MUTATIONS THAT ALTER THE TRANSMEMBRANE SIGNALING PATHWAY IN AN ATP BINDING CASSETTE (ABC) TRANSPORTER

The maltose transport system of Escherichia coli is a well-characteriz ed member of the ATP binding cassette transporter superfamily. Members of this familyshare sequence similarity surrounding two short sequenc es (the Walker A. and B sequences) which constitute a nucleotide bindi ng pocket. It is likely that the energy from binding and hydrolysis of ATP is used to accomplish the translocation of substrate from one loc ation to another. Periplasmic binding protein-dependent transport syst ems, Like the maltose transport system of E.coli, possess a water-solu ble ligand binding protein that is essential for transport activity. I n addition to delivering ligand to the membrane-bound components of th e system on the external face of the membrane, the interaction of the binding protein with the membrane complex initiates a signal that is t ransmitted to the ATP binding subunit on the cytosolic side and stimul ates its hydrolytic activity. Mutations that alter the membrane comple x so that it transports independently of the periplasmic binding prote in also result in constitutive activation of the ATPase. Genetic analy sis indicates that, in general, two mutations are required for binding protein-independent transport and constitutive ATPase. The mutations alter residues that cluster to specific regions within the membrane sp anning segments of the integral membrane components MalF and MalG. Ind ividually, the mutations perturb the ability of MBP to interact produc tively with the membrane complex. Genetic alteration of this signallin g pathway suggeststhat other agents might have similar effects. These could be potentially useful for modulating the activities of ABC trans porters such as P-glycoprotein orCFTR, that are implicated in disease.