Abstact

A number of mutations at Gly134 of the periplasmic ribose-binding protein of Escherichia coli were examined by a combined biochemical and structural approach. Different mutations gave rise to different patterns of effects on the chemotaxis and transport functions. The smallest residue (alanine) had the least effect on transport, whereas large hydrophobic residues had the smallest effect on chemotaxis. Comparison of the x-ray crystal structure of the G134R mutant protein (2.5-A resolution) to that of the wild type (1.6-A resolution) showed that the basic structure of the protein was unaltered. The loss of chemotaxis and transport functions in this and similar mutant proteins must therefore be caused by relatively simple surface effects, which include a change in local main chain conformation. The loss of chemotaxis and transport functions resulting from the introduction of an alanine residue at position 134 was suppressed by an additional isoleucine to threonine mutation at residue 132. An x-ray structure of the I132T/G134A double mutant protein (2.2-A resolution) showed that the changes in local structure were accompanied by a diffuse pattern of structural changes in the surrounding region, implying that the suppression derives from a combination of sources.