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J. Biol. Chem., Vol. 279, Issue 30, 31197-31204, July 23, 2004

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The DNase-I Binding Loop of Actin May Play a Role in the Regulation of Actin-Myosin Interaction by Tropomyosin/Troponin^*

Joanna Moraczewska, Joanna Gruszczynska-Biegala, Maria J. Redowicz, Sofia Yu. Khaitlina¶, and Hanna Strzelecka-Golaszewska||

From the Department of Muscle Biochemistry, Nencki Institute of Experimental Biology, 3 Pasteur Street, 02-093 Warsaw, Poland, the Kazimierz Wielki University of Bydgoszcz, Institute of Biology and Environmental Protection, 85-064 Bydgoszcz, Poland, and the ^¶Department of Cell Culture, Institute of Cytology, 194064 St Petersburg, Russia

Various lines of evidence suggest that communication between tropomyosin and myosin in the regulation of vertebrate-striated muscle contraction involves yet unknown changes in actin conformation. Possible participation of loop 38–52 in this communication has recently been questioned based on unimpaired Ca²⁺ regulation of myosin interaction, in the presence of the tropomyosin-troponin complex, with actin cleaved by subtilisin between Met⁴⁷ and Gly⁴⁸. We have compared the effects of actin cleavage by subtilisin and by protease ECP32, between Gly⁴² and Val⁴³, on its interaction with myosin S1 in the presence and absence of tropomyosin or tropomyosin-troponin. Both individual modifications reduced activation of S1 ATPase by actin to a similar extent. The effect of ECP cleavage, but not of subtilisin cleavage, was partially reversed by stabilization of interprotomer contacts with phalloidin, indicating different pathways of signal transmission from the N- and C-terminal parts of loop 38–52 to myosin binding sites. ECP cleavage diminished the affinity to tropomyosin and reduced its inhibition of acto-S1 ATPase at low S1 concentrations, but increased the tropomyosin-mediated cooperative enhancement of the ATPase by S1 binding to actin. These effects were reversed by phalloidin. Subtilisin-cleaved actin more closely resembled unmodified actin than the ECP-modified actin. Limited proteolysis of the modified and unmodified F-actins revealed an allosteric effect of ECP cleavage on the conformation of the actin subdomain 4 region that is presumably involved in tropomyosin binding. Our results point to a possible role of the N-terminal part of loop 38–52 of actin in communication between tropomyosin and myosin through changes in actin structure.

Received for publication, January 23, 2004 , and in revised form, May 20, 2004.

^* This work was supported by grants from the State Committee for Scientific Research (Poland) (to H. S.-G.) (6 P04A 01417) and the Nencki Institute and by a grant from the Russian Foundation for Basic Research (to S. Yu. K.) (02-04-48253). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^|| To whom correspondence should be addressed. Tel.: 48-22-589-2309; Fax: 48-22-822-5342; E-mail: hannas{at}nencki.gov.pl.

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