Myosin-actin interaction, troponin inhibitory action

The inhibitory action of troponin I on actomyosin-tropomyosin represents the depression of contractile interaction of myosin-actin by troponin-tropomyosin in the absence of Ca. In the absence of tropomyosin, the inhibitory action of troponin I is weak, whereas about 80% of the ATPase of actomyosin is inhibited by troponin I in the presence of... 

The discussion above stresses the importance of the CN4 region (residues 96-116) of troponin I in the inhibition of contractile interaction of myosin-actin in the presence of tropomyosin. The amino acid sequence of the CN4 region is shown in Fig. 3. Talbot and Hodges (1981) synthesized 12 peptide analogs of the CN4 sequence and examined their inhibitory action on actomyosin ATPase activity. The absence of residues 115 and 116 did not affect the activity, whereas the absence of residue 114 significantly decreased the inhibitory action. As to the N-terminal portion of the peptide, residues 96-103 were not essential, but the absence of Lys-105 decreased the inhibitory activity. A peptide containing the region of residues Lys-105-Val-l 14 showed about half of the inhibitory action of troponin I. The authors concluded that Lys-105 and the bulky side chain at Val-114 are essential for the inhibitory action of troponin I (Fig. 3). 

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). 

In the absence of troponin I inhibits the thin filament from interacting with myosin. Troponin T is also involved in this inhibition. Troponin T, on one hand, binds to tropomyosin and, on the other hand, binds to troponin I. The action of troponin T is mosdy to adjust the relation of troponin I to tropomyosin—actin for the inhibitory state in the absence of Ca. ... 

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