scholarly journals Myosin motors that cannot bind actin leave their folded OFF state on activation of skeletal muscle

2021 ◽  
Vol 153 (11) ◽  
Author(s):  
Massimo Reconditi ◽  
Elisabetta Brunello ◽  
Luca Fusi ◽  
Marco Linari ◽  
Vincenzo Lombardi ◽  
...  
Keyword(s):  
Skeletal Muscle ◽  
Binding Sites ◽  
Muscle Activation ◽  
Sarcomere Length ◽  
Rapid Change ◽  
Thick Filament ◽  
Actin Binding ◽  
Thin Filaments ◽  
Thick Filaments ◽  
Myosin Motors

The myosin motors in resting skeletal muscle are folded back against their tails in the thick filament in a conformation that makes them unavailable for binding to actin. When muscles are activated, calcium binding to troponin leads to a rapid change in the structure of the actin-containing thin filaments that uncovers the myosin binding sites on actin. Almost as quickly, myosin motors leave the folded state and move away from the surface of the thick filament. To test whether motor unfolding is triggered by the availability of nearby actin binding sites, we measured changes in the x-ray reflections that report motor conformation when muscles are activated at longer sarcomere length, so that part of the thick filaments no longer overlaps with thin filaments. We found that the intensity of the M3 reflection from the axial repeat of the motors along the thick filaments declines almost linearly with increasing sarcomere length up to 2.8 µm, as expected if motors in the nonoverlap zone had left the folded state and become relatively disordered. In a recent article in JGP, Squire and Knupp challenged this interpretation of the data. We show here that their analysis is based on an incorrect assumption about how the interference subpeaks of the M3 reflection were reported in our previous paper. We extend previous models of mass distribution along the filaments to show that the sarcomere length dependence of the M3 reflection is consistent with <10% of no-overlap motors remaining in the folded conformation during active contraction, confirming our previous conclusion that unfolding of myosin motors on muscle activation is not due to the availability of local actin binding sites.

scholarly journals Bridgelike interconnections between thick filaments in stretched skeletal muscle fibers observed by the freeze-fracture method.

1986 ◽  
Vol 102 (3) ◽  
pp. 1093-1098 ◽  
Author(s):  
S Suzuki ◽  
G H Pollack
Keyword(s):  
Skeletal Muscle ◽  
Muscle Fibers ◽  
Freeze Fracture ◽  
Thick Filament ◽  
Rapid Freezing ◽  
Semitendinosus Muscle ◽  
Thin Filaments ◽  
Thick Filaments ◽  
Cross Bridges ◽  
Half Length

The ultrastructure of frog semitendinosus muscle was explored using the freeze-fracture, deep-etch, rotary-shadowing technique. Mechanically skinned fibers were stretched to decrease or eliminate the overlap of thick and thin filaments before rapid freezing with liquid propane. In relaxed, contracting, and rigor fibers, a significant number of bridgelike interconnections, distinct from those observed in the M-region, were observed between adjacent thick filaments in the non-overlap region. Their half-length and diameter corresponded approximately to the known dimensions of the cross-bridge (or myosin S-1). The interconnection may thus be formed by the binding of two apposed cross-bridges projecting from adjacent thick filaments. Fixation with 0.5% glutaraldehyde for 5-10 min before freezing effectively preserved these structures. The results indicate that the interconnections are genuine structures that appear commonly in stretched muscle fibers. They may play a role in stabilizing the thick filament lattice, and possibly in the contractile process.

scholarly journals A mechanical model of the half-sarcomere which includes the contribution of titin

2018 ◽  
Author(s):  
Pertici ◽  
M. Caremani ◽  
M. Reconditi
Keyword(s):  
Mechanical Model ◽  
Sarcomere Length ◽  
Thick Filament ◽  
Theoretical Treatment ◽  
Complete Model ◽  
Active Muscle ◽  
Spring Element ◽  
Thick Filaments ◽  
Myosin Motors

ABSTRACTThe evidence, in both resting and active muscle, for the presence of an I-band spring element like titin that anchors the Z line to the end of the thick filament did not yet produce a proper theoretical treatment in a complete model of the half-sarcomere. The textbook model developed by A.F. Huxley and his collaborators in 1981, which provides that the half-sarcomere compliance is due to the contribution of the compliances of the thin and thick filaments and actin-attached myosin motors, predicts that at any sarcomere length the absence of attached motors results in an infinite half-sarcomere compliance, in contrast with the observations. Growing evidence for the presence of a titin-like I-band spring urges the 1981 model to be implemented to include the contribution of this element in the mechanical model of the half-sarcomere. The model described here represents a tool for the interpretation of measurements of half-sarcomere compliance at long sarcomere lengths and for investigations of the possible role of titin as the mechano-sensor in thick filament regulation.

Distribution of c-protein isoforms in sarcomeres of developing chick skeletal muscle

1988 ◽  
Vol 46 ◽  
pp. 226-227
Author(s):  
D. A. Fischman ◽  
J. E. Dennis ◽  
T. Obinata ◽  
H. Takano-Ohmuro
Keyword(s):  
Skeletal Muscles ◽  
Thick Filament ◽  
Protein Isoforms ◽  
Actin Binding ◽  
Striated Muscles ◽  
C Protein ◽  
Sarcomeric Protein ◽  
Thick Filaments ◽  
Cross Bridges ◽  
Bridge Bearing

C-protein is a 150 kDa protein found within the A bands of all vertebrate cross-striated muscles. By immunoelectron microscopy, it has been demonstrated that C-protein is distributed along a series of 7-9 transverse stripes in the medial, cross-bridge bearing zone of each A band. This zone is now termed the C-zone of the sarcomere. Interest in this protein has been sparked by its striking distribution in the sarcomere: the transverse repeat between C-protein stripes is 43 nm, almost exactly 3 times the 14.3 nm axial repeat of myosin cross-bridges along the thick filaments. The precise packing of C-protein in the thick filament is still unknown. It is the only sarcomeric protein which binds to both myosin and actin, and the actin-binding is Ca-sensitive. In cardiac and slow, but not fast, skeletal muscles C-protein is phosphorylated. Amino acid composition suggests a protein of little or no αhelical content. Variant forms (isoforms) of C-protein have been identified in cardiac, slow and embryonic muscles.

scholarly journals What makes skeletal muscle striated? Discoveries in the endosarcomeric and exosarcomeric cytoskeleton

2018 ◽  
Vol 42 (4) ◽  
pp. 672-684 ◽  
Author(s):  
Jack A. Rall
Keyword(s):  
Skeletal Muscle ◽  
Muscle Fiber ◽  
Intermediate Filament ◽  
Sarcomere Length ◽  
Thick Filament ◽  
Skeletal Muscle Fiber ◽  
Elastic Behavior ◽  
Filament Length ◽  
Iconic Images ◽  
Elastic Protein

One of the most iconic images in biology is the cross-striated appearance of a skeletal muscle fiber. The repeating band pattern shows that all of the sarcomeres are the same length. All of the A bands are the same length and are located in the middle of the sarcomeres. Furthermore, all of the myofibrils are transversely aligned across the muscle fiber. It has been known for 300 yr that skeletal muscle is striated, but only in the last 40 yr has a molecular understanding of the striations emerged. In the 1950s it was discovered that the extraction of myosin from myofibrils abolished the A bands, and the myofibrils were no longer striated. With the further extraction of actin, only the Z disks remained. Strangely, the sarcomere length did not change, and these “ghost” myofibrils still exhibited elastic behavior. The breakthrough came in the 1970s with the discovery of the gigantic protein titin. Titin, an elastic protein ~1 µm in length, runs from the Z disk to the middle of the A band and ensures that each sarcomere is the same length. Titin anchors the A band in the middle of the sarcomere and may determine thick-filament length and thus A-band length. In the 1970s it was proposed that the intermediate filament desmin, which surrounds the Z disks, connects adjacent myofibrils, resulting in the striated appearance of a skeletal muscle fiber.

Natural Variability in the Length of Thin and Thick Filaments in Single Fibres From a Crab, Portunus Depurator

1970 ◽  
Vol 6 (2) ◽  
pp. 559-592
Author(s):  
CLARA FRANZINI-ARMSTRONG
Keyword(s):  
Thick Filament ◽  
Natural Variability ◽  
Thin Filaments ◽  
Thick Filaments ◽  
Striation Spacing ◽  
Free Region ◽  
Cross Striation ◽  
Single Fibres ◽  
Length Changes ◽  
Medium Type

The carpopodite flexor of the walking legs of the crab Portunus depurator contains fibres belonging to 3 groups. These are characterized by differences in the cross-striation spacing. Fibres having sarcomeres of approximately 4, 5 and 7 µm are here called short, medium and long sarcomere types, respectively. Within individual fibres belonging to any of the groups the length of the A band is not constant. Up to 25 % length differences have been measured in A bands belonging even to the same fibril. The bridge-free regions of the thick filaments are not always in the centre, so that the filaments are often asymmetric. Analogally, the L line, resulting from the alignment of the bridge-free regions of the thick filaments, may be asymmetrically placed in the Z band. The length of the bridge-free region in crab thick filaments is 60 nm, while the corresponding region in vertebrate thick filaments is 120 nm. This is discussed in terms of a possible model of the filament. The length of the thin filaments is proportional to that of the thick filaments in the corresponding portion of the sarcomere. When two A bands of different length occur in adjacent positions along the fibril, the Z line is not a centre of symmetry. The ratio of thin to thick filament number is variable in individual fibrils. In general, the ratio is higher in the medium sarcomere type fibres than in the short sarcomere type. Stretched and shorter portions of single fibres of the medium type have been examined and the A-band length populations compared. From such a study it can be deduced that passive length changes occur in crab fibres by sliding of thin and thick filaments.

An ultrastructural study of crossbridge arrangement in the fish skeletal muscle thick filament

1989 ◽  
Vol 94 (3) ◽  
pp. 391-401
Author(s):  
R.W. Kensler ◽  
M. Stewart
Keyword(s):  
Skeletal Muscle ◽  
Fourier Transforms ◽  
Thick Filament ◽  
Fish Muscle ◽  
X Ray Diffraction ◽  
X Ray ◽  
Thick Filaments ◽  
Gold Fish ◽  
Diffraction Patterns ◽  
Cross Bridges

A procedure has been developed for isolating gold-fish skeletal muscle thick filaments that preserves the near-helical arrangement of the myosin cross-bridges under relaxing conditions. These filaments have been examined by electron microscopy and computer image analysis. Electron micrographs of the negatively stained filaments showed a clear periodicity associated with the crossbridges, with an axial repeat every 42.9 nm. Computed Fourier transforms of the negatively stained filaments showed a series of layer lines confirming this periodicity, and were similar to the X-ray diffraction patterns of fish muscle obtained by J. Hartford and J. Squire. Analysis of the computed transform data and filtered images of the isolated fish filaments demonstrated that the myosin crossbridges lie along three strands. Platinum shadowing demonstrated that the strands have a right-handed orientation, and computed transforms and filtered images of the shadowed filaments suggest that the crossbridges are perturbed both axially and azimuthally from an ideal helical arrangement.

scholarly journals LOCALIZATION OF MYOSIN FILAMENTS IN SMOOTH MUSCLE

1968 ◽  
Vol 37 (1) ◽  
pp. 105-116 ◽  
Author(s):  
Robert E. Kelly ◽  
Robert V. Rice
Keyword(s):  
Smooth Muscle ◽  
Cross Section ◽  
Striated Muscle ◽  
Thick Filament ◽  
Longitudinal Axis ◽  
Thin Filaments ◽  
Chicken Gizzard ◽  
Thick Filaments ◽  
Myosin Filaments ◽  
Muscle Myosin

Thick myosin filaments, in addition to actin filaments, were found in sections of glycerinated chicken gizzard smooth muscle when fixed at a pH below 6.6. The thick filaments were often grouped into bundles and run in the longitudinal axis of the smooth muscle cell. Each thick filament was surrounded by a number of thin filaments, giving the filament arrangement a rosette appearance in cross-section. The exact ratio of thick filaments to thin filaments could not be determined since most arrays were not so regular as those commonly found in striated muscle. Some rosettes had seven or eight thin filaments surrounding a single thick filament. Homogenates of smooth muscle of chicken gizzard also showed both thick and thin filaments when the isolation was carried out at a pH below 6.6, but only thin filaments were found at pH 7.4. No Z or M lines were observed in chicken gizzard muscle containing both thick and thin filaments. The lack of these organizing structures may allow smooth muscle myosin to disaggregate readily at pH 7.4.

scholarly journals Changes in thick filament length in Limulus striated muscle.

1977 ◽  
Vol 75 (2) ◽  
pp. 366-380 ◽  
Author(s):  
M M Dewey ◽  
B Walcott ◽  
D E Colflesh ◽  
H Terry ◽  
R J Levine
Keyword(s):  
Horseshoe Crab ◽  
Striated Muscle ◽  
Thick Filament ◽  
Filament Length ◽  
Thin Filaments ◽  
Thick Filaments ◽  
Wide Range ◽  
The Difference

Here we describe the change in thick filament length in striated muscle of Limulus, the horseshoe crab. Long thick filaments (4.0 microns) are isolated from living, unstimulated Limulus striated muscle while those isolated from either electrically or K+-stimulated fibers are significantly shorter (3.1 microns) (P less than 0.001). Filaments isolated from muscle glycerinated at long sarcomere lengths are long (4.4 microns) while those isolated from muscle glycerinated at short sarcomere lengths are short (2.9 microns) and the difference is significant (P less than 0.001). Thin filaments are 2.4 microns in length. The shortening of thick filaments is related to the wide range of sarcomere lengths exhibited by Limulus telson striated muscle.

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