# Titin’s P-zone domains A164–167 are essential for thick filament structural arrangement

## TL;DR

Deleting specific domains in the protein titin disrupts muscle structure and function, showing their essential role in organizing thick filaments in muscle cells.

## Contribution

The study identifies the structural role of titin’s P-zone domains A164–A167 in organizing thick filaments and maintaining titin conformations.

## Key findings

- Deleting domains A164–A167 in titin causes altered thick filament architecture and disrupted α and β conformations.
- Muscle function deficits and increased passive stiffness were observed in the TtnΔA164–167 mouse model.
- Structural changes suggest a loss of a myosin helical repeat due to the deletion.

## Abstract

Hoover Browne et al. report that deleting the P-zone domains A164–167 from titin in a novel mouse model reveals their critical role in muscle function and thick filament structure, including maintaining titin’s α and β conformations.

The sarcomeric protein titin plays a central role in thick filament structure and function through its modular A-band domains, including the understudied P-zone, which links the C-zone to the M-band. To investigate the first four domains of titin’s P-zone (A164–A167), we deleted them in a mouse model (TtnΔA164–167). Echocardiography and cardiomyocyte mechanics revealed mild changes to diastolic function and enlargement of the heart, but preserved contractility. The EDL muscle showed contractile deficits at the whole muscle level and increased passive stiffness at the myofiber level. Immunoelectron and super-resolution microscopy revealed altered thick filament architecture, including a ∼40-nm shift of titin and myosin binding protein-C epitopes toward the M-band, disruption of titin’s α and β conformations, and shorter thick filaments. The structural changes are consistent with the loss of a myosin helical repeat. These findings establish a key structural role of titin’s P-zone domains A164–A167 in templating thick filament protein arrangement, including the importance of titin’s α and β conformations.