Calcium-induced splitting of connectin filaments into β-connectin and a 1,200-kDa subfragment

When rabbit skeletal muscle myofibrils were treated with a solution containing 0.1 mM Ca²⁺ and 30 μg of leupeptin/ml, α-connectin, which forms very thin filaments in myofibrils, was split into β-connectin and a 1,200-kDa subfragment. A part of β-connectin located near the junction between β-connectin and the subfragment seems to have an affinity for calcium ions and to be susceptible to the binding of large amounts of calcium ions. The calcium- binding site on β-connectin is localized near the N₂ line in the I band, and the subfragment Is localized adjacent to the Z disk. It is possible that connectin filaments change their elasticity during the contraction-relaxation cycle of skeletal muscle at the physiological concentration of calcium ions. Because postmortem skeletal muscles lose their elasticity and become plastic in association with the calcium-specific splitting of connectin filaments, the splitting is considered to be a factor in meat tenderization during postrigor ageing.