Processive vertebrate myosin-dependent contraction of formaldehyde-fixed S. japonicus actomyosin rings. (A) Schematic representation of the experimental design. (B) Formaldehyde-fixed rings incubated with ATP do not contract (top). Addition of a processive myosin does not induce ring contraction without ATP (bottom). (C) Contraction of the fixed rings incubated with a processive myosin and ATP (nine rings).