552 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



€. Muscle Fibres (Actomyosin) 



Proteins of muscle fibres. Fresh striated muscle contains about 20 % 

 of protein. On extraction of minced muscle with water, the soluble 

 protein myogen is dissolved; but this protein does not appear to be 

 involved in contractility. 



The contractile substance is contained in the insoluble fraction. If 

 treated with a slightly alkaline salt solution the protein myosin can be 

 extracted. Its I.E. P. is 5.3. Szent-Gyorgyi (1943) succeeded in 

 obtaining crystallized myosin and established in this way that ordinary 

 myosin solutions are contaminated with another protein, actin. Under 

 the electron microscope crystallized myosin appears to be a fibrillar 

 protein (Astbury, 1947/49). Its molecular weight is 1.5 million 

 (Snellman and Erdos, 1948). 



When myosin is properly extracted from muscle tissue, the main 

 part of the acfin remains in the residue. After drying this solid fraction 

 with acetone, the actin can be dissolved (I.E. P. 4.7). The solution is 

 perfectly clear and has a low viscosity. When left in the presence of 

 KCl, it becomes more viscous and ultimately turns into a thick thixo- 

 tropic gel. This gelation is due to a transformation of globular protein. 

 Both modifications are visible in the electron microscope. The 

 globular actin has been called G-actin and the fibrillar modification 

 F-actin. The particles of G-actin are ellipsoidal with the dimensions 

 300 A X 100 A. Rosza, Szent-Gyorgyi and Wyckoff (1949) have 

 shown how these particles form F-actin in situ by linear aggregation. 

 The filaments of F-actin are 100 A thick and appear to be segmented 

 with a period of 300 A. They aggregate laterally forming cross- 

 striated bands. 



The G-actin as seen in the electron microscope would have a 

 molecular weight of about 1.5 million, whereas, according to measure- 

 ments in the ultracentrifuge, it consists of only 4 Svedberg units 

 (MW = 70,000; Straub, 1948). The particles of G-actin visible in 

 the electron microscope therefore consist of about 24 actin molecules. 



Neither myosin nor F-actin is contractile. But if these two proteins 

 are brought together they react with each other forming the con- 

 tractile substance F-actomyosin. There is an optimal reaction with 

 a ratio of 2.5 parts myosin to i part F-actin. Snellman and Erdos 

 (1949) conclude from this fact that there is a stoichiometrical ratio 

 of these two components in the contractile muscle protein. 



