PROBES BOUND TO MYOSIN CYS-707 ROTATE DURING LENGTH TRANSIENTS IN CONTRACTION

It is widely conjectured that muscle shortens because portions of myos in molecules (the ''cross-bridges'') impel the actin filament to which they transiently attach and that the impulses result from rotation of the cross-bridges. Crystallography indicates that a cross-bridge is a rticulated-consisting of a globular catalytic/actin-binding domain and a long lever arm that may rotate. Conveniently, a rhodamine probe wit h detectable attitude can be attached between the globular domain and the lever arm, enabling the observer to tell whether the anchoring reg ion rotates. Well-established signature effects observed in shortening are tension changes resulting from the sudden release or quick stretc h of active muscle fibers. In this investigation we found that closely correlated with such tension changes are changes in the attitude of t he rhodamine probes. This correlation strongly supports the conjecture about how shortening is achieved.