Backbone Dynamics of a symmetric Calmodulin dimer in complex with the calmodulin-binding domain of the basic-Helix-Loop-Helix transcription factor SEF2-1/E2-2 - A highly dynamic complex
Göran Larsson 1, Juergen Schleucher 1, Jacqueline Onions 2, Stefan Herman 3, Thomas Grundstrom 1 and Sybren S. Wijmenga 4*
1 Umea University, Sweden
2 Umea University,Sweden
3 Karolinska Institute, Sweden
4 Radboud University
* To whom correspondence should be addressed. E-mail: sybrenw{at}sci.kun.nl.
Submitted on November 8, 2004
Revised on January 5, 2005
Accepted on 4 May 2005
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Abstract |
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Calmodulin (CaM) interacts specifically as a dimer with some dimeric basic-Helix-Loop-Helix (bHLH) transcription factors via a novel high affinity binding-mode. Here we report a study of the backbone dynamics by 15N-spin relaxation on the CaM dimer in complex with a dimeric peptide that mimics the CaM binding region of the bHLH transcription factor SEF2-1. The relaxation data were measured at multiple magnetic fields, and analysed in a model-free manner using in-house written software designed to detect ns internal motion. Besides picosecond motions, all residues also experience internal motion with an effective correlation time of ca. 2.5 ns with squared order parameter (S2) of ca 0.75. Hydrodynamic calculations suggest that this can be attributed to motions of the N- and C-terminal domains of the CaM dimer in the complex. Hydrodynamic calculations also suggest the possible presence of monomer internal motion. Moreover, residues with significant exchange broadening are found. They are located in structural 'hotspots' and have similar exchange times (ca. 50 ms), suggesting a cooperative mechanism probably caused by protein:protein interactions. The dynamic features presented here, support the conclusion that the conformationally heterogeneous bHLH mimicking peptide trapped inside the CaM dimer exchanges between different binding sites on both ns- and ms-time scales. Nature has thus found a way to specifically recognise a relatively ill-fitting target. This novel mode of target-specific binding, which neither belongs to lock-and-key - nor induced-fit binding, is characterised by dimerization and continuous exchange between multiple flexible binding alternatives.
Key Words:
15N relaxation, Dynamics, NMR, calmodulin, exchange, nanosecond motion
