Progress in the identification of interaction sites on the periplasmic maltose binding protein from E coli

The periplasmic maltose binding protein (MBP) is required for the high affinity transport of maltose and maltodextrins and for chemotaxis towards these sugars. In these functions, MBP interacts with proteins of the cytoplasmic membrane: MalF and MalG for transport, Tar for chemotaxis. A large number of MBP mutations have been isolated by us and other laboratories. We grouped these mutations into classes depending on the interactions affected and we represented the corresponding residues on the 3-D model for MBP so as to further identify the sites of MBP interacting with the MalF-MalG complex and with the Tar protein. MBP (like the other binding proteins) is composed of 2 lobes enclosing a cleft where the substrate binds. The face of the protein opposite the cleft seems to interact neither with MalF-MalG nor with Tar. The other face, corresponding to the cleft, contains sites for interactions with MalF-MalG and Tar. These sites appear to cover both sides of the cleft and may overlap in part. The present definition of the interaction sites suggests further that MBP has different in vivo orientations when it interacts with MalF-MalG or with Tar. This work constitutes an additional step in combining the use of genetic and structural analysis to define the interaction sites on MBP. Because of the structural similarities between periplasmic binding proteins, the regions of interaction defined could be relevant for other members of this family.