Microscopic observations on the interaction of heavy meromyosin-S-1 and actin in myofibrils.

The binding of the proteolytic myosin fragment, HMM-S-1, to the non-overlapping part of actin filaments in intact myofibrils can be demonstrated under the phase contrast microscope as a contrast reversal of striation. The same effect can be seen on ghost myofibrils (after myosin extraction) where the whole length of the I-filaments is bare. HMM-S-1-loaded ghost myofibrils contracted upon addition of ATP in agrement with the recent report of Oplatka et al. (1974a, b) but under the same conditions ghost myofibrils not treated with HMM-S-1) also contracted. If the ghosts were prepared under conditions more favourable to myosin extraction, contraction became nil or negligible even when we loaded then ghosts with S-1. Thus we attribute the effect described by Oplatka's group to a small numer of residual myosin filaments in the ghosts.