MEMBRANE TOPOLOGY OF THE L-RHAMNOSE-H+ TRANSPORT PROTEIN (RHAT) FROM ENTEROBACTERIA

The L-rhamnose-H+ symporter (RhaT) is a 344-amino acid integral membra ne protein, found in many Enterobacteria, which couples the uptake of the sugar L-rhamnose with the inward movement of protons. Based on its hydropathy profile and the application of von Heijne's ''positive ins ide'' rule (von Heijne, G. (1992) J. Mol. Biol. 225, 487-494), a model of the L-rhamnose-H+ symport protein (RhaT) is proposed containing 10 transmembrane helices with the NH2 and COOH termini in the periplasm. This model was tested by the creation of random beta-lactamase (Bla) fusions. The data from 33 unique, randomly generated, RhaT-Bla fusions and from 5 site-specific fusions supported the proposed topology betw een transmembrane helices 2-10. However, the localization of the putat ive first hydrophilic loop and the NH2 terminus was not possible becau se the beta-lactamase fusions in this region were shown to be unreliab le indicators of the topology of RhaT.