An improved purification of lactose permease.

The integral membrane protein lactose permease of Escherichia coli was purified to homogeneity in detergent micelles. No other proteins could be detected in the final product. The molar ratio of lactose permease to lipid was less than 0.2 in detergent solution, thus the preparation was essentially lipid-free. All molecules were functionally active as shown by substrate binding.