FIG. 7.

Structure-function relationships in P5-Cys mutants. All images are alpha-carbon backbone traces of P5, with key residues in space-filled representation. The same shading convention is used throughout: light gray residues indicate biochemical defects caused by the cysteine replacement itself, and black residues indicate that modification of a cysteine at that position caused the biochemical defect. The upper left and middle images correspond to the P5 orientation in the upper middle image of Fig. 4; the next three images correspond to the P5 orientation in the upper left image of Fig. 4. The image at bottom right corresponds to the P5 orientation in the bottom right image of Fig. 4. (A) P5-Cys sites that cause CheW-binding defects. Left: residues in loops and turns. The N and C termini and the subdomain interface are indicated. Both of the next two images show all other P5 residues implicated in CheW binding. Middle: same orientation as the left image. The four labeled sites may modulate conformational motions between subdomains. The boxed residues correspond to the putative CheW target site shown in the upper middle image of Fig. 4. Right: the boxed area corresponds to the putative binding target for CheW shown in the upper left image of Fig. 4. Some of the labeled sites may be direct binding targets; others may modulate the conformation of the binding surface. (B) P5-Cys sites that specifically affect receptor-mediated activation of CheA (see text for discussion). Underlined labels denote sites that cause a substantial activation defect before 5-FM modification and another substantial reduction in activity after modification. (C) P5-Cys sites that primarily affect receptor-mediated deactivation. The unmodified mutant proteins assemble CheA-activating ternary complexes but cannot deactivate in response to an attractant stimulus. All such sites are located in subdomain 2. Labeled residues in the first image reside in loop 1; labeled residues in the second image reside near the subdomain interface. In the right image, the loop 1 residues are not shown, to better illustrate the packing interactions and backbone connections of the other residues.