The x-ray structure of a mutant (Gly72 to Asp) of the Escherichia coli ribose-binding protein with altered transport function has been solved and refined to 2.2-A resolution with a conventional R-factor (R-factor = [formula: see text]) of 16.0% and good stereochemistry. Comparison with the wild type ribose-binding protein shows that the structure is disturbed little at the actual mutation site, but quite appreciably in a neighboring loop. Changes in the surface of the protein at the site of mutation, however, seem to explain the functional effects. A corresponding mutation of the related glucose/galactose-binding protein has different structural and functional effects due to the different structural context of the mutation site in that protein. These results are consistent with the concept that these proteins have slightly different ways of interacting with the membrane components in transport and chemotaxis.