The leader region of pre-maltose binding protein binds amphiphiles. A model for self-assembly in protein export

Maltose binding protein, like most periplasmic proteins, is resistant to a variety of proteinases. Treatment of pre-maltose binding protein with trypsin, chymotrypsin, or proteinase K removes an amino-terminal domain of the same approximate size as the leader sequence without degrading the mature portion of the protein. In addition, pre-maltose binding protein is as active as mature in binding maltose (Ferenci, T., and Randall, L.L. (1979) J. Biol. Chem. 254, 9979-9981). By these criteria, the precursor and mature proteins are in the same conformation except for the exposed leader sequence on the precursor. We have compared the ability of these proteins to interact with amphipaths, such as detergents. The precursor protein binds to Triton X-100, while the mature protein does not. We propose that the leader domain is responsible for detergent binding. Mutations in the leader region of the precursor which block export in vivo prevent detergent binding in vitro. A mutant with a mild export defect can still bind detergent. This correlation between detergent binding by precursors with related leaders and export efficiency of each precursor suggests that hydrophobic partition of the leader may initiate pre-protein transfer across the membrane.