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Calcium-sensitive modulation of the actomyosin ATPase by fodrin
Abstract:
Fodrin, a spectrin-like protein isolated from brain, is a long flexible molecule which binds calmodulin and cross-links F-actin. The effects of fodrin on the actin-activated ATPase of myosin have been examined. When added after ATP, fodrin inhibited the actomyosin ATPase. Two to three times as much fodrin was required for inhibition in the presence of Ca2+ as in its absence. Complete inhibition in the absence of Ca2+ occurred at about one fodrin to 200 actins. Inhibition does not appear to result from fodrin cross-linking F-actin, and, thereby, preventing the myosin filaments from reaching the actin filaments; but cross-linking may promote inhibition by trapping the myosin filaments within the cross-linked F-actin. When added before ATP, fodrin stimulated the actomyosin ATPase almost 3-fold in the presence of Ca2+ and by less than 50% in the absence of Ca2+. Stimulation is thought to result from fodrin cross-linking F-actin. After several minutes the stimulations in Ca2+ were greatly reduced, and in the absence of Ca2+ the actomyosin ATPases were substantially inhibited. Whether added before or after ATP, fodrin inhibited the actin-activated ATPase of myosin subfragment 1. This inhibition was also slightly Ca2+ sensitive.
Insights
Brain fodrin, a spectrin-like protein, modulates actomyosin ATPase activity. It inhibits ATPase when added after ATP and stimulates it when added before, with Ca2+ sensitivity influencing these effects.
Area of Science:
- Biochemistry
- Cell Biology
- Molecular Motors
Background:
- Fodrin is a spectrin-like protein found in the brain.
- It is known to bind calmodulin and cross-link F-actin.
- Its influence on myosin's actin-activated ATPase activity is not fully understood.
Purpose of the Study:
- To investigate the effects of brain fodrin on the actin-activated ATPase activity of myosin.
- To determine the role of calcium ions (Ca2+) in these interactions.
Main Methods:
- Biochemical assays measuring actomyosin ATPase activity.
- Varying the order of addition of ATP, fodrin, and actin.
- Investigating the influence of Ca2+ on fodrin's effects.
Main Results:
- Fodrin inhibited actomyosin ATPase when added after ATP, requiring more protein in the presence of Ca2+.
- Fodrin stimulated actomyosin ATPase when added before ATP, particularly in the presence of Ca2+.
- Fodrin inhibited myosin subfragment 1's actin-activated ATPase activity in a Ca2+-sensitive manner.
Conclusions:
- Fodrin's interaction with F-actin influences myosin ATPase activity in a complex, Ca2+-dependent manner.
- Fodrin may trap myosin filaments within cross-linked actin networks, contributing to inhibition.
- The findings provide insights into the regulatory mechanisms of muscle contraction and cytoskeletal dynamics.