# Tropomodulin–Tropomyosin Interplay Modulates Interaction Between Cardiac Myosin and Thin Filaments

## TL;DR

This study explores how tropomodulin interacts with tropomyosin to regulate actin-myosin interactions in heart muscle, revealing isoform-specific effects.

## Contribution

The study identifies specific interactions between Tmod isoforms and cardiac tropomyosin isoforms, revealing their impact on actin-myosin dynamics.

## Key findings

- Tmod1 and Tmod4 form stable complexes with cardiac tropomyosin isoforms Tpm1.1 and Tpm1.2.
- Tmod1 reduces calcium sensitivity of actin-myosin sliding velocity in a tropomyosin isoform- and myosin type-dependent manner.
- Tmod1 affects thin filament activation differently depending on the tropomyosin isoform and myosin type.

## Abstract

Tropomodulin (Tmod) is an actin-binding protein that interacts with tropomyosin and the actin filament at the pointed end. The influence of Tmod on the thin filament activation in the myocardium is not clear. We studied the interactions of Tmod1 and Tmod4 with the cardiac tropomyosin isoforms Tpm1.1 and Tpm1.2 using size-exclusion chromatography, a pull-down assay, and cross-linking with glutaraldehyde. We found that Tmod1 and Tmod4 form complexes with both Tpm1.1 and Tpm1.2, indicating durable interactions between these proteins. The effects of both Tmods on the actin–myosin interaction were studied using an in vitro motility assay. Tmod did not affect the sliding velocity of bare F-actin. Tmod1 slightly dose-dependently decreased the sliding velocity of F-actin–Tpm1.1 filaments and had no effect on the velocity of F-actin–Tpm1.2 filaments. With ventricular myosin, Tmod1 reduced the calcium sensitivity of the sliding velocity of thin filaments containing Tpm1.1 but did not affect it with filaments containing Tpm1.2. With atrial myosin, Tmod1 decreased the calcium sensitivity of the sliding velocities of thin filaments containing both Tpm1.1 and Tpm1.2. We can conclude that Tmod takes part in the regulation of actin–myosin interactions in the myocardium through interactions with Tpm. The effect of Tmod on the activation of thin filaments depends on the protein isoforms.