40-kDa Protein from Thin Filaments of the Mussel Crenomytilus grayanus Changes the Conformation of F-Actin during the ATPase Cycle

---

V. V. Sirenko¹, A. H. Simonyan¹, A. V. Dobrzhanskaya², N. S. Shelud’ko², and Y. S. Borovikov^1*

¹Institute of Cytology, Russian Academy of Sciences, Tikhoretsky pr. 4, 194064 St. Petersburg, Russia; fax: (812) 297-0341; E-mail: boroviko@mail.cytspb.rssi.ru

²Zhirmunsky Institute of Marine Biology, Far Eastern Branch of the Russian Academy of Sciences, ul. Palchevskogo 17, 690059 Vladivostok, Russia; fax: (423) 231-0900; E-mail: sheludko@stl.ru

^* To whom correspondence should be addressed.

Received June 26, 2012; Revision received November 9, 2012

---

Polarized fluorimetry was used to study in ghost muscle fibers the influence of a 40-kDa protein from the thin filaments of the mussel Crenomytilus grayanus on conformational changes of F-actin modified by the fluorescent probes 1,5-IAEDANS and FITC-phalloidin during myosin subfragment (S1) binding in the absence of nucleotides and in the presence of MgADP or MgATP. The fluorescence probes were rigidly bound with actin, which made the absorption and emission dipoles of the probes sensitive to changes in the orientation and mobility of both actin monomer and its subdomain-1 in thin filaments of the muscle fiber. On modeling different intermediate states of actomyosin, the orientation and mobility of oscillators of the dyes were changed discretely, which suggests multistep changes in the actin conformation during the cycle of ATP hydrolysis. The 40-kDa protein influenced the orientation and mobility of the fluorescent probes markedly, suppressing changes in their orientation and mobility in the absence of nucleotides and in the presence of MgADP, but enhancing these changes in the presence of MgATP. The calponin-like 40-kDa protein is supposed to prevent formation of the strong binding state of actomyosin in the absence of nucleotides and in the presence of MgADP but to activate formation of this state in the presence of MgATP.

---

KEY WORDS: calponin-like muscle protein of mussel, ghost fibers, ATP analogs, intermediate states of actomyosin, conformational changes of myosin subfragment-1

DOI: 10.1134/S0006297913030097

---