Electron microscopic observations on the interaction of the myosin head subunit with actin in myofibrils.

Glycerol-extracted rabbit psoas muscle fibres were treated with a solution containing the head subunits of myosin. Interaction of the isolated myosin heads with actin filaments in situ was indicated by an increase in the density of the I-band and by an increased diameter of actin filaments alongside their whole length. We conclude that actin filaments are able to interact with a considerably larger number of myosin molecules than that available in the myofibril under in vivo conditions.