Contractile proteins from striated muscle

Most of our knowledge about contractile proteins comes from the extensive studies of the actomyosin from striated muscle. For more detailed accounts the reader is referred to one of the recent reviews on the subject (Weber, 1957 Perry, I960 Szent-Gyorgyi, 1960). 

Fresh uterus contains from 1.1 to 3.5 % contractile protein, which is extractable under the same conditions as the actomyosin from striated muscle (Table III). It is interesting to note that the solubility of this material changes in the course of gravidity, the actomyosin from nongravid uterus passing into solution at an ionic strength of 0.3-0.4 n, independent of the amount of ATP present. The material from gravid uterus is already soluble at 0.2-0.3 and its solubility is influenced by ATP (Ledermair, 1959). 

Smooth muscle differs from skeletal muscle in various ways. Smooth muscles—which are found, for example, in blood vessel walls and in the walls of the intestines—do not contain any muscle fibers. In smooth-muscle cells, which are usually spindle-shaped, the contractile proteins are arranged in a less regular pattern than in striated muscle. Contraction in this type of muscle is usually not stimulated by nerve impulses, but occurs in a largely spontaneous way. Ca (in the form of Ca -calmodulin see p.386) also activates contraction in smooth muscle in this case, however, it does not affect troponin, but activates a protein kinase that phosphorylates the light chains in myosin and thereby increases myosin s ATPase activity. Hormones such as epinephrine and angiotensin II (see p. 330) are able to influence vascular tonicity in this way, for example. 

The contraction of ascidian smooth muscle was found to be regulated through the troponin-tropomyosin system. But the action of troponin components was different from that of troponin of vertebrate striated muscles (Endo and Obinata, 1981). In this system, the inhibitory action of troponin I (MW 24,000) is less remarkable compared with vertebrate skeletal troponin I, and troponin C (MW 18,000) does not neutralize the inhibition by troponin I. But upon further addition of troponin T (MW 33,000) in the concomitant presence of all three components and tropomyosin, the contractile interaction of myosin and actin is activated. In this case, the action of troponin T has some similarity with that of the above-mentioned cardiac troponin T hybridized with skeletal troponin C-I. Since actomyosin, without these regulatory proteins, is inhibited regardless of Ca concentration, Ca " and troponin-tropomyosin are activators for contraction of actomyosin in ascidian smooth muscle. In this respect, the type of Ca + regulation of ascidian smooth muscle is the same as that for vertebrate smooth muscles which do not contain troponin (Ebashi, 1980). 

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