Biophysical investigations of engineered polyproteins: implications for force data

Dynamic force spectroscopy is rapidly becoming a standard biophysical technique. Significant advances in the methods of analysis of force data have resulted in ever more complex systems being studied. The use of cloning systems to produce homologous tandem repeats rather than the use of endogenous multidomain proteins has facilitated these developments. What is poorly addressed are the physical properties of these constructed polyproteins. Are the properties of the individual domains in the construct independent of one another or attenuated by adjacent domains? We present data for a construct of eight fibronectin type III domains from the human form of tenascin that exhibits approximately 1 kcal mol(-1) increase in stability compared to the monomer. This effect is salt and pH dependent, suggesting that the stabilization results from electrostatic interactions, possibly involving charged residues at the interfaces of the domains. Kinetic analysis shows that this stabilization reflects a slower unfolding rate. Clearly, if domain-domain interactions affect the unfolding force, this will have implications for the comparison of absolute forces between types of domains. Mutants of the tenascin 8-mer construct exhibit the same change in stability as that observed for the corresponding mutation in the monomer. And when Phi-values are calculated for the 8-mer construct, the pattern is similar to that observed for the monomer. Therefore, mutational analyses to resolve mechanical unfolding pathways appear valid. Importantly, we show that interactions between the domains may be masked by changes in experimental conditions.