Multivesicular bodies (MVBs) are an obligatory step in the trafficking of cell surface receptors for degradation. In the Dec. 23, 2010, issue of Cell, Taelman et al. show that during canonical Wnt signaling the enzyme Glycogen Synthase Kinase 3 (GSK3, shown in red in the diagram) is bound by the Wnt receptor complex, becoming sequestered from the cytoplasm inside MVBs. In this way, the Wnt growth factor inhibits GSK3 activity, causing the stabilization of many proteins. This "enzyme imprisonment" mechanism links the basic intracellular membrane trafficking machinery to cell signaling.