he bacterial cyclic AMP receptor protein (CRP) has been studied extensively as a model allosterically controlled transcription factor, despite the lack of a crystal structure of its inactive (unliganded) form. We report the crystal structure at 2.0 Å resolution of the unliganded CRP dimer from M. tuberculosis H37Rv (Mt), in which the protein is known to regulate over a hundred genes. The overall structure is similar to that seen for the cAMP-bound E. coli protein, but the homodimer is profoundly asymmetric, with a root-mean-square deviation of 3.5 Å between all C positions in the two chains, and extensive asymmetric interactions involving both main chain and side chains. The two C-domains have numerous residues with completely different local environments and hydrogen bond interactions, especially in the DNA-binding and hinge regions. The observed level of asymmetry is far beyond the normal range for homodimeric crystal structures, and beyond that observed in E. coli CRP structures. The structure is compared with that of E. coli CRP in its cAMP-bound state. We suggest that the observed structural asymmetry may serve to prevent DNA binding in the absence of cAMP, and that the cAMP-induced allosteric transition switches the dimer to a more symmetric, DNA-binding-competent form.
Citation: Journal of Biological Chemistry
Pub Type: Journals
allostery, asymmetry, catabolite activator protein (CAP), crystal, cyclic AMP, dimorphism, DNA-binding, transcription factor, x-ray diffraction