Muscle contracts by sliding myosin filaments relative to actin filaments. The my

Question

Muscle contracts by sliding myosin filaments relative to actin filaments. The myosin head group hydrolyzes ATP to get the energy for its globular motor doman to move a long lever-arm domain. Suzuki et al. (1998) used GFP and its mutated relative blue fluorescent protein (BFP) in a fluorescence resonance energy transfer (FRET) experiment to demonstrate that during the working stroke the lever-arm domain titlts against the motor domain. They constructed a fusion protein in which GFP was attached through three glycines to the N-terminal end of the myosin and BFP was attached in the same way to the C-terminal end. Exciting the BFP causes the GFP to fluoresce with an efficiency of 0.333 after the ATP is hydrolyzed to ADP, and this corresponds to a distance of 3.8 nm. Before the hydrolysis, the FRET efficiency is 0.082. How far do the two flurophores move rleative to each other during the working stroke?

Explanation / Answer

The relation between the efficiency of the FRET process and the distance between the acceptor and donor flourophores is as follows:

E = Ro^6 / (Ro^6 + R^6)

where, E = efficiency

Ro = distance between donor and acceptor at 50% efficiency

R = actual distance between donor and acceptor

At an efficiency of 0.333:

0.333 = Ro^6 / (Ro^6 + 3.8^6)

Thus,

Ro = 1.001 nm

Now using the same formula and the value of Ro;

For an efficiency of 0.082,

0.082 = 1.001^6 / (1.001^6 + R^6)

R = 1.172 nm

Thus the distance moved relative to each other during the working stroke is:

3.8 - 1.172 = 2.628 nm

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