Figure 2.
Fig. 2. (A) Ribbon diagrams of the nucleotide-free
scallop S1 structure (Lower) and of scallop S1-VO[4] (Upper)
oriented such that the lower 50-kDa subdomains of these two
structures superimpose. An arrow indicates the approximate
direction of the actin filament axis relative to this subdomain,
deduced from an electron microscope study of S1-decorated actin
(16). The position of the ELC in the scallop nucleotide-free
structure is very close to that found in the electron-microscope
maps of actin decorated with vertebrate smooth muscle myosin S1
under rigor conditions (16). No data are available to indicate
how S1 binds to actin in the prepower stroke state; for
illustrative purposes only, we have chosen to orient this
structure by assuming that the interactions with the lower
50-kDa subdomain would be conserved. The lever arm is positioned
at 90° and
25° to the actin filament axis in the transition-state and
near-rigor structures, respectively. (Note that for measuring
angles, the lever-arm position is taken as a straight line drawn
from the N-terminal side of the lever-arm helix to the sharp
bend near the C terminus.) (B) Schematic drawings of the
transition-state and the near-rigor conformations of scallop
myosin from an interpretation of the structures seen in A. The
rotation of the converter (green)/relay (yellow) module during
the power stroke is amplified by the lever arm (scallop blue
helix, light chains omitted for clarity). The direction of the
movement of the subdomains in the transition between the two
states is indicated with black arrows. Although the subdomains
of the MD are similar in different isoforms, differences are
seen in the lever-arm position. To illustrate this point, the
position of the lever arm found in smooth muscle MDE (purple
helix, Upper) and that of chicken striated muscle myosin S1
(purple helix, Lower) is compared with the positions found for
scallop myosin in the transition state and near-rigor state,
respectively. Differences in the bending of the heavy-chain
helix at the junction between the converter and the lever arm
result in markedly different orientations for the lever arm of
these structures representing the same state. (C) Schematic
drawing of an orthogonal view of the structures seen in A. In
this orientation, the actin filament axis is approximately
perpendicular to the page, and one can thus estimate the
azimuthal component of the movement of the lever arm. This
component is very small in the case of scallop. In contrast,
bending of the heavy-chain helix at the pliant region in smooth
MDE in the transition-state conformation could lead to a large
azimuthal component during the power-stroke in this myosin.
Comparison of the transition-state and near-rigor conformations
in this view reveals changes in the position of the upper and
lower 50-kDa subdomains related to differences in both the
conformation of switch II and the actin-binding site.
90° and
25° to the actin filament axis in the transition-state and
near-rigor structures, respectively. (Note that for measuring
angles, the lever-arm position is taken as a straight line drawn
from the N-terminal side of the lever-arm helix to the sharp
bend near the C terminus.) (B) Schematic drawings of the
transition-state and the near-rigor conformations of scallop
myosin from an interpretation of the structures seen in A. The
rotation of the converter (green)/relay (yellow) module during
the power stroke is amplified by the lever arm (scallop blue
helix, light chains omitted for clarity). The direction of the
movement of the subdomains in the transition between the two
states is indicated with black arrows. Although the subdomains
of the MD are similar in different isoforms, differences are
seen in the lever-arm position. To illustrate this point, the
position of the lever arm found in smooth muscle MDE (purple
helix, Upper) and that of chicken striated muscle myosin S1
(purple helix, Lower) is compared with the positions found for
scallop myosin in the transition state and near-rigor state,
respectively. Differences in the bending of the heavy-chain
helix at the junction between the converter and the lever arm
result in markedly different orientations for the lever arm of
these structures representing the same state. (C) Schematic
drawing of an orthogonal view of the structures seen in A. In
this orientation, the actin filament axis is approximately
perpendicular to the page, and one can thus estimate the
azimuthal component of the movement of the lever arm. This
component is very small in the case of scallop. In contrast,
bending of the heavy-chain helix at the pliant region in smooth
MDE in the transition-state conformation could lead to a large
azimuthal component during the power-stroke in this myosin.
Comparison of the transition-state and near-rigor conformations
in this view reveals changes in the position of the upper and
lower 50-kDa subdomains related to differences in both the
conformation of switch II and the actin-binding site.