The effect of protein complexation on the mechanical stability of Im9

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 (similar to 20 pN). Complexation of E9 had little effect on the mechanical resistance of Im9 (unfolding force similar to 30 pN) despite the high avidity of this complex (K-d similar to 10 fM).