Conservation of residues involved in sugar/H(+) symport by the sucrose permease of Escherichia coli relative to lactose permease.

Building a three-dimensional model of the sucrose permease of Escherichia coli (CscB) with the X-ray crystal structure lactose permease (LacY) as template reveals a similar overall fold for CscB. Moreover, despite only 28% sequence identity and a marked difference in substrate specificity, the structural organization of the residues involved in sugar-binding and H(+) translocation is conserved in CscB. Functional analyses of mutants in the homologous key residues provide strong evidence that they play a similar critical role in the mechanisms of CscB and LacY.