Probing Muscle Myosin Motor Action: X-Ray (M3 and M6) Interference Measurements Report Motor Domain Not Lever Arm Movement

X-ray Diffraction Evidence for Low Force Actin-Attached and Rigor-Like Cross-Bridges in the Contractile Cycle

“…Thus, our results show for the 3-state model (C in Figure 2b) that 48% + 20% of the heads are in the off and weak-binding states, that 32% of the heads are in the first stereospecific and rigor-like states and that these 32% of heads together produce about 96% of the tension. Note that in our previous analysis of the meridional diffraction pattern [41] and the effects of length steps, we also concluded that the motor domains, which dominate the diffraction patterns, would have 70% (off and weak) not moving with the step and 30% (strong) moving as the filaments move. The two sets of results are closely compatible.…”

“…In other words, the states in Figure 12b,c would look rather similar on the equator because of the relatively small mass of the lever arm compared to the motor domain. As discussed in the Introduction, we previously [41] came to the same conclusion that the lever arm would make little contribution to the meridional diffraction pattern at low resolution because of its small mass.…”

“…In thinking about what this might mean, we believe [41] as do others that active muscle contains a mixture of at least three myosin head structural states; some strongly attached heads (AM.ADP.Pi, AM.ADP, AM), some detached, resetting heads (M.ATP to M.ADP.Pi) and thirdly some heads in a state (AM.ADP.Pi*; possibly weak binding, pre-powerstroke heads) which would give a relatively strong A(11) peak, an A(10) not far from its relaxed value and strong outer intensities at the A(30), A(31) and A(32) positions. This latter state may be the non-force-producing initial actin-attached state in fish muscle as suggested by Harford and Squire [15].…”

“…These diffraction patterns carry a wealth of information, but they can sometimes be over-interpreted. For example, in a series of papers, the so-called M3 reflection of the meridian of diffraction patterns from frog muscles (the M3 corresponds to the 14.3 nm repeat of “crowns” of myosin heads in the myosin filaments), was used in an attempt to deduce the angles that the lever arms of myosin heads make on the motor domain under different conditions [27,28,29,30,31,32,33,34,35,36,37,38,39,40]; for further references see [41]. Unfortunately, this was not unambiguous, and we showed [41] that these results were simply reporting the relative axial positions of different motor domains in different populations of myosin heads without specifying the location of the lever arm.…”

Obituary: Professor Gerald Elliott

“…Piazzesi et al 2002;Reconditi et al 2004;Huxley et al 2006a, b and many more) were interpreted almost entirely in terms of movements of the lever arms of the myosin heads. It was not until others looked at alternative interpretations of these observations (Knupp et al 2009) that it was shown these observed effects were almost entirely due to relative movements of the motor domains of the myosin heads, some attached to actin and some detached, and that sadly these intensities revealed little about the behaviour of the lever arms. It is this kind of testing of alternatives that we should be doing all the time if we don't want to lead ourselves down blind alleys, and it is what we need to do to take the field forward.…”