# Calcium‐dependent cooperativity and stability of Titin's tandem I82‐I83 domains

## TL;DR

This study explores how calcium affects the stability and cooperative behavior of two domains in the muscle protein titin, which is important for muscle function and disease.

## Contribution

The study reveals calcium-dependent cooperativity and structural changes in the tandem I82-I83 domains of titin.

## Key findings

- The tandem I82-I83 domains show cooperative unfolding as a single event.
- Calcium increases the stability of the I82-I83 domains.
- NMR structure shows a tighter interface between I82 and I83 compared to crystal structure.

## Abstract

The muscle protein titin spans half a sarcomere, from M‐line to Z‐disk, and is essential for both active and passive stretch. The N2A region of titin plays a critical role in various regulatory processes through its binding interactions. Located at the C‐terminus of the N2A region, adjacent to the PEVK region, are the I82 and I83 domains, which are key to binding calpain/p94. However, this interaction is absent in the mdm‐mouse model, which contains an 83‐amino acid deletion spanning the C‐terminus of the I83 domain and the N‐terminus of the PEVK region, leading to muscular dystrophy with myositis. This mdm‐deletion disrupts the structure of the I83 domain, preventing normal force enhancement in the presence of calcium and inhibiting eccentric contractions. Our lab has demonstrated that the I83 domain exhibits calcium sensitivity at concentrations similar to those found in active muscle. In this current study, we further demonstrate that the tandem I82‐I83 domains exhibit cooperative unfolding, as seen by a single unfolding event, and that calcium enhances the stability of the tandem I82‐I83 domains. The NMR structure of this construct exhibits a tighter interface between I82 and I83 than is observed in the crystal structure, suggesting that the two structures might represent the structure in the relaxed state versus the structure under force. The calcium response of these domains is hypothesized to affect the function of the N2A region during muscle activation.