FIGURE 5. Structural model for high constitutive activity of T127L/S128I CRP.

The hypothesized local structure of T127L/S128I CRP (A) was derived from the structure of active cAMP-bound WT CRP (B) (Protein Data Bank code 1CGP). In this view along a portion of the C-helices, the DNA-binding domains are in the distance. The T127L/S128I CRP structure is not meant to represent the actual one, but rather to indicate a reasonable set of Leu¹²⁷ and Ile¹²⁸ conformers that might explain its high constitutive activity. In each figure, two subunits are differently colored. In A, the side chains of Leu⁶¹, Leu¹²⁷, and Ile¹²⁸ residues are shown. In B, the side chains of Leu⁶¹, Thr¹²⁷, and Ser¹²⁸ residues and cAMP molecules are shown, and the known interactions (adopted from Ref. 14) are also indicated. The structure was generated using Swiss-PdbViewer Version 3.7.