Studies of the Maltose Transport System Reveal a Mechanism for Coupling ATP Hydrolysis to Substrate Translocation without Direct Recognition of Substrate

The ATPase activity of the maltose transporter (MalFGK2) is dependent on interactions with the maltose-binding protein (MBP). To determine whether direct interactions between the translocated sugar and MalFGK2 are important for the regulation of ATP hydrolysis, we used an MBP mutant (sMBP) that is able to bind either maltose or sucrose. We observed that maltose-and sucrose-bound sMBP stimulate equal levels of MalFGK2 ATPase activity. Therefore, the ATPase activity of MalFGK2 is coupled to translocation of maltose solely by interactions between MalFGK2 and MBP. For both maltose and sucrose, the ability of sMBP to stimulate the MalFGK2 ATPase was greatly reduced compared with wild-type MBP, indicating that the mutations in sMBP have interfered with important interactions between MBP and MalFGK2. High resolution crystal structure analysis of sMBP shows that in the closed conformation with bound sucrose, three of four mutations are buried, and the fourth causes only a minor change in the accessible surface. In contrast, in the open form of sMBP, all of the mutations are accessible, and the main chain of Tyr(62)-Gly(69) is destabilized and occupies an alternative conformation due to the W62Y mutation. On this basis, the compromised ability of sMBP to stimulate ATP hydrolysis by MalFGK2 is most likely due to a disruption of interactions between MalFGK2 and the open, rather than the closed, conformation of sMBP. Modeling the open sMBP structure bound to MalFGK2 in the transition state for ATP hydrolysis points to an important site of interaction and suggests a mechanism for coupling ATP hydrolysis to substrate translocation that is independent of the exact structure of the substrate.