On the Working Stroke Elicited by Steps in Length and Temperature View Full Text


Ontology type: schema:Chapter     


Chapter Info

DATE

1998

AUTHORS

Gabriella Piazzesi , Natalia Koubassova , Malcolm Irving , Vincenzo Lombardi

ABSTRACT

Muscle contraction is generally thought to involve tilting of the light chain region of the myosin head. This could account for 5-10 nm of axial displacement as it moves from nearly perpendicular to the filament axis (the state at the beginning of the working stroke) to the rigor conformation (at the end of the working stroke). According to the kinetic model of Huxley and Simmons1, the extent that a cross-bridge progresses through the working stroke depends on the mechanical conditions. A large tilting occurs only when the fibre is allowed to shorten. Evidence for such tilting was provided by the changes in intensity of the third myosin meridional reflection (M3) following a step release2. However, there is little change in the M3 intensity when a force increase is elicited by a 10°C temperature jump3, and these results were interpreted to indicate that tilting is not the structural transition responsible for force generation. Here we present a simulation of the changes in the intensity of the M3 reflection elicited by step changes in either length or temperature, based on the atomic model of the actin-myosin head complex. The results show that the same set of assumptions for the motions associated with the working stroke can predict the response to both kinds of perturbation. The main difference is due to the larger extent of the working stroke elicited by the length step.The elementary process which drives force generation and filament sliding in muscle is thought to originate from a structural change, the working stroke, in the myosin head attached to actin1,4. The recent resolution of the atomic structure of the acto-myosin complex5,6 allowed this model to be refined: a substantial tilting of the light chain binding region of the myosin head can account for 5–10 nm of axial displacement as it moves from nearly perpendicular to the filament axis (the state at the beginning of the working stroke) to the rigor conformation (the state at the end of the working stroke). How far a cross-bridge progresses through the working stroke depends on the load imposed on it1. During the rise of tension in an isometric contraction or the tension redevelopment following a length step, the extent of the working stroke depends on the position of the cross-bridge along the overlap region7,8. In this case the cross-bridges generate force against the total sarcomere compliance, the sum of the filament compliance and the cross-bridge compliance. At the plateau of an isometric tetanus the steady force T0 is maintained by asynchronous cross-bridge interactions without change in filament strain, and a freshly attached cross-bridge generates force against only its own compliance.Recent structural9,10 and mechanical11 evidence suggests that the cross-bridge compliance is ∼2 nm per T0, much smaller than previously assumed1. This implies some revision of kinetic models for force generation that are based on the Huxley-Simmons paradigm12. In general, the smaller the cross-bridge compliance the higher the barrier of mechanical energy which prevents the cross-bridge from proceeding through the working stroke in isometric conditions, and the larger the structural difference between the state responsible for the isometric force and the state responsible for maintaining force during filament sliding.Thus the observation that there is little movement of the myosin heads at the isometric plateau13 is not necessarily in contradiction with the tilting head model. On the contrary, this model is strongly supported by measurements on single muscle fibres of the changes in the intensity of the third order myosin meridional reflection (M3, at spacing 1/14.5 nm−1, sensitive to axial movement of the myosin heads) in response to rapid length steps. The intensity of the M3 reflection changed with the same time course as the mechanical manifestation of the working stroke, the quick tension recovery elicited by the length step2,14,15. These results suggested that the heads are oriented near the perpendicular to the filament axis at the isometric plateau, in contrast with the more parallel conformation in rigor. Depending on the extent of the working stroke elicited by the step, the heads may pass through the perpendicular and eventually assume a more parallel orientation. When the temperature of an active isometric fibre from frog muscle is suddenly increased from 6°C to 16°C, causing a 1.7-fold rise in isometric tension, there is no substantial change in the intensity of M3 reflection3. This result was interpreted by the authors as evidence that tilting is not the structural basis of force generation, but rather “merely a consequence of the change in force induced by the length perturbation”. However we show below that the lack of change in the M3 intensity in the experiment of Bershitsky et al.3 can be quantitatively explained by the tilting head model. Changes in the intensity of the M3 reflection were calculated on the basis of a model for the structural changes elicited in the myosin heads by step changes in length or in temperature. The results show that the same set of assumptions for the motions associated with the working stroke and the elastic distortion of the myosin head can predict the responses to both kinds of perturbations. The starting point for the calculations was the atomic model for the structure of the myosin head bound to the actin filament5. This was considered to correspond to the configuration in a rigor fibre without any applied strain. The elastic distortion responsible for the 2 nm/T0 compliance in the cross-bridges was modelled as bending of the light chain, or neck, region of the myosin head, as a uniform cantilever clamped at residue 770 (the beginning of the long α-helix). The working stroke was modelled as a rotation of the neck region about residue 707, in the motor unit6. The length of the lever arm (the distance between residue 707 and the C-terminal of the heavy chain) is then 9.5 nm. The intensity of the M3 reflection was calculated from the 14.5 nm Fourier component of the mass density projection of the myosin heads onto the filament axis. Since shortening of about 1 nm is necessary to maximize the intensity of the M3 reflection15, we assume that at the isometric plateau the neck region is rotated 8° from the perpendicular to the filament axis, or 30° away from its rigor conformation. More... »

PAGES

259-264

Identifiers

URI

http://scigraph.springernature.com/pub.10.1007/978-1-4684-6039-1_30

DOI

http://dx.doi.org/10.1007/978-1-4684-6039-1_30

DIMENSIONS

https://app.dimensions.ai/details/publication/pub.1053676143

PUBMED

https://www.ncbi.nlm.nih.gov/pubmed/9889837


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Here we present a simulation of the changes in the intensity of the M3 reflection elicited by step changes in either length or temperature, based on the atomic model of the actin-myosin head complex. The results show that the same set of assumptions for the motions associated with the working stroke can predict the response to both kinds of perturbation. The main difference is due to the larger extent of the working stroke elicited by the length step.The elementary process which drives force generation and filament sliding in muscle is thought to originate from a structural change, the working stroke, in the myosin head attached to actin1,4. The recent resolution of the atomic structure of the acto-myosin complex5,6 allowed this model to be refined: a substantial tilting of the light chain binding region of the myosin head can account for 5–10 nm of axial displacement as it moves from nearly perpendicular to the filament axis (the state at the beginning of the working stroke) to the rigor conformation (the state at the end of the working stroke). How far a cross-bridge progresses through the working stroke depends on the load imposed on it1. During the rise of tension in an isometric contraction or the tension redevelopment following a length step, the extent of the working stroke depends on the position of the cross-bridge along the overlap region7,8. In this case the cross-bridges generate force against the total sarcomere compliance, the sum of the filament compliance and the cross-bridge compliance. 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Changes in the intensity of the M3 reflection were calculated on the basis of a model for the structural changes elicited in the myosin heads by step changes in length or in temperature. The results show that the same set of assumptions for the motions associated with the working stroke and the elastic distortion of the myosin head can predict the responses to both kinds of perturbations. The starting point for the calculations was the atomic model for the structure of the myosin head bound to the actin filament5. This was considered to correspond to the configuration in a rigor fibre without any applied strain. The elastic distortion responsible for the 2 nm/T0 compliance in the cross-bridges was modelled as bending of the light chain, or neck, region of the myosin head, as a uniform cantilever clamped at residue 770 (the beginning of the long α-helix). The working stroke was modelled as a rotation of the neck region about residue 707, in the motor unit6. The length of the lever arm (the distance between residue 707 and the C-terminal of the heavy chain) is then 9.5 nm. The intensity of the M3 reflection was calculated from the 14.5 nm Fourier component of the mass density projection of the myosin heads onto the filament axis. Since shortening of about 1 nm is necessary to maximize the intensity of the M3 reflection15, we assume that at the isometric plateau the neck region is rotated 8° from the perpendicular to the filament axis, or 30° away from its rigor conformation.
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21 M3 reflection
22 T0
23 acto-myosin
24 al
25 arm
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28 atomic model
29 atomic structure
30 authors
31 axial displacement
32 axis
33 barriers
34 basis
35 calculations
36 cantilever
37 cases
38 chain
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40 changes
41 compliance
42 components
43 conditions
44 configuration
45 conformation
46 consequences
47 contraction
48 contradictions
49 contrary
50 contrast
51 course
52 cross-bridge compliance
53 cross-bridge interaction
54 density projections
55 differences
56 displacement
57 distortion
58 elastic distortion
59 elementary processes
60 energy
61 et al
62 evidence
63 experiments
64 extent
65 fibers
66 filament axis
67 filament compliance
68 filament strain
69 filaments
70 force
71 force generation
72 force increases
73 frog muscle
74 generate forces
75 generation
76 head
77 head model
78 increase
79 intensity
80 interaction
81 isometric conditions
82 isometric contraction
83 isometric fibers
84 isometric force
85 isometric plateau
86 isometric tension
87 isometric tetanus
88 kind
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90 kinetic model
91 lack
92 lack of change
93 large extent
94 large tilting
95 length
96 length perturbations
97 length step
98 lever arm
99 light chain
100 light chain region
101 little change
102 little movement
103 load
104 main difference
105 manifestations
106 measurements
107 mechanical conditions
108 mechanical energy
109 mechanical manifestation
110 meridional reflection
111 model
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113 movement
114 muscle
115 muscle contraction
116 muscle fibers
117 myosin
118 myosin heads
119 myosin meridional reflection
120 neck
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122 observations
123 order
124 orientation
125 overlap
126 own compliance
127 parallel conformation
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129 perpendicular
130 perturbations
131 plateau
132 point
133 position
134 process
135 projections
136 quick tension recovery
137 rapid length steps
138 recent resolution
139 recovery
140 redevelopment
141 reflection
142 region
143 resolution
144 response
145 results
146 revision
147 rigor
148 rigor conformation
149 rigor fibers
150 rise
151 rise of tension
152 rotation
153 same set
154 same time course
155 sarcomere compliance
156 set
157 simulations
158 single muscle fibers
159 starting point
160 state
161 step
162 step change
163 strains
164 stroke
165 structural basis
166 structural changes
167 structural differences
168 structural transition
169 structure
170 substantial changes
171 substantial tilting
172 such tilting
173 sum
174 temperature
175 tension
176 tension recovery
177 tension redevelopment
178 tetanus
179 third order
180 tilting
181 time course
182 transition
183 uniform cantilever
184 working stroke
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