UNLIGANDED MALTOSE-BINDING PROTEIN TRIGGERS LACTOSE TRANSPORT IN AN ESCHERICHIA-COLI MUTANT WITH AN ALTERATION IN THE MALTOSE TRANSPORT-SYSTEM

Escherichia coli accumulates malto-oligosaccharides by the maltose tra nsport system, which is a member of the ATP-binding-cassette (ABC) sup erfamily of transport systems. The proteins of this system are LamB in the outer membrane, maltose-binding protein (MBP) in the periplasm, a nd the proteins of the inner membrane complex (MalFGK(2),), composed o f one MalF, one MalG, and two MalK subunits. Substrate specificity is determined primarily by the periplasmic component, MBP. However, sever al studies of the maltose transport system as well as other members of the ABC transporter superfamily have suggested that the integral inne r membrane components MalF and MalG may play an important role in dete rmining the specificity of the system. We show here that residue L334 in the fifth transmembrane helix of MalF plays an important role in de termining the substrate specificity of the system. A leucine-to-trypto phan alteration at this position (L334W) results in the ability to tra nsport lactose in a saturable manner. This mutant requires functional MalK-ATPase activity and the presence of MBP, even though MBP is incap able of binding lactose. The requirement for MBP confirms that unligan ded MBP interacts,vith the inner membrane MalFGK, complex and that MBP plays a crucial role in triggering the transport process.