Invited
Forced unfolding of tandem proteins within polyproteins exhibit sequential dependency

The unique arrangement of polyproteins, comprised from proteins arrayed in tandem, enables their specific physiological function under mechanical loads through partial unfolding and extension. Probed with single molecule force spectroscopy (SMFS), sequential unfolding-times of N proteins within a polyprotein are considered as a random-variable. In contrast to the intuitive expectation from a random-variable that describes time between events, this variable is reported not to follow the exponential distribution. So far, various interpretations and statistical means to explain this deviation have been suggested, all sharing an underlying assumption that such sequential unfolding is characterized by unfolding events that are independent of each other and identically distributed (iid). According to this assumption, the unfolding kinetics of a polyprotein made of N proteins, is equivalent to the unfolding kinetics of such N individual proteins. Here we demonstrate that the introduction of an external load, as an unfolding agent, introduces correlations between the sequential events during the unfolding along the polyprotein. This observation confronts the iid assumption. With the use of continuous time random walk approach we show that polyprotein unfolding exhibits anomalous subdiffusive transport. Put together with free-energy reconstruction of the whole unfolding polyprotein, we provide physical explanation for this nontrivial behavior, according to which the elongating polypeptide chain with each unfolding event intervenes with the sequential unfolding probabilities.