Actin is a bit easier to understand than myosin... that's because it doesn't do as many things.

    You already know that actin is a microfilament that is made up of two long chains of g-actin twisted together.  Each g-actin molecule (each green circle above) in each chain has a myosin-binding site on it; that is the site where the myosin head can attach to the actin microfilament.  Because each g-actin molecule has this binding site, you should be able to picture that there are many, many places along the actin microfilament where myosin can attach.

    However, the myosin-binding sites on actin are not always available to myosin.  This is important, because it would not be a good thing if our myosin and actin were always grabbing on to each other-- that would cause constant tension and rigidity in our muscles.  Therefore, the availability of these myosin-binding sites is regulated by other molecules.  Those other molecules are:  tropomyosin and troponin.

    Tropomyosin and troponin are both proteins.  Tropomyosin is a long, threadlike protein that wraps around actin.   Tropomyosin wraps actin in such a way that it lies directly over all the myosin-binding sites on actin.  So now you know how these sites are blocked-- physically, by tropomyosin.  See Figure 9.9 in your textbook to see how tropomyosin interacts with the actin filament.

    How are the myosin-binding sites on actin ever available, then?  Well, obviously, we are going to have to move the tropomyosin out of the way.  That's why troponin is there!  Troponin is a much smaller, globular-type of protein that sits on top of tropomyosin.  It just sits there, unless calcium ions come along.  You see, calcium ions (Ca²⁺) interact with troponin.  And when this happens, the troponin in the troponin-Ca²⁺ complex undergoes a conformational change... and changes shape.  As troponin changes shape, it drags tropomyosin around with it, pulling tropomyosin clear of the myosin-binding sites on actin.  Both tropomyosin and troponin, and how they move with Ca²⁺ around, is shown in your Interactive Physiology CD in slides 3, 4, and 11 - 14.

Summary of dynamic properties of the thin filament:

1. Actin has myosin-binding sites that are covered by tropomyosin.
2. Troponin is attached to tropomyosin.
3. In the presence of Ca²⁺, troponin undergoes a conformational change.
4. Tropomyosin is pulled out of the way of the myosin-binding sites by the troponin-Ca²⁺ complex.
5. Actin is free to interact with myosin.

Please notice that these dynamic properties of the thin filament, as well as any interaction with myosin, all depends on Ca²⁺.

Here's a pretty picture showing how the myosin head (or cross-bridge) and actin attach to one-another... in this image, the actin filament is running up and down, while the myosin head is coming in from the right.  Also, no tropomyosin or troponin are shown.

© 2006 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.