Introduction to Muscle Contraction, Part 2

The Myosin Cross Bridge has two Confirmations

According to the Lymn-Taylor scheme, the myosin cross bridge would be expected to have two discernible conformations: (1) when it first attaches to actin with the products of hydrolysis still bound with the lever at the beginning of the working stroke; and (2) at the end of the working stroke when the phosphate and ADP are released. This sequence is often referred to as the power stroke. The end state is referred to as rigor, since it is the state muscle enters on ATP depletion. It is also called strong because it binds to actin quite tightly. The initial state is called the weak binding state because of its low affinity for actin (see Geeves and Conibear, 1995). We might anticipate that these two states of the myosin cross bridge might exist independently from actin and indeed protein crystallography shows this to be the case.

The chicken S1 structure was solved without bound nucleotide. Furthermore, the chicken S1 crystal structure fits excellently into the electron micrograph reconstructions of the strong actin-myosin nucleotide-free interaction (decorated actin). Therefore the crystal structure of chicken S1 would appear to represent the end of the power stroke or rigor state.

In addition, Rayment et al have studied a crystalline fragment of the dictyostelium myosin II cross-bridge which has been truncated after residue 761 (equivalent to gg781). The truncation eliminates the lever arm and the associated light chains (which aids crystallization). However, the converter domain is still present. The crystal structures of the 761 construct have been determined with a number of ATP analogs, particularly ADP.BeFx (Fisher et al. 1995) and ADP.vanadate (Smith and Rayment 1996). ADP.vanadate complexes are used as analogs of the transition state or possibly of the ADP.π state.

While the ADP.BeFx state looks similar to rigor, the ADP.vanadate structure shows, compared to the chicken rigor structure dramatic changes in shape of the S1 structure, There is a closing of the 50K upper/lower domain cleft, particularly around the γ-phosphate binding pocket, and large movements in the C-terminal region. The 50K upper/lower domains rotate a few degrees w.r.t. each other around the helix gg648-666 in a way which closes the nucleotide binding pocket (Fig 5) - a movement of some 5Å. At the same time the outer end of the long helix (the so called switch 2 helix, residues gg475-507) bends out 24° at residue V497. This is coupled to a rotation of the converter domain (gg711-781) by 70°. The fulcrum is provided by the mutual rotation of the distal part of the SH1-SH2 helix around the distal part of the switch 2 helix.

A model of this new state is shown in Fig. 6. The coordinates of the missing lever arm have been generated from the chicken coordinates by superimposing the converter domains (for comparison Fig. 7), we have generated the corresponding diagram from the coordinates with ADP.BeFx bound in the active site (Fisher et al. 1995) which, for reasons stated below, we take to be the ADP state. On comparing Figs. 6 and 7 one sees that the end of the lever arm has moved through 12nm along the actin helix axis, which is greater than most estimates of the size of the power stroke. Therefore, it would appear that the ADP.vanadate state is indeed a model of the anticipated beginning of power stroke state.

The mechanism for coupling the movement of the lever arm with the status of the nucleotide binding pocket revealed by this structure suggests that the two events are tightly coupled: pocket closed, lever up (beginning) - pocket open, lever down (end).