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The Effect of Protein Complexation on the Mechanical Stability of Im9

Eleanore Hann ^* , Nadine Kirkpatrick , Colin Kleanthous , D. Alastair Smith  , Sheena E. Radford ^*  and David J. Brockwell ^* 

^* Institute of Molecular and Cellular Biology and Astbury Centre for Structural Molecular Biology, University of Leeds, United Kingdom; Department of Biology, University of York, United Kingdom; and School of Physics and Astronomy, University of Leeds, United Kingdom

Correspondence: Address reprint requests and inquiries to D. J. Brockwell, E-mail: brock{at}bmb.leeds.ac.uk.

Force mode microscopy can be used to examine the effect of mechanical manipulation on the noncovalent interactions that stabilize proteins and their complexes. Here we describe the effect of complexation by the high affinity protein ligand E9 on the mechanical resistance of the simple four-helical protein, Im9. When concatenated into a construct of alternating I27 domains, Im9 unfolded below the thermal noise limit of the instrument (20 pN). Complexation of E9 had little effect on the mechanical resistance of Im9 (unfolding force 30 pN) despite the high avidity of this complex (K_d 10 fM).