Mechanical regulation of Titin N2B-us conformation and its binding to FHL2

The 572 amino acids unique sequence on titin N2B element (N2B-us) is known to regulate the passive elasticity of muscle as an elastic spring. It also serves as a hub for cardiac hypertrophic signaling by interacting with multiple proteins such as FHL1(Sheikh et al , 2008), FHL2(Lange et al , 2002), and Erk2(Perkin et al , 2015). N2B-us is thought to be an intrinsically disordered region. In addition, N2B-us bears force; therefore, the functions of N2B-us are likely regulated by mechanical stretching. In the work, we investigated the conformation of N2B-us as well as its force-dependent interaction with FHL2 using a combination of AlphaFold2 predictions and single-molecule experimental validation. Surprisingly, a stable alpha/beta structural domain (~115 a.a.) was predicted and confirmed in N2B-us, which can be mechanically unfolded at forces greater than 5 pN. More than twenty FHL2 LIM domain binding sites were predicted to spread throughout N2B-us including the regions cryptic in the structural domain. Mechanosensitive binding of FHL2 to N2B-us is revealed in single-molecule manipulation experiments. Together, the results unveil several previously unknown aspects of the N2B-us conformations and its force-dependent interactions with FHL2, which provides new insights into the physiological functions of the force-bearing N2B-us region.