Work Done by Titin Protein Folding Assists Muscle Contraction

Publication date: Available online 4 February 2016
Source:Cell Reports
Author(s): Jaime Andrés Rivas-Pardo, Edward C. Eckels, Ionel Popa, Pallav Kosuri, Wolfgang A. Linke, Julio M. Fernández
Current theories of muscle contraction propose that the power stroke of a myosin motor is the sole source of mechanical energy driving the sliding filaments of a contracting muscle. These models exclude titin, the largest protein in the human body, which determines the passive elasticity of muscles. Here, we show that stepwise unfolding/folding of titin immunoglobulin (Ig) domains occurs in the elastic I band region of intact myofibrils at physiological sarcomere lengths and forces of 6–8 pN. We use single-molecule techniques to demonstrate that unfolded titin Ig domains undergo a spontaneous stepwise folding contraction at forces below 10 pN, delivering up to 105 zJ of additional contractile energy, which is larger than the mechanical energy delivered by the power stroke of a myosin motor. Thus, it appears inescapable that folding of titin Ig domains is an important, but as yet unrecognized, contributor to the force generated by a contracting muscle.

Graphical abstract

Teaser

Titin, the largest protein in the human body, is responsible for muscle elasticity, while myosin motors are thought to provide the sole source of contractile energy. Here, we find that titin unfolding occurs at forces below 10 pN and that subsequent refolding can produce substantial amounts of work that assist in muscle contraction.


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