Evidence for novel 30,000-50,000Mr cofactor in the activation of muscle.

Abstract A new approach is described for reconstituting a fully desensitized skeletal muscle fiber to restore its contractility. These studies revealed a novel regulatory cofactor, 30–50,000Mr by filtration (26–55kDa by SDS PAGE). It was shown to be critical for the Ca 2+ -activation in the physiological milieu. The cofactor was present in skeletal and cardiac muscles as well as in brain, but not in kidney and liver. The cofactor may be a second Ca 2+ switch in a dual-regulation scheme for vertebrate muscle, or could provide an essential link in the cross-bridge cycle beyond activation.