Tandem repeating modular proteins avoid aggregation in single molecule force spectroscopy experiments.

We have used single molecule force spectroscopy to explore the unfolding and refolding behavior of the immunoglobulin-like I27 protein in aqueous 2,2,2-trifluoroethanol (TFE). In bulk solution experiments, a 28% v/v TFE solution has previously been observed to enhance intermolecular attractions and lead to misfolding and aggregation of tandem modular proteins of high sequence identity. In our single molecule experiments, however, we measure successful refolding of the polyprotein I27(8) in all TFE solutions up to 35% v/v. Using a single molecule micromanipulation technique, we have shown that refolding of a polyprotein with identical repeats is not hindered by the presence of this cosolvent. These experimental results provide new insight into the properties of tandem repeating proteins and raise interesting questions as to the evolutionary success of such proteins in avoiding misfolding and aggregation.