Rough process of the experiment

The key of the experiment is to attach the labeled fluorescent actin filaments to the myosin motors so that we can observe the motility of the actin filaments under microscope. This video demonstrates the key step roughly.

 

Explanation of the motility assay of fluorescent actin filaments

Myosin motors can be attached to the surface of the glass slide. These fluorescent actin filaments will bind to the motor domain of the attached myosins. When ATP is added, myosin motors move the actin filaments. These rabid movements can be observed under fluorescent microscope. As the actin filaments appear to crawl across the slide.
This video explains the principle of the motility of actin filaments.

Motility of actin filaments

This video shows clearly about the motility of actin filaments. The quality of the video is fairly nice.

Model for myosin action

The binding of ATP dissociates myosin from actin. ATP hydrolysis then induces a conformational change that displaces the myosin head group. This is followed by binding of the myosin head to a new position on the actin filament and release of ADP and P_i. The return of the myosin head to its original conformation drives actin filament sliding.

Introduction of myosin thick filaments

The thick filaments of muscle consist of several hundred myosin molecules, associated in a parallel staggered array by interactions between their tails. The globular heads of myosin bind actin, forming cross-bridges between the thick and thin filaments. It is important to note that the orientation of myosin molecules in the thick filaments reverses at the M line of the sarcomere. The polarity of actin filaments (which are attached to Z discs at their plus ends) similarly reverses at the M line, so the relative orientation of myosin and actin filaments is the same on both halves of the sarcomere. As discussed later, the motor activity of myosin moves its head groups along the actin filament in the direction of the plus end. This movement slides the actin filaments from both sides of the sarcomere toward the M line, shortening the sarcomere and resulting in muscle contraction.


The organization of myosin thick filaments:

Thick filaments are formed by the association of several hundred myosin II molecules in a staggered array. The globular heads of myosin bind actin, forming cross-bridges between the myosin and actin filaments. The orientation of both actin and myosin filaments reverses at the M line, so their relative polarity is the same on both sides of the sarcomere.

Introduction of Myosin II

The type of myosin present in muscle (myosin II) is a very large protein (about 500 kd) consisting of two identical heavy chains (about 200 kd each) and two pairs of light chains (about 20 kd each). Each heavy chain consists of a globular head region and a long α-helical tail. The α-helical tails of two heavy chains twist around each other in a coiled-coil structure to form a dimer, and two light chains associate with the neck of each head region to form the complete myosin II molecule.
The structure of Myosin II:

The myosin II molecule consists of two heavy chains and two pairs of light chains (called the essential and regulatory light chains). The heavy chains have globular head regions and long α-helical tails, which coil around each other to form dimers.

Actin, Myosin and Cell Movement

Actin filaments, usually in association with myosin, are responsible for many types of cell movements. Myosin is the prototype of a molecular motors—a protein that converts chemical energy in the form of ATP to mechanical energy, thus generating force and movement. The most striking variety of such movement is muscle contraction, which has provided the model for understanding actin-myosin interactions and the motor activity of  myosin molecules. However, interactions of actin and moysin are responsible not only for muscle contraction but also for a variety of movements of nonmuscle cells, including cell division, so these interactions play a central role in cell biology. Moreover, the actin cytoskeleton is responsible for the crawling movements of cells across a surface, which appear to be driven directly by actin polymerization as well as actin-myosin interactions.
The structure of actin-myosin:

As shown in the figure. The myosin filaments lie next to the actin filaments and have the capability to temporarily bind to the actin, causing the muscle to move. This binding capability provided the basis of the sliding filament model of muscle contraction. This model proposes the myosin head binds to the actin filament and then rotates to a different position, possibly as much as a forty five degree change, which can be accomplished by a change in structure. The myosin head is comparable to a stretched spring held in place by free energy generated by ATP hydrolysis.

The following video shows the sliding-filament model composed by actin and myosin. Myosin is thick, actin is thin.