Membrane microdomains (lipid rafts) are now recognized as critical for proper compartmentalization of insulin signaling, but their role in the pathogenesis of insulin resistance has not been investigated. Detergent-resistant membrane microdomains (DRMs), isolated in the low-density fractions, are highly enriched in cholesterol, glycosphingolipids and various signaling molecules. Tumor necrosis factor alpha (TNFalpha) induces insulin resistance in type 2 diabetes, but its mechanism of action is not fully understood. In other studies we have found a selective increase in the acidic glycosphingolipid ganglioside GM3 in 3T3-L1 adipocytes treated with TNFalpha, suggesting a specific function for GM3. In the DRMs from TNFalpha-treated 3T3-L1 adipocytes, GM3 levels were doubled compared with results in normal adipocytes. Additionally, insulin receptor (IR) accumulations in the DRMs were diminished, whereas caveolin and flotillin levels were unchanged. Furthermore, insulin-dependent IR internalization and intracellular movement of the IR substrate 1(IRS-1) were both greatly suppressed in the treated cells, leading to an uncoupling of IR-IRS-1 signaling. GM3 depletion was able to counteract the TNFalpha-induced inhibitions of IR internalization and accumulation into DRMs. Together, these findings provide compelling evidence that in insulin resistance the insulin metabolic signaling defect can be attributed to a loss of IRs in the microdomains due to an accumulation of GM3.