THE ESCHERICHIA-COLI MANNITOL PERMEASE AS A MODEL FOR TRANSPORT VIA THE BACTERIAL PHOSPHOTRANSFERASE SYSTEM

The bacterial phosphoenolpyruvate-dependent carbohydrate phosphotransf erase system (PTS) consists of several proteins whose primary function s are to transport and phosphorylate their substrates. The complexity of the PTS undoubtedly reflects its additional roles in chemotaxis to PTS substrates and in regulation of other metabolic processes in the c ell. The PTS permeases (Enzymes II) are the membrane-associated protei ns of the PTS that sequentially recognize, transport, and phosphorylat e their specific substrates in separate steps, and the Escherichia col i mannitol permease is one of the best studied of these proteins. It c onsists of two cytoplasmic domains (EIIA and EIIB) involved in mannito l phosphorylation and an integral membrane domain (EIIC) which is suff icient to bind mannitol, but which transports mannitol at a rate that is dependent on phosphorylation of the EIIA and EIIB domains. Recent r esults show that several residues in a hydrophilic, 85-residue segment of the EIIC domain are important for the binding, transport, and phos phorylation of mannitol. This segment may be at least partially expose d to the cytoplasm of the cell. A model is proposed in which this regi on of the EIIC domain is crucial in coupling phosphorylation of the EI IB domain to transport through the EIIC domain of the mannitol permeas e.