Slip Sliding Away: Load-dependence of Velocity Generated by Skeletal Muscle Myosin Molecules in the Laser Trap
Edward P. Debold 1, Joseph B. Patlak 1 and David M. Warshaw 1*
1 University of Vermont
* To whom correspondence should be addressed. E-mail: warshaw{at}physiology.med.uvm.edu.
Submitted on August 19, 2005
Revised on August 24, 2005
Accepted on 29 August 2005
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Abstract |
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Skeletal muscle's ability to shorten and lengthen against a load is a fundamental property, presumably reflecting the inherent load-dependence of the myosin molecular motor. Here we report the velocity of a single actin filament translocated by a mini-ensemble (~8) of skeletal myosin heads under constant loads up to 15pN in a laser trap assay. Actin filament velocity decreased with increasing load hyberbolically with unloaded velocity and stall force differing by a factor of 2 with [ATP] (30 vs 100 µM). Analysis of actin filament movement revealed that forward motion was punctuated with rapid backward 60nm slips, with the slip frequency increasing with load. At stall force, myosin-generated forward movement was balanced by backward slips, while at loads greater than stall, myosin could no longer sustain forward motion, resulting in negative velocities as in eccentric contractions of whole muscle. Thus, the force-velocity relationship of muscle reflects both the inherent load-dependence of the actomyosin interaction and the balance between forward and reverse motion observed at the molecular level.
Key Words:
Force-velocity, molecular motor, optical tweezers, single molecule
