Please note these web pages are part of an assignment for a graduate course in Advanced Biochemistry and Molecular Biology BCMB8010 at the University of Georgia. Questions should be directed to Laura Harris laharris@uga.edu Myosin II: An Essential Protein of Muscle Contraction Abstract In striated muscle, the striations seen under the microscope are in reality filaments of actin and myosin.  Myosin is also known as the "thick filament"; actin is the "thin filament" (1). In general, myosin heads bind to the actin filaments and slide the actin directionally down the cell.  When the actin filaments in the muscle tissue move together as the result of a stimulus a muscle contraction begins.  After the contraction takes place the myosin unbinds from the actin and the muscle cells reach a resting state (1).             The myosin structure forms a filamentous section and a crossbridge section (1). The crossbridges have the responsibility of binding to actin while the filamentous section is primarily structural (2). The crossbridges have two heads with a separate actin and ATP binding site on each head (3). The presence of the ATP binding site suggests that ATP is a necessary energy source for the contractile reaction.             While there are several types of myosin, myosin II is the form most commonly found (3). The full myosin II structure consists of six protein subunits:  two heavy chains and four light chains.  The heavy chains have their N terminus in the globular head and extend into a straight portion that makes a stable complex with other myosin molecules to make a linear filament (4). The four light chains do not have a covalent linkage to the heavy chains and have functions that are, as yet, not well understood (4). The light chains are present just above the myosin head and before the filament region (4).             In the contractile reaction with actin the heads of myosin first bind to actin as the result of an intercellular signal for contraction.  The heads then move to form a contraction as actin is pulled towards other actins coming from the other side of the cell.  This reaction uses stored energy present in the myosin light chain from a previous contraction that cleaved ATP.  The ADP and Pi from that reaction are still attached to the heads.  When these are released another ATP can bind to the heads and be cleaved to store energy for another contraction (1). In this manner the myosin will be ready for the contractile signal and can respond quickly.             Molecules present on actin can inhibit the binding of the myosin heads to regulate cell contraction.  The protein tropomyosin binds to actin and covers the binding site for myosin on actin.  Tropomyosin is held in place by the protein troponin, which has a binding site for calcium.  When calcium binds to troponin, the tropomyosin changes its location and myosin binds to its binding sites on the actin filament (1). It is in this way that muscle contraction by myosin can be regulated.  In some lower organisms, calcium is the only regulator of muscle contraction (5). References Vander, A., Sherman, J., and D. Luciano. Human Physiology:  The Mechanisms of Body Function 8th ed. McGraw Hill. 2001. Yu, F. et al. Effects of thyroid hormone receptor gene disruption on myosin isoform expression in mouse skeletal muscles. Am J Physiol Regul Integr Comp Physiol. 278: R1545-R1554. 2000. Tanaka-Takiguchi, Y et al. The Elongation and Contraction of Actin Bundles are Induced by Double-headed Myosins in a Motor Concentration-dependent Manner.  Journal of Molecular Biology. Volume 341, Issue 2. 467-476. 2004 Szczesna-Cordary, D. Regulatory Light Chains of Striated Muscle Myosin. Structure, Function and Malfunction. Current Drug Targets – Cardiovascular and Haematological Disorders. 187-197. 2003. Colegrave, M. et al.  (2003)  Evaluation of the symmetric model for myosin-linked regulation: effect of site-directed mutations in the regulatory light chain on scallop myosin.  Journal of Biochemistry.  Vol 374, Issue 1.  89-96. PDF Version of Full Report CHIME Files Oral Report Figures Back to the BCMB8010 Homepage