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Gross structural features of myosin head during sliding movement of actin as studied by quick-freeze deep-etch
1Department of Fine Morphology, University of Tokyo, Japan.
Abstract:
With quick-freeze deep-etch electron microscopy coupled with mica-flake technique, I showed previously that myosin subfragment-1 (S1) attached to F-actin in the presence of ATP is short and rounded, in contrast to its elongated and tilted appearance under rigor condition [J. Biochem. 106, 751-770 (1989)]. I further indicated that each head of heavy meromyosin (HMM) changes its configuration in a likely manner as above by the addition of various nucleotides, i.e. heads were pear-shaped in the absence of nucleotide, in a ball-on-a-stick appearance when complexed with ADP and strongly kinked to the particular direction in the presence of ATP or ADP.Vi [J. Muscle Res. Cell Motility 12, 313 (1991)]. Such morphological data not only corroborates the independent biophysical evidences suggesting gross conformational changes of myosin head upon binding ATP or ADP.Vi, but also provide strong evidence for the distinct polarity in the structure of each myosin head. Negatively stained image of chemically cross-linked acto-S1 also included cross-bridges sharply kinked to the same direction, confirming the above observation. Attempts were made to examine if such conformational change of myosin cross-bridge occurs during actomyosin superprecipitation. Samples were quick-frozen during rapid turbidity-increasing phase where actin filaments actively slide past myosin heads. The resultant image included actin-attached myosin heads all in a kinked configuration with the same polarity as observed for HMM. Several heads associated with a single actin filament were bent to the same direction suggesting that myosin heads might be in a kinked configuration with distinct polarity during contraction.
Insights
Myosin heads change shape dramatically when interacting with actin filaments, shifting from elongated to kinked configurations during muscle contraction. This conformational change is crucial for muscle movement and shows distinct polarity.
Area of Science:
- Muscle Physiology
- Molecular Biology
- Biophysics
Background:
- Myosin subfragment-1 (S1) and heavy meromyosin (HMM) are key components of muscle contraction.
- Previous studies suggested myosin heads undergo conformational changes upon nucleotide binding.
Purpose of the Study:
- To investigate the conformational changes of myosin cross-bridges during actomyosin superprecipitation.
- To provide morphological evidence for myosin head configuration changes during muscle contraction.
Main Methods:
- Quick-freeze deep-etch electron microscopy coupled with mica-flake technique.
- Analysis of negatively stained chemically cross-linked acto-S1 complexes.
- Cryo-electron microscopy of actomyosin samples during superprecipitation.
Main Results:
- Myosin heads (S1 and HMM) exhibit distinct conformations depending on nucleotide presence (ATP, ADP, ADP.Vi).
- Actin-attached myosin heads during actomyosin superprecipitation showed a kinked configuration with consistent polarity.
- Morphological data supports biophysical evidence of myosin head conformational changes and reveals distinct head polarity.
Conclusions:
- Myosin heads undergo significant conformational changes, adopting a kinked structure with specific polarity during muscle contraction.
- These findings corroborate existing biophysical data and offer direct structural evidence for myosin's role in muscle movement.