Actin amino acid composition

The fifth was a molecular biologist, who smiled sweetly and pointed out that all the others had missed the point. The frog jumps because of the biochemical properties of its muscles. The muscles are largely composed of two interdigitated filamentous proteins, actin and myosin, and they contract because the protein filaments slide past each other. This property of the actin and myosin is dependent on the amino acid composition of the two proteins, and hence on chemical, and thus on physical properties. In the last analysis, the molecular biologist insisted, following James Watson, we are all nothing but subatomic particles. 

Actin. Rabbit muscle G-actin is globular with a molecular weight of 4.2 X 104. In the presence of salts it is polymerized into F-action (34). The principal properties of fish actin (35-37,40, 43,44), including amino acid composition (41), are similar to rabbit actin, but fish actin is more readily extracted from wet muscle by salt solutions as a viscous solution of actomyosin (22,35,36,45). 

Kuroda et al. (1981) have discovered an interesting protein very similar to actin in molecular size and amino acid composition. They called it eu-actinin (42 kDa). eu-Actinin is located in Z lines and it interacts with F-actin to form bundles (Kuroda and Masaki, 1984). Unlike a-actinin, eu-actinin inhibits the onset of superprecipitation of actomyosin. Actin-binding protein (ABP, also called filamin 240 kDa X 2 Stossel and Hartwig, 1975), which causes gelation of actin filaments, has also been shown to exist in Z lines (Corner and Lazarides, 1981 Bechtel, 1979). 

Study of the molecular biology of calcium regulation of muscle contraction was initiated by the discovery of a new protein factor sensitizing actomyosin to calcium ions (Ebashi, 1963 Ebashi and Ebashi, 1964). This protein factor was called native tropomyosin, because of its similarity in amino acid composition to tropomyosin, which had been discovered earlier (Bailey, 1946, 1948). It was soon found that this factor is a complex of tropomyosin and a new globular protein, termed troponin (Ebashi and Kodama, 1965 Ebashi et al., 1968). Thus four proteins, i.e., myosin, actin, troponin, and tropomyosin, are involved in calcium-regulated physiological muscle contraction (Ebashi et al., 1968, 1969 Ebashi and Endo, 1968). The contractile interaction between myosin and actin is depressed by troponin and tropomyosin in the absence of calcium ions. When calcium ion acts on troponin, this depression is removed and the contractile interaction is then activated (Figs. 1 and 2). 

The amino acid composition of connectin is listed in Table IV. It is an acidic protein rather similar to actin. There are hardly any recognizable differences in amino acid composition between j8-connectin and a-con-nectin-rich samples. It is of some interest to note that C protein (MW 135 kDa) located on myosin filaments has an amino acid composition similar to connectin (Offer et al., 1973). Immunological tests showed that the two proteins are distinguishable. 

RNA-binding motif Actinin-type actin-binding motifs DNA-binding motifs Ribosomal-protein motifs Prokaryotic DNA-binding motifs N-myristoylation motif Amino acid composition... 

Troponin is a heterotrimeric protein involved in the regulation of striated and cardiac muscle contraction. Most troponin in the cell is bound to the actin-tropomyosin complex in the muscle fibril. The three subunits of troponin consist of troponin-C, troponin-T, and troponin-l, each with a specific function in the regulatory process. Troponin-T and troponin-l exist as different isoforms in cardiac and skeletal muscle (sequences with a different amino acid composition), thus allowing the development of specific antibodies against each form. As a consequence, either cardiac troponin-T or cardiac troponin-l may be rapidly measured in blood samples by immunoassay with a good degree of specificity. 

No new X-ray data have become available since the summary in this journal by Bailey in 1944. The X-ray examination of actin (Astbury ei al., 1947) has shown indirectly that the wide angle diagram of myosin (actomyosin-L-myosin mixture) is that of the L-myosin component. The relation between the X-ray data and amino acid analysis was also thoroughly discussed by Bailey (1944) in the same review. Few modifications in the amino-acid composition have been made since that time, and the present position cf. Bailey, 1948) is given in Table X. 

Recently small fibrils known as microtubules have been shown to be present in the cytoplasm. During nuclear division these aggregate in parallel bundles radiating from the centriole and form the spindle fibres. Microtubules have also been isolated from cilia and flagella. They are composed of globular units having a molecular weight of 60000 and an amino acid composition which resembles that of the muscle protein actin. 

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