Crystallographic studies of the maltodextrin-binding protein from Escherichia coli
Rodseth, Lynn Elaine
Doctor of Philosophy
Crystals were prepared for several studies of the periplasmic maltodextrin-binding protein (MBP) from E. coli. This protein is part of the bacterial transport system for starch metabolites. The complex of MBP with maltotetraose was refined and analysed. The maltotetraose conformation has surprising implications, as it is unusual in terms of the sugar literature but the same as that of maltoheptaose bound to glycogen phosphorylase a (Goldsmith et al., Journal of Molecular Biology 156:411-427, 1983). The complex suggests explanations for an unexpectedly low binding affinity for maltotetraose. MBP was co-crystallized with alpha-cyclodextrin and beta-cyclodextrin; data has been collected from the latter crystal. Crystals and data were also obtained for sugar-free MBP. The current model of transport by this system suggests a conformational change in MBP upon sugar binding which is recognized by the transport system. A mutation in the hinge region of MBP produced a ligand-specific effect on transport; crystals of this mutant with two different ligands were grown. Two further experiments use MBP for general studies of protein structure. First, two deletions on the surface of MBP remove secondary structural elements without affecting the function of the protein (Duplay et al., Journal of Molecular Biology 194:663-678, 1987). Both were crystallized, and data from one was collected and structural analysis begun. Finally, a mutant produced by inserting the C3 neutralizing epitope from type 1 poliovirus into one of the surface deletion sites was crystallized. The structure was examined carefully, but the insertion was found to be disordered.