Contributionof Z-line constituents to the formation of the contraction bands of chicken myofibrils on addition of Mg2+-ATP.

1. Z-line constituents contributing to the formation of the contraction bands in chicken myofibrils were investigated by using the rabbit calcium-activated factor (CAF). 2. CAF digestion hampered the formation of the contraction bands and increased the amount of soluble proteins which are apparently derived from the Z-line. Among several proteins released by CAF digestion, a 95,000 dalton component, which corresponds to alpha-actinin, was predominant, followed by a 220,000 dalton component and several other proteins. 3. An apparent inverse relationship between the formation of the contraction bands and the release of Z-line proteins was observed. It is concluded that the presence of Z-line, including at least the two components mentioned above, is essential for the formation of the contraction bands.