Dependence of maltose transport and chemotaxis on the amount of maltose-binding protein.

Maltose-binding protein (MBP) is essential for maltose transport and chemotaxis in Escherichia coli. To perform these functions it must interact with two sets of cytoplasmic membrane proteins, the MalFGK transport complex and the chemotactic signal transducer Tar. MBP is present at high concentrations, on the order of 1 mM, in the periplasm of maltose-induced or malTc constitutive cells. To determine how the amount of MBP affects transport and taxis, we utilized a series of malE signal-sequence mutations that interfere with export of MBP. The MBP content in shock fluid from cells carrying the various mutations ranged from 4 to 23% of the malE+ level. The apparent Km for maltose transport varied by less than a factor of 2 among malE+ and mutant strains. At a saturating maltose concentration 9% (approximately 90 microM) of the malE+ amount of MBP was required for half-maximal uptake rates. Transport exhibited a sigmoidal dependence on the amount of periplasmic MBP, indicating that MBP may be involved in a cooperative interaction at some stage of the transport process. The chemotactic response to a saturating maltose stimulus exhibited a first-order dependence on the amount of periplasmic MBP. Thus, interaction of a single substrate-bound MBP with Tar appears sufficient to initiate a chemotactic signal from the transducer. A half-maximal chemotactic response occurred at 25% of the malE+ MBP level, suggesting that in vivo the KD for binding of maltose-loaded MBP to Tar is quite high (approximately 250 microM).