In myofibrils, titin (also called connectin) molecules span from Z line to M line and constitute a third filament system containing an elastic domain in the I band. This giant protein is particularly sensitive to proteolysis in situ. Treatment of rabbit skeletal myofibrils with exogenous proteinases induces a release of titin fragments, which are detected in the soluble myofibrillar fraction. The cleavage of titin occurs at specific points localized at the proximity of Z line and could lead to a concomitant release of alpha-actinin.

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