The HIV-1 encoded virus protein U (VpU) is required for efficient viral release from human host cells and for induction of CD4 degradation in the endoplasmic reticulum. The cytoplasmic domain of the membrane protein VpU (VpUcyt) is essential for the latter activity. The structure and dynam-ics of VpUcyt were characterized in the presence of membrane simulating dodecylphosphatidylcholine (DPC) micelles by high-resolution liquid state NMR.