IDENTICAL MUTATIONS AT CORRESPONDING POSITIONS IN 2 HOMOLOGOUS PROTEINS WITH NONIDENTICAL EFFECTS

The x-ray structure of a mutant (Gly72 to Asp) of the Escherichia coli ribose-binding protein with altered transport function has been solve d and refined to 2.2-angstrom resolution with a conventional R-factor (R-factor = SIGMA\\F(obs)\ - \F(calc)\\/SIGMA\F(obs)) of 16.0% and goo d stereochemistry. Comparison with the wild type ribose-binding protei n shows that the structure is disturbed little at the actual mutation site, but quite appreciably in a neighboring loop. Changes in the surf ace of the protein at the site of mutation, however, seem to explain t he functional effects. A corresponding mutation of the related glucose /galactose-binding protein has different structural and functional eff ects due to the different structural context of the mutation site in t hat 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.