Seminar

Regulation of muscle elasticity via redox reactions coupled to the un/folding dynamics of titin domains

David Giganti, PhD

The giant protein titin acts as a molecular spring responsible for the passive elasticity of skeletal and cardiac muscles. Because of its high cost of expression and assembly in the sarcomere, the long titin filament has a slow turnover. This raises an important question: how is titin rapidly adjusting its stiffness during muscle activity? We propose that titin elasticity is finely regulated via a sophisticated interplay between redox reactions and mechanical unfolding and elongation of tandem repeat immunoglobulin (Ig) domains. We have used sequence and structural analyses to identify a conserved triad of buried and neighboring cysteines in the Ig domains of titin. Single molecule force spectroscopy enabled direct identification of the three potential disulfide bonds that can be established by the cysteine triad. We find that disulfide bonds decrease the mechanical stability of the parent Ig domains limiting their mechanical extension in a position-dependent manner. Furthermore, we show that mechanical unfolding of Ig domains triggers redox reactions whereby disulfide bonds can be cleaved and/or exchanged causing further extensions of the polypeptide chain. In addition, titin disulfides critically accelerate protein refolding. Our results suggest that hundreds of versatile and switchable disulfide bonds contribute to setting the elasticity of striated muscle by finely tuning the stiffness of titin.