUG (Tether containing UB, Xenopus/Amphibian, domain for GLUT4) protein contains a UB, Xenopus/Amphibian, domain for GLUT4. In truncated form, TUG acts in a dominant-negative manner to inhibit insulin-stimulated GLUT4 redistribution in Chinese hamster ovary cells and 3T3-L1 adipocytes. Full-length TUG forms a comple, Xenopus/Amphibian, specifically with GLUT4. In 3T3-L1 adipocytes this comple, Xenopus/Amphibian, is present in unstimulated cells and is largely disassembled by insulin. Endogenous TUG is localized with the insulin-mobilizable pool of GLUT4 in unstimulated 3T3-L1 adipocytes and is not mobilized to the plasma membrane by insulin.