Twitchin of mollusc smooth muscles can induce "catch"-like properties in human skeletal muscle: support for the assumption that the "catch" state involves twitchin linkages between myofilaments.

Molluscan catch muscles can maintain tension with low or even no energy utilization, and therefore, they represent ideal models for studying energy-saving holding states. For many decades it was assumed that catch is due to a simple slowing of the force-generating myosin head cross-bridge cycles. However, recently evidences increased suggesting that catch is rather caused by passive structures linking the myofilaments in a phosphorylation-dependent manner. One possible linkage structure is the titin-like thick filament protein twitchin, which could form bridges to the thin filaments. Twitchin is known to regulate the catch state depending on its phosphorylation state. Here, we found that twitchin induces a catch-like stiffness in skinned human skeletal muscle fibres, when these fibres are exposed to this protein. Subsequent phosphorylation of twitchin reduces the stiffness. These findings support the assumption that catch of molluscan smooth muscle involves twitchin linkages between thick and thin filaments.