Actin participation in actomyosin contraction

1. 1. Pellicular fibers made of actin-free myosin show no ability to contract, at either pH 7.6 or 9.0. 2. 2. Fibers of actomyosin contract strongly and optimally at pH 7.6, but do not contract at pH 9.0. 3. 3. With increasing actin content, up to approximately 27%, the actomyosin fibers exhibit an increasing contractile activity. 4. 4. Fibers formed of actomyosin and of myosin hydrolyze ATP enzymically, the former at 7.6, the latter at pH 6.4 and 9.5. 5. 5. Myosin fibers, formed and maintained in the cold, do not contract when the temperature is raised in the presence of ATP, whereas actomyosin fibers under the same conditions, contract strongly with the rise in temperature. 6. 6. It is concluded that myosin without actin does not contract, and that actin is a specific requirement for contraction in the muscle protein model system.