Contractile proteins actomyosin type

A great variety of contractile proteins of the actomyosin type have been described. They are found in many different animal species and in different tissues. No attempt will be made here to give a complete list this seems the more permissible as a detailed review on the subject has been published (Perry, 1960). 

Most likely, proteins of the actomyosin type also exist in other smooth muscles. litiegg and Strassner (1963), for instance, have isolated from arterial walls a protein which is soluble at high ionic strengths and which exhibits ATPase activity. More detailed ijiformation about this material is not yet available, and therefore a comparison with the other contractile proteins is not possible at the present time. 

The discussion of the similarities and dissimilarities of thrombosthenin and other contractile proteins will be restricted to the group of actomyosin-like proteins. As already mentioned, contractile substances of other types have not yet been isolated in a purified form and most of our knowledge about them comes from studies of glycerol-extracted cell models, which have not yet been prepared from blood platelets. Lastly, there can be little doubt that the mode of action of these other systems must be quite different from thrombosthenin. 

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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