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Determination of the Conformations of cAMP Receptor Protein and Its T127L,S128A Mutant with and without cAMP from Small Angle Neutron Scattering Measurements*

Small angle neutron scattering (SANS) measurements were performed on solutions of cAMP receptor protein (CRP) and on solutions of the T127L,S128A double mutant of CRP (CRP*) in D2O K3PO4 buffer containing 0.5 m KCl, in the absence and presence of 3′,5′ cyclic adenosine monophosphate (cAMP). Energy-minimized structures of the CRP were calculated by minimization of the x-ray crystallographic structure of CRP in either the exclusively “closed” form where the α-helices of the carboxyl-terminal domain are folded close to the amino-terminal domain and in the exclusively “open” form where the α-helices of the carboxyl-terminal domain are folded away from the amino-terminal domain. Neutron scattering models show that the CRP SANS data follow closely the data curve predicted for unligated CRP in the open form, whereas the cAMP-ligated data are more in agreement with the data predicted for the minimized cAMP-ligated CRP structure in the closed form. Thus, it appears that CRP undergoes a conformational change from the open form to the closed form in solution upon ligation with cAMP. The SANS data from the CRP* and cAMP-ligated CRP* are coincidental, which implies that there is very little structural difference between the two species of CRP*. This is in agreement within vivo results, which show that whereas CRP activates transcription in the cell only in the presence of cAMP, CRP* activates transcription in the absence of cAMP, implying that CRP* is already in the correct conformation for the activation of transcription.