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Role of Actin C-Terminus in Regulation of Striated Muscle Thin Filament

Magorzata liwiska, Radosaw Skórzewski and Joanna Moraczewska

Kazimierz Wielki University in Bydgoszcz, Department of Experimental Biology, Bydgoszcz, Poland

Correspondence: Address reprint requests to Joanna Moraczewska, Kazimierz Wielki University in Bydgoszcz, Dept.of Experimental Biology, Chodkiewicza 30, 85-064 Bydgoszcz, Poland. E-mail: moraczjo{at}ukw.edu.pl.

In striated muscle, regulation of actin-myosin interactions depends on a series of conformational changes within the thin filament that result in a shifting of the tropomyosin-troponin complex between distinct locations on actin. The major factors activating the filament are Ca²⁺ and strongly bound myosin heads. Many lines of evidence also point to an active role of actin in the regulation. Involvement of the actin C-terminus in binding of tropomyosin-troponin in different activation states and the regulation of actin-myosin interactions were examined using actin modified by proteolytic removal of three C-terminal amino acids. Actin C-terminal modification has no effect on the binding of tropomyosin or tropomyosin-troponin + Ca²⁺, but it reduces tropomyosin-troponin affinity in the absence of Ca²⁺. In contrast, myosin S1 induces binding of tropomyosin to truncated actin more readily than to native actin. The rate of actin-activated myosin S1 ATPase activity is reduced by actin truncation both in the absence and presence of tropomyosin. The Ca²⁺-dependent regulation of the ATPase activity is preserved. Without Ca²⁺ the ATPase activity is fully inhibited, but in the presence of Ca²⁺ the activation does not reach the level observed for native actin. The results suggest that through long-range allosteric interactions the actin C-terminus participates in the thin filament regulation.