Identification of a mechanical rheostat in the hydrophobic core of protein L

The ability of proteins and their complexes to withstand or respond to mechanical stimuli is vital for cells to maintain their structural organisation, to relay external signals and to facilitate unfolding and remodelling. Force spectroscopy using the atomic force microscope allows the behaviour of single protein molecules under an applied extension to be investigated and their mechanical strength to be quantified. protein L, a simple model protein, displays moderate mechanical strength and is thought to unfold by the shearing of two mechanical sub-domains. Here, we investigate the importance of side-chain packing for the mechanical strength of protein L by measuring the mechanical strength of a series of protein L variants containing single conservative hydrophobic volume deletion mutants. Of the five thermodynamically destabilized variants characterised, only one residue (I60 V) close to the interface between two mechanical sub-domains was found to differ in mechanical properties to wild type (Delta F(I60 V-WT)=-36 pN at 447 nm s(-1), Delta x(uI60V-WT)=0.2 nm). Phi-value analysis of the unfolding data revealed a highly native transition state. To test whether the number of hydrophobic contacts across the mechanical interface does affect the mechanical strength of protein L, we measured the mechanical properties of two further variants. protein L L10F, which increases core packing but does not enhance interfacial contacts, increased mechanical strength by 13+/-11 pN at 447 nm s(-1). By contrast, protein L I60F, which increases both core and cross-interface contacts, increased mechanical strength by 72+/-13 pN at 447 nm s(-1). These data suggest a method by which nature can evolve a varied mechanical response from a limited number of topologies and demonstrate a generic but facile method by which the mechanical strength of proteins can be rationally modified.