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J. Biol. Chem., Vol. 282, Issue 6, 3632-3639, February 9, 2007

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A C-helix Residue, Arg-123, Has Important Roles in Both the Active and Inactive Forms of the cAMP Receptor Protein^*

Hwan Youn, Robert L. Kerby, Junseock Koh, and Gary P. Roberts¹

From the Departments of Bacteriology and Biochemistry, University of Wisconsin, Madison, Wisconsin 53706

The cAMP receptor protein (CRP) of Escherichia coli exists in an equilibrium between active and inactive forms, and the effector, cAMP, shifts that equilibrium to the active form, thereby allowing DNA binding. For this equilibrium shift, a C-helix repositioning around the C-helix residues Thr-127 and Ser-128 has been reported as a critical local event along with proper 4/5 positioning. Here we show that another C-helix residue, Arg-123, has a unique role in cAMP-dependent CRP activation in two different ways. First, Arg-123 is important for proper cAMP affinity, although it is not critical for the conformational change with saturating amounts of cAMP. Second, Arg-123 is optimal for stabilizing the inactive conformation of CRP when cAMP is absent, thereby allowing a maximal range of regulation by cAMP. However, Arg-123 does not appear to be critical for a functional response to cAMP, as has been proposed previously (Berman, H. M., Ten Eyck, L. F., Goodsell, D. S., Haste, N. M., Korney, A., and Taylor, S. S. (2005) Proc. Natl. Acad. Sci. U. S. A. 102, 45–50). Based on mutagenic evidence, we also propose the basis for the stabilization of the inactive form to be through a salt interaction between Asp-68 and Arg-123.

Received for publication, July 11, 2006 , and in revised form, December 6, 2006.

^* This work was supported by the College of Agricultural and Life Sciences at University of Wisconsin-Madison and by National Institutes of Health Grants GM53228 (to G. P. R.) and GM23467 (to M. T. Record, Jr.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Table 1.

¹ To whom correspondence should be addressed. Tel.: 608-262-3567; Fax: 608-262-9865; E-mail: groberts{at}bact.wisc.edu.

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