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Structural changes induced in scallop heavy meromyosin molecules by Ca2+ and ATP
1Department of Cell Biology, University of Massachusetts Medical School, Worcester 01655.
Journal of Muscle Research and Cell Motility
|August 1, 1992
Summary
Calcium and ATP alter scallop myosin head structure, influencing muscle contraction regulation. These ions affect heavy meromyosin (HMM) conformation, suggesting a role beyond immediate contractile activity.
Area of Science:
- Muscle physiology
- Biochemistry
- Structural biology
Background:
- Scallop adductor muscle utilizes myosin-linked regulation.
- Heavy meromyosin (HMM) is a key component of myosin.
- Understanding HMM structure is crucial for muscle function.
Purpose of the Study:
- To investigate the effects of calcium (Ca2+) and adenosine triphosphate (ATP) on scallop HMM structure.
- To determine how these ions influence HMM conformation and susceptibility to enzymatic digestion.
Main Methods:
- Physicochemical methods
- Ultrastructural methods
- Papain digestion as a structural probe
- High-performance liquid chromatography (HPLC) gel filtration
- Rotary-shadowed electron microscopy
Main Results:
- High Ca2+ levels increased papain digestion susceptibility at the head/tail junction in the presence of ATP.
- HPLC revealed two HMM fractions at low Ca2+ (ATP present), with one predominating at high Ca2+.
- Electron microscopy indicated two HMM conformations: 'heads-down' (bent towards tail) and 'heads-up' (away from tail).
- The proportion of 'heads-down' molecules decreased with increasing Ca2+.
- HMM behaved like the high Ca2+ state in the absence of ATP or high salt.
Conclusions:
- Scallop myosin heads exist in at least two conformations, regulated by Ca2+, ATP, and salt concentration.
- The conformational change equilibrium is likely too slow for direct roles in rapid muscle contraction.
- 'Heads-down' conformation may relate to inactive myosin states or filament assembly.