Differences in stability and calcium sensitivity of the Ig domains in titin's N2A region

Titin is a large filamentous protein that spans half a sarcomere, from Z-disk to M-line. The N2A region within the titin molecule exists between the proximal immunoglobulin (Ig) region and the PEVK region and protein–protein interactions involving this region are required for normal muscle function. The N2A region consists of four Ig domains (I80–I83) with a 105 amino acid linker region between I80 and I81 that has a helical nature. Using chemical stability measurements, we show that predicted differences between the adjacent Ig domains (I81–I83) correlate with experimentally determined differences in chemical stability and refolding kinetics. Our work further shows that I83 has the lowest ΔG_unfolding, which is increased in the presence of calcium (pCa 4.3), indicating that Ca²⁺ plays a role in stabilizing this immunoglobulin domain. The characteristics of N2A's three Ig domains provide insight into the stability of the binding sites for proteins that interact with the N2A region. This work also provides insights into how Ca²⁺ might influence binding events involving N2A.