Mechanical responses of the mechanosensitive unstructured domains in cardiac titin

Titin is one of the three main filaments in cardiac sarcomere. Besides a chain of Ig domains, cardiac titin also contains a proline (P), glutamate (E), valine (V), lysine (K) (PEVK) domain and a cardiac-specific N2B domain, both are largely unstructured. While they are believed to be involved in the elastic (PEVK and N2B) and the trophic (N2B) functions of the heart, their mechanical responses in physiological level of forces remains poorly understood.

In order to gain understanding on their mechanical responses, we used magnetic tweezers to investigate their force responses from 1 to 30 pN. We confirmed that in vitro the PEVK domain is intrinsically disordered within the force range. Surprisingly, we discovered a mechanosensitive folded element in the disordered region of N2B, ∼84 amino acids in length, which has a large folding energy of approximately -10 k_B T. Based on the force responses of PEVK and N2B domains, as well as an approximated force-dependent unfolding and refolding rates of titin Ig domains, we show that the tension in cardiac titin fluctuates within 5 pN during cardiac contraction and extension cycle using Gillespie simulation algorithm. Exceptionally, the simulation shows that deletion of N2B domain results in 10-fold increase in peak force.

Our results highlight a critical role that N2B may potentially play in regulating tension on cardiac titin.

The study provides new insights into the tension regulatory role of unstructured domains in the elastic function of the heart, which has broad implication in diastolic dysfunction and cardiac trophic mechanisms. In addition, the method can be applied to probing other unstructured mechanosensitive proteins/domains.