Abstract
At a resting sarcomere length of approximately 2.2 µm bony fish muscles put into rigor in the presence of BDM (2,3-butanedione monoxime) to reduce rigor tension generation show the normal arrangement of myosin head interactions with actin filaments as monitored by low-angle X-ray diffraction. However, if the muscles are put into rigor using the same protocol but stretched to 2.5 µm sarcomere length, a markedly different structure is observed. The X-ray diffraction pattern is not just a weaker version of the pattern at full overlap, as might be expected, but it is quite different. It is compatible with the actin-attached myosin heads being in a different conformation on actin, with the average centre of cross-bridge mass at a higher radius than in normal rigor and the myosin lever arms conforming less to the actin filament geometry, probably pointing back to their origins on their parent myosin filaments. The possible nature of this new rigor cross-bridge conformation is discussed in terms of other well-known states such as the weak binding state and the ‘roll and lock’ mechanism; we speculate that we may have trapped most myosin heads in an early attached strong actin-binding state in the cross-bridge cycle on actin.
Highlights
Rigor is the state into which a muscle enters in the absence of ATP (Adenosine triphosphate)
The pattern consists of a set of layer-lines and meridionals which can be indexed on a repeat of approximately 2145 Å, which is 5 × 429 and 3 × 715 Å
The first few of these beating layer-lines were identified by Haselgrove [16] and since they have been observed at several other spacings, reported by Squire and Harford [8]; Yagi [11]; Koubassova and Tsaturyan [12]; and references therein
Summary
Rigor is the state into which a muscle enters in the absence of ATP (Adenosine triphosphate). The myosin heads, projecting from the thick filaments, form permanent strong bonds with the actin monomers in the thin filaments. These strongly attached states of myosin heads on actin monomers in the absence of ATP are known as rigor complexes and are thought to be similar in configuration to the AM step in the cross-bridge cycle in contracting muscle [1]. These strong rigor attachments cause the muscle to become stiff as in rigor mortis. Higher resolution structures have been determined by Holmes et al [4] and von der Ecken et al [5]
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