PROBING PROTEIN-PROTEIN INTERACTIONS - THE RIBOSE-BINDING PROTEIN IN BACTERIAL TRANSPORT AND CHEMOTAXIS

A number of mutations at Gly(134) of the periplasmic ribose binding pr otein of Escherichia coli were examined by a combined biochemical and structural approach. Different mutations gave rise to different patter ns of effects on the chemotaxis and transport functions. The smallest residue (alanine) had the least effect on transport, whereas large hyd rophobic residues had the smallest effect on chemotaxis. Comparison of the x-ray crystal structure of the C134R mutant protein (2.5 Angstrom resolution) to that of the wild type (1.6-Angstrom resolution) showed that the basic structure of the protein was unaltered. The loss of ch emotaxis and transport functions in this and similar mutant proteins m ust therefore be caused by relatively simple surface effects, which in clude a change in local main chain conformation. The loss of chemotaxi s and transport functions resulting from the introduction of an alanin e residue at position 134 was suppressed by an additional isoleucine t o threonine mutation at residue 132. An x-ray structure of the I132T/G 134A double mutant protein (2.2-Angstrom resolution) showed that the c hanges in local structure were accompanied by a diffuse pattern of str uctural changes in the surrounding region, implying that the suppressi on derives from a combination of sources.